Correlating Feather Structure, Wettability, and Robustness with Ecological Behavior of
Aquatic Birds
By
Jesus 0. Guardado
Submitted to the Department of Materials Science and Engineering
in Partial Fulfillment of the Requirements for the Degree of
ARCBHVES
Bachelor of Science
at the
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
Massachusetts Institute of Technology
NOV 10 2015
June 2011
LIBRARIES
0 2011 Jesus 0. Guardado. All rights reserved.
The author hereby grants to MIT permission to reproduce and to
distribute publicly paper and electronic copies of this thesis document in whole or
in part in any medium now known or hereafter created.
Signature of Author..........................................
Signature redacted
Departmrof44p'rials Science and Engineering
May 6, 2011
Certified by....................................................
redacted
'Signature
Snrec
Robert E. Cohen
St. Laurent Professor of Chemical Engineering
Theis'Spervisor
Certified by......................................Signafure
redacted
Michael F. Rubner
TDK Professor of Materials Science and Engineering
Course III ThesifrS ervisor
Accepted by..........................
.Signature
redacted
Lionel C. Kimerling
Thomas Lord Professor of Materials Science and Engineering
Chairman, Undergraduate Thesis Committee
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Correlating Feather Structure, Wettability, and Robustness with Ecological
Behavior of Aquatic Birds
By
Jesus 0. Guardado
Submitted to the Department of Materials Science and Engineering
on May 6, 2011 in Partial Fulfillment of the Requirements for the
Degree of Bachelor of Science in Materials Science and Engineering
ABSTRACT
In nature, aquatic birds can interact with water without their feathers being easily wetted; some
species dive tens of meters and emerge to spread their wings to dry. In past studies attempting to
connect such ecological behavior and feather structure, the typical approach of microscopy has
demonstrated the difficulty in characterizing specimens as delicate and complex as feathers by
visual techniques alone. In this work, the question was addressed of how various species balance
the wettability problem with the need to dive to various depths or to remain on or near the water
surface as dictated by their feeding habits. Texture of wing feathers from six different species of
aquatic birds was characterized by measuring contact angles and applying the previously
developed framework of the effective spacing ratio, D*, and robustness factor, A *, according to
the Cassie-Baxter relation for composite interfaces. This "effective microscopy" technique was
successfully employed to assess the wettability and robustness of bird feather textures. The
observable water-related behaviors of diving, wing-spreading, shallow foraging, and dabbling for
the species studied were explained as partly determined by feather structure, exhibiting effectiveD* analysis as an adequate technique for characterizing complex, textured surfaces, fabricated or
natural.
Thesis Supervisor: Professor Robert E. Cohen
Title: St. Laurent Professor of Chemical Engineering
Course III Thesis Supervisor: Professor Michael F. Rubner
Title: TDK Professor of Materials Science and Engineering
3
This page was intentionally left blank.
4
Table of Contents
1
2
LIST OF FIGURES .............................................................................................................................
LIST OF TABLES ...............................................................................................................................
ACKNOW LEDGEM ENTS...................................................................................................................8
6
7
INTRODUCTION ..............................................................................................................................
9
BACKGROUND ..............................................................................................................................
11
2.1
ECOLOGICAL BEHAVIOR OF INTEREST: BIRD INTERACTIONS WITH FLUIDS .................................................
2.1.1
2.1.2
PROFILES OF AQUATIC BIRD SPECIES STUDIED ..................................................................................
2.2
2.2.1
2.2.2
2.2.3
2.2.4
Phalacrocoracidae: cormorants and shags.....................................................................
Anhinga rufa: African darter .........................................................................................
Tadorna tadorna: common shelduck..............................................................................
Anas platyrhynchos: mallard.........................................................................................
2.3
EFFECTIVE MICROSCOPY TO CHARACTERIZE SURFACES IN TERMS OF D ..................................................
2.4
W ETTABILITY, COMPOSITE INTERFACES, AND TEXTURED SURFACES ........................................................
11
14
16
18
18
20
21
21
22
24
Cassie-Baxter (CB) relation and D*................................................................................
Robustness parameter, A*, and breakthrough pressure, Pb............................................
25
27
2.5
IMPACT OF OPENNESS OF THE W EAVE ON SURFACE WETTABILITY..........................................................
29
2.6
IMPACT OF OPENNESS OF THE WEAVE ON BREAKTHROUGH PRESSURE, P6 ........................
31
2.4.1
2.4.2
3
Past attempts to characterize feather topography ........................................................
Difficulties of characterizing topography of bird feathers .............................................
. . . .. . . . . . . . . . .. . . . . . . . .
EXPERIM ENTAL PROCEDURES ......................................................................................................
BIRD FEATHER SPECIMENS ...........................................................................................................
COATING M ETHODOLOGY............................................................................................................
3.1
3.2
3.2.1
3.2.2
3.3
Fluorodecyl POSS (polyhedral oligomeric silsesquioxane) and Tecnoflon *1 solution ......
Coating sm ooth and textured surfaces.........................................................................
SURFACE CHARACTERIZATION ......................................................................................................
3.3.1
3.3.2
Contact angle measurements ........................................................................................
Scanning electron microscopy (SEM)..............................................................................
RESULTS........................................................................................................................................
4
34
34
35
35
36
37
37
37
38
4.1
APPARENT CONTACT ANGLES FOR BIRD FEATHERS FROM EACH SPECIES...................................................
38
4.2
"EFFECTIVE MICROSCOPY" OF FEATHERS WITH D*.............................................................................
40
4.3
SCANNING ELECTRON MICROSCOPY (SEM ) OF SELECTED FEATHERS ......................................................
43
4.4
CRITICAL DIVING DEPTH FROM ROBUSTNESS PARAMETER, A , AND BREAKTHROUGH PRESSURE, Ps................. 47
5
DISCUSSION..................................................................................................................................48
5.1
APPARENT CONTACT ANGLES USED TO ASSESS WETTABILITY OF BIRD FEATHERS ........................................
48
5.2
EFFECTIVE MICROSCOPY (1-D MODEL) CHARACTERIZES FEATHER TEXTURE .............................................
49
5.3
EFFECTIVE D* IS BETTER INDICATOR OF FEATHER WETTABILITY THAN D* FROM GEOMETRY .............................
50
5.4
ROLE OF WETTABILITY (O Aov) AND FEATHER TEXTURE (D*) ON ECOLOGICAL BEHAVIOR..............................
51
5.5
ROLE OF ROBUSTNESS FACTOR, A*, AND BREAKTHROUGH PRESSURE, PB, ON ECOLOGICAL BEHAVIOR................ 54
6
CONCLUSIONS ..............................................................................................................................
56
7
FUTUR E W ORK .............................................................................................................................
57
8
REFERENCES .................................................................................................................................
59
9
APPENDICES .................................................................................................................................
61
9.1
APPENDIX A: M ATLAB* CODE FOR D* FITTING PROCEDURE ...............................................................
61
9.2
APPENDIX B: SLIDING ANGLES MEASURED FOR SELECTED BIRDS AND LIQUIDS..............................................
62
5
List of Figures
FIGURE 1. BIRD FEATHERS INTERACTING W ITH LIQUIDS .........................................................................
11
FIGURE 2. EFFECT OF SURFACE CHEMISTRY ON THE WETTABILITY OF A FLAT SURFACE BY WATER AND OCTANE ....... 13
FIGURE 3: ECOLOGICAL BEHAVIORS OF INTEREST OF AQUATIC BIRDS.........................................................14
FIGURE 4. ATTEMPT TO CHARACTERIZE FEATHERS BY D* = (r + d) / r VALUES VISUALLY OBTAINED VIA MICROSCOPY
..........................................................................................................................................
15
FIGURE 5. SEM MICROGRAPHS (100X MAGNIFICATION) OF REED CORMORANT FEATHERS FROM THE SIX DIFFERENT
PRIMARY AND BODY CONTOUR FEATHER CATEGORIES.......................................................................16
FIGURE 6. GENERALIZED NON-WETTING DIAGRAM FOR TEXTURED SURFACES: DIP-COATED, WOVEN MESHES........23
FIGURE 7. SCHEMATIC DIAGRAMS OF THE INTERFACE FORMED BY A WATER DROPLET ..................................
FIGURE
25
8. SCHEMATIC DIAGRAM OF REPRESENTATIVE D REGIMES FOR A PARALLEL ARRAY OF CYLINDERS............27
FIGURE 9. EFFECT OF LENGTH SCALE, R, ON THE ROBUSTNESS OF A TEXTURED SURFACE ..............................
29
FIGURE 10. M ODEL OF BIRD FEATHER W ETTED BY OILS ...........................................................................
31
FIGURE 11. M ODEL OF BIRD FEATHER SUBMERGED IN WATER...................................................................33
FIGURE 12. PHOTOGRAPHS OF WING, BREAST, AND BELLY FEATHERS FOR THE SIX BIRD SPECIES.......................35
FIGURE 13. CHEMICAL STRUCTURE OF FLUORODECYL POSS AND TECNOFLON* FLUOROELASTOMER.................36
FIGURE 14. GENERALIZED NON-WETTING DIAGRAM FOR DIP-COATED BIRD FEATHERS FROM EACH SPECIES...........41
FIGURE 15. SCANNING ELECTRON MICROGRAPHS FOR TOPOGRAPHY OF WING FEATHERS OF AFRICAN DARTER AND
COM M ON SHELDUCK ...........................................................................................................
47
FIGURE 16. MATLAB* CODE USED TO ESTIMATE THE DIMENSIONLESS SPACING RATIO, D*, FOR EACH BIRD SPECIES
..........................................................................................................................................
6
61
List of Tables
TABLE 1. A LIST OF THE SIX BIRD SPECIES STUDIED, INCLUDING SCIENTIFIC AND COMMON NAMES AND METRICS FOR
DIVING BEHAVIO R ................................................................................................................
18
TABLE 2. A CATALOG OF THE TWELVE BIRD FEATHER SPECIMEN SUPPLIED BY DR. ANDREW PARKER (BMNH,
LONDON), CONTAINING SCIENTIFIC NAMES, COMMON NAMES, AND IDENTIFICATION CODES...................34
TABLE 3. AVERAGE, APPARENT ADVANCING AND RECEDING CONTACT-ANGLE DATA (O*ADv AND O
REc,
RESPECTIVELY)
FOR WING FEATHERS OF EACH SPECIES AND FOR CHEMICALLY IDENTICAL, FLAT, FLUORODECYL POSS SURFACE
39
(0E) W ITH VARIOUS LIQUIDS...................................................................................................
SPECIES..................................................42
TABLE 4. EFFECTIVE SPACING RATIO, D , VALUES FOR EACH BIRD
TABLE 5. AVERAGE, APPARENT ADVANCING AND RECEDING CONTACT-ANGLE DATA
(e*ADV
AND
0
*REC,
RESPECTIVELY)
WITH VARIOUS LIQUIDS FOR WING FEATHERS OF THE SPECIES WITH HIGHEST AND LOWEST D , AFRICAN DARTER
AND COMMON SHELDUCK, RESPECTIVELY. FOR EACH SPECIES, LIQUIDS ARE LISTED
IN ORDER OF DECREASING
SURFACE TENSION................................................................................................................
TABLE 6.
RGEOMETRY
CALCULATED
AND
D
42
AS MEASURED FROM SCANNING ELECTRON MICROGRAPHS AND CORRESPONDING,
FOR BARBS AND BARBULES OF AFRICAN DARTER AND COMMON SHELDUCK FEATHERS47
DGEOMETRY
GEOMETRY
TABLE 7. COMPILATION OF FITTED D , CALCULATED A , EXPECTED PB, EQUIVALENT BREAKTHROUGH SWIMMING
VELOCITY, AND BREAKTHROUGH DEPTH FOR EACH BIRD SPECIES.......................................................48
TABLE 8. EFFECTIVE D VALUES
(D*EFF)
FROM THIS WORK AND D
= (R + D) / R VALUES FROM THE LITERATURE
.......................................................................................................................................
50
TABLE 9. CATALOG FOR THE SIX SPECIES OF METRICS OF INTEREST FOR THIS STUDY FOR ANALYZING BIRD-WATER
INTERACTIONS. SPECIES ARE ARRANGED IN ORDER OF INCREASING D, DECREASING A, AND DECREASING,
PREDICTED HB- ----------.
-- - - - --
- - --...............................................................................................
52
TABLE 10. SLIDING ANGLE (() MEASUREMENTS FOR WATER AND ETHYLENE GLYCOL ON WING FEATHERS FROM
SPECIES IN THE PHALACROCORACIDAE FAMILY . ..........................................................................
7
62
Acknowledgements
I thank God for an able mind, which He has caused to give Him thanks. I thank my
advisor, Prof Robert Cohen, for accepting me as an undergraduate student in his lab and being
so supportive and amiable. I also thank Prof Michael Rubner for his help throughout, from my
introduction into to the Cohen lab and even before, and for accepting me as my Course 3 advisor.
I thank my graduate student advisor, Shreerang Chhatre, for working so closely with me and
always taking the time to clarify and explain things, and through whom I have increasingly
grown to appreciate the ways a researcher and engineer approaches problems. From the Cohen
lab, I thank Siddarth Srinivasan for all his technical help, in particular with SEM images,
goniometer troubleshooting, and general conversations. Also from the Cohen lab, I thank
Jonathan DeRocher for good lab safety and generally helping keep things operating and Adam
Meuler for insightful conversations about my project and broader topics. I thank Dr. Andrew
Parker for his collaboration and provision of the bird feathers-and for his hospitality during my
time in Oxford. Thank you also to Dr. Joseph M. Mabry from the Edwards Air Force Base, CA
for his contribution of the POSS materials.
I thank Daisy, my wife-to-be, for reminding me of God's sovereignty and what truly
matters. I thank my parents, brother, and sister, for their love and labor in general and
specifically during my four years at MIT. I thank my younger sister, who gladly proofread my
papers since I was in high school and was spared from doing the same for my thesis. Finally, I
thank MIT and the Department of Materials Science and Engineering and its people for an
enriching four years.
8
1
Introduction
In nature, aquatic birds interact with water with ease, and their feathers are not wetted;
ducks are a prime example. Remarkably, some aquatic bird species dive tens of meters and
emerge to spread their wings to dry. These behaviors and other bird interactions with water are
of great interest. To better understand these behaviors, bird-feather topography is examined.
The typical approach of microscopy in past studies has shown the topography of feathers is
difficult to characterize because they make for delicate, fluffy specimens with structure too
complex to characterize by microscopy alone.
In this project, the texture of feathers belonging to several aquatic birds from six different
species was studied. This thesis is broadly divided into two sections. For the first section of this
study, a single, dimensionless parameter (D*) that characterizes surface texture-effectively
acting as a "microscope"- and was obtained for each species using contact angle measurements
and the Cassie-Baxter relationship for textured surfaces. Complementarily, resistance against
water penetration was subsequently computed. In previous work in the Cohen group, this
resistance to wetting has been characterized in terms of the breakthrough pressure or the
robustness parameter (A *), and in this work, the threshold for non-wetting was also defined by
calculating A * for wing feathers from each species.
As the contrast across past studies reveals, even after natural specimens are characterized,
difficult questions of function often remain. For the second section of this thesis, the effective
spacing ratio, D*, and the robustness factor, A *, served to correlate feather structure with the
ecological behavior of these aquatic birds, specifically their diving depths and the wingspreading behaviors some exhibit for drying under sunlight. Beyond a particular threshold
depth, we expect water to wet bird feathers.
9
Consequently, the question is addressed of how the various species of birds balance the
wettability problem with the need to dive to various depths or to remain on or near the surface as
dictated by their feeding habits. In addition to diving, other water-related behavior of aquatic
birds such as wing-spreading, shallow foraging, and dabbling were linked to the calculated
metrics, namely the effective spacing ratio, robustness factor, and breakthrough depth. The
"effective microscopy" technique was successfully employed to correlate bird feather texture
with wettability and robustness, and to elucidate the role feather texture plays in the waterrelated behavior of aquatic birds.
10
2
2.1
Background
Ecological Behavior of Interest: Bird Interactions with Fluids
Plumage encloses an insulating layer of air close to the bird's body and ensures adequate
thermoregulation. Aquatic birds, then, require highly water-repellent, or hydrophobic, feathers
to prevent water from penetrating into the air layer. Bird feathers are naturally water-repellent
due to the surface chemistry and structure of their feathers, which along with behavior help
counter the effects of hydrostatic pressure. The base component of feathers, the fibrous,
structural protein keratin, is inherently water-repellent, and the natural oils the uropygial glands
of birds produce and secrete, help repel water. Smeared throughout the feathers during preening,
oils from the uropygial glands have been shown to chiefly function to maintain supple and strong
feathers and not primarily to repel water [1]. Figure 1 emphasizes how water easily flows off the
back of an aquatic bird, but oil does not. The increased wettability of bird feathers by oils is an
unfortunate vulnerability often made most visible by ecological disasters such as oil spills, like
the crude-oil leak in 2010 from a BP drilling rig in the Gulf of Mexico.
Water (y7,= 72.1 mN/m)
(d)
(c)
(b)
(a)
Figure 1. Bird feathers interacting with liquids. A healthy duck (a) interacting with water,
which beads and forms relatively high contact angles on the surface of aquatic bird feathers (b)
compared to oil that can come into contact with a bird's coat (c), soiling it by fully wetting the
plumage. (d) Rapeseed oil as an example of lower-surface-tension liquids, which more easily
wet a bird feather surface. Water colored with methylene blue and rapeseed oil colored with oil
red. Images adapted from http://griffyclan007.wordpress.com/2010/06/21/bp-oil-spill-disaster,
http://www.conservationinstitute.org, and [2], adapted with permission.
11
Surface chemistry affects a surfaces ability to repel liquids. Wettability, or the degree to
which a liquid wets a surface, is determined by the contest between intermolecular, cohesive
forces within the liquid and interfacial, adhesive forces between liquid and solid-surface
molecules. The chemical constituents of the liquid and the solid surface both matter. Strictly
considering surface chemistry, if a glass slide were coated with nature's keratin, for example, a
water droplet on such a surface would form a contact angle in the range of 80 to 1000 [3]. As
Figure 1 and Figure 2 illustrate, oils and other lower-surface-tension liquids more easily wet such
a surface and display a smaller contact angle, which is the angle between the line tangent to the
curved liquid/vapor interface and the solid surface Figure 2 shows how adhesive interactions
between a liquid droplet and a flat, solid surface can dramatically vary based on the solid-surface
energy, which is dictated by surface chemistry. A water droplet and an octane droplet are placed
on flat, chemically different surfaces with decreasing surface tensions. In this example, octane
was selected as a representative hydrocarbon and one of the main components of gasoline rather
than treating the surface with many different oils. For the same surface, a water droplet will
display consistently higher contact angles than an oil droplet due to the oil's lower surface
tension and, thus, relatively weaker cohesive interactions and stronger adhesive interactions
(Figure 2a). Whether with a fluorodecyl POSS or keratin surface, for instance, this result is
observed. Moreover, when surface chemistry is changed, the contact angle formed (shown as a
red line tangent to the curved air/liquid interface in the images in Figure 2) decreases
monotonically for both water and octane with decreasing solid-surface energy (Figure 2b and
Figure 2c). Surface chemistry alone, however, plays a limited role in achieving high contact
angles on a flat surface. Even coatings with low solid-surface energies may offer a mix of results
due to specific interactions resulting from non-ideal polar, and nonpolar (dispersion) hydrogen-
12
bond-donating, and hydrogen-bond-accepting contributions to the solid-surface energy by the
compounds on a surface, as previous work characterizing the low-solid-surface-energy coating
utilized in this thesis has shown [4]. Along with chemical composition, the other major
contribution to liquid repellency comes from structure, or topography, the determining factor for
whether water droplets will flow off a feather surface or wet and penetrate between the barbs and
barbules.
(c) Octane droplet
(a)
Oil droplet on...
Water droplet on...
OE, water (0)
180
Air
oane (0)
180
Air
Fluorodecyl POSS
Poly(dimethyl siloxane).
PDMS
Keratin
110
Poly(methyl methacrylate).
PMMA
70
65 -- Fluorodecyl POSS
Hydrophilic polymers
_-
Clean glass
0
<10
-0
PMMA / PDMS.
Keratin
Figure 2. Effect of surface chemistry on the wettability of a flat surface by water and octane.
(a) Advancing contact angles formed on flat surfaces (OE) of different chemical composition by
water (liquid-surface tension, y1v = 72.1 mN/m) and octane (yIv = 21.6 mN/m) droplets. A surface
of keratin, a basic component of bird feathers, gives lower contact angles than a surface coated
with fluorodecyl POSS, a very low-surface-tension molecule (ys, ~ 10 mN/m). (b) OE values of
a water droplet on different flat surfaces are greater than the OE values formed by (c) octane on
the same coated, flat surfaces. Adapted from [5].
13
2.1.1
Pastattempts to characterizefeather topography
Deep-diving birds, such as cormorants, shags, and other aquatic birds routinely dive up to
tens of meters for food and are known to subsequently dry their wings by spreading them in
sunlight for extended periods of time (Figure 3). Noting these behaviors, researchers have
attempted to correlate it to the structure of bird feathers, with notable efforts for the cormorant
and darter, but there is a lack of consensus [6-8]. Particularly, in a 1968 study, Rijke correlated
the two by reporting on the wing-spreading behaviors of cormorants and studying feather barbs
(c)
(b)
(a)
Figure 3: Ecological behaviors of interest of aquatic birds. (a) A great cormorant with a
successful capture. (b) a cormorant diving, likely while foraging, after which it will likely
emerge to dry its wings (c) in sunlight while in a wing-spreading posture. Images from (a)
http://ibc.lynxeds.com/photo/great-cormorant-phalacrocorax-carbo/fishing-doha-habour, (b)
http://commons.wikimedia.org/wiki/File:Cormorantdiving for-foodinMorroBay.jpg, (c)
http://tolweb.org/Phalacrocoracidae/26338.
and barbules, which he characterized by applying an idealization of cylinders of radius R and
inter-cylinder spacing 2D and the dimensionless spacing ratio, D* = (R + D) / R , a model used
for describing textiles (Figure 4) [6,8]. Rijke thought in terms of D*, as schematically presented
in Figure 4a; to study the feather structure of various species, he employed optical microscopy
and photography to measure barb spacing, 2D, and diameter, 2R, from which he calculated D*
values for various species (Figure 4b), including four of those also studied in this present work.
In his study of breast feathers from terrestrial families and fully aquatic families, Rijke also
14
identified the two areas of interest, water repellency (wettability) and resistance to water
penetration (robustness), noting bird feathers of terrestrial birds were generally less wettable,
whereas feathers of aquatic birds were generally more robust. One of his main conclusions, that
D* for the feathers is correlated to diving, swimming, and wing-spreading behavior, was refuted
within one-and-a-half decades by Elowson, who specifically claimed wing-spreading posture
(b)
(a)
Species
Water
r
Air
Iwo10
Mallard (A nas platyrhynchos)
African Shelduck (Tadorna cana)
Reed Cormorant (Phalacrocorax
africanus)
Bank Cormorant (P. neglectus)
Cape Cormorant (P. capensis)
Great Cormorant (P. carbo)
African Darter (Anhinga rufa)
5.9
5.8
4.3
4.5
4.4
4.8
4.5
.
(c)
(r + d)/r
Figure 4. Attempt to characterize feathers by D* =(r + d) /r values visually obtained via
microscopy. (a) Schematic diagram of cross-section of feather barbs modeled as cylinders
(barbules not shown) covered by a water droplet of much larger size; r is radius of barbules, d is
half-length between barbules. (b) Rijke's spacing-ratio data for various bird species. (c)
Photomicrographs representative of breast feathers from two different species of dippers
(Cinclidae) examined by Rijke. Scale bar, 300 m. [8], [9] adapted with permission.
is independent of D* based on comparisons of contact angle measurements and estimations of D*
using scanning electron microscopy (SEM) [10]. Elowson manually measured R and D values
from such micrographs to calculate D*, finding poor agreement between measured contact angles
and expected contact angles back-calculated from the D* values obtained from the simple 1 -D
textile model. Subsequently, Rijke presented a rebuttal to Elowson's objections, specifically
15
addressing the validity of the textile model and the general applicability of the Cassie-Baxter
relation derived from physico-chemical principles [11]. Most recently, in 2010, he and
collaborator Jesser reapplied the D* = (R + D) / R to another aquatic bird family, employing
digital photography, similar to previous studies [9]. The photomicrographs of breast feathers of
dippers (Cinclidae) in Figure 4c come from that latest study and typify the problematic
complexity and fragility of feather topography, even as probed by recent approaches. They also
hint at the shortcomings of techniques that primarily rely on visual measurement to characterize
such complex textures.
2.1.2 Difficulties of characterizingtopography of birdfeathers
As evidenced by the literature history, the complicated structure of feather texture makes
uncertain D* values obtained solely by visual measurement and fails to capture the overall
feather topography. An excerpt from several scanning electron micrographs from Elowson's
results, Figure 5 exemplifies the complexity of feather structure, and the consequent difficulty in
characterizing it. Feather structure is typically hierarchical. The features of feather structure are
most generally: a main shaft (rachis), barbs (ramus) that branch out of the main shaft, and
Figure 5. SEM micrographs (100x magnification) of reed cormorant feathers from the six
different primary and body contour feather categories. Images capture the main shaft or axis
(rachis), the barbs or branches (ramus), and the barbules, or minute filaments that branch out in
dendritic fashion from the barbs. The feather categories, from left to right, top to bottom: DOV,
distal primary outer vane; DIV, distal primary inner vane; BK, back; POV, proximal primary
outer vane; PIV, proximal primary inner vane; and BR, breast. [10]
16
barbules that extend from the barbs and often form an interlocking microstructure. Barbules
compose the majority of the surface area in these bird feathers. In many cases, because of the
intricate feather topography and the small area probed by microscopy, Elowson was forced to
report R and D from different areas on a feather as he established D* visually. Elowson's work
also dealt with several feather categories, including those coming from the wing, breast, and
other areas of the bird's body, as the acronyms within the panels of Figure 5 describe. This
broad approach makes sense considering the diversity of nature, even when just considering the
expected differences between body feathers and flight feathers, for example. However, for the
present study, feathers samples all come from wings, selected because they are more structurally
ordered and because the focus on diving and wing-spreading made wing feathers the most
relevant choice for both phenomena.
Subsequent works by other researchers have also dealt with bird diving behavior,
buoyancy, and the wettability of the plumage as a whole, including considerations of energetic
consequences and heat losses [12,13], yet these approaches focus even less on characterizing
actual feather structure. Even for the purposes of comparing wettability, which is most
commonly done with contact angle values, the contact angle formed on a chemically equivalent,
flat surface is not accurately known.
More specifically, no approach to-date has considered an "effective D*" as obtained
through the method employed in this thesis to characterize feather texture. There seems to be a
connection between D* and ecological behavior, but the connection as of yet is unclear. We seek
to elucidate the correlation between the details of feather texture and the behavioral response of
birds.
17
2.2
Profiles of Aquatic Bird Species Studied
In this project, the texture of feathers from several aquatic birds from six different species
was studied. The wing feathers studied came from six different species of aquatic birds, as
tabulated in Table 1 (see also Section 3). Three of the species were chosen from the same
Phalacrocoracidaefamily: the reed cormorant, great cormorant, and European shag. The rest of
the species each come from distinct families and were chosen because of what is known from
observation about their diving and feather-drying behavior. These three other species are: the
African darter, common shelduck, and mallard.
Table 1. A list of the six bird species studied, including scientific and common names and
metrics for diving behavior.
Common name
Scientific name
Divig deth ivin sped Wng- References for:
Diving speed
spading
diving, wingDiig depth
spreading
Reed cormorant
Phalacrocorax
africanus
Great cormorant
Phalacrocoraxcarbo
European shag
Phalacrocorax
aristotelis
African darter
Anhinga rufa
Common shelduck
Tadorna tadorna
5 - 6*
0.7-0.85
Y
[13], [6,10]
4.7, < 10"
1.1-2.1
Y
[14], [6]
33 -35
1.7-1.9
Y
[15], [6]
Y
[16], [10]
N
[10]
<5
Dabbling
0.19
0.16t
Dabbling
Mallard
Anas platyrhynchos
Dabbling
Dabbling
N
[10]
* from neutral buoyancy experiments, not natural observation; 7 horizontal traveling speed, not diving speed;
I usually < 10 m, but can dive to depths of 35 m [17]; Wing-spreading: Y = predictably, N = never
2.2.1
Phalacrocoracidae: cormorants and shags
The Phalacrocoracidaefamily is subsumed under the broad Pelecaniformes order and
consists of several large water birds with long bodies, long necks, and webbed feet. They
possess iridescent-dark back plumage and light or dark front plumage, varying from species to
species. Their wingspans are commonly in the range of 3 to 5 ft (0.9 to 1.5 m) [18].
Phalacrocoracidae contains the cormorants and shags, which are seen worldwide and exist in
greatest diversity in tropical and temperate zones. They inhabit marine as well as inland waters,
18
and species can be migratory or sedentary. Their wing morphology makes them agile fliers over
short distances [17]. Those in marine habitats feed primarily on fish, whereas the diets of inland
birds can include fish, frogs, aquatic insects, and water snakes. Their foraging generally relies
on underwater food supplies, and cormorants and shags dive, catch a fish with their mouths, and
hold it in a pouch akin to that of the pelican.
Phalacrocoraxare excellent swimmers. The birds surface dive and use their feet to
propel themselves into the air before turning to dive head-first (Figure 3b). The pre-dive leap
supplies considerable momentum, and after submergence, the bird continues its dive by
simultaneous, feet kicks [19]. After water activities, these birds are known for their wingspreading behavior by which they hold their wings extended and dry their feathers [6,10].
Phalacrocoraxafricanus, or the reed cormorant, is so named due to its location; it is a
native of regions throughout Africa, mainly inland [20]. The diving behavior of this bird is not
well-documented, and the one study with definitive numbers on diving depth comes from neutral
buoyancy experiments with submerged carcasses rather than from natural tracking or observation
[13].
The great cormorant (Phalacrocoraxcarbo) is relative large in size-largest of the six
North American cormorants, for instance-and in distribution. The species is the most broadly
distributed of the cormorants and nearly cosmopolitan, occupying the northwest-Atlantic coasts
and also breeding in Europe, Asia, Africa and Australia. It typically nests on cliff ledges and
feeds in sheltered, inshore waters. Like others in its family, the great cormorant is a fast flier
over short distances at speeds of about 50 km/hour and up to 93 km/hour. This bird will
typically dive to depths less than 10 m and captures fish at shallow water less than 20 m deep,
but because it feeds primarily on bottom-living fish, it does surface dive up to 35 m [17].
19
The phalacrocoraxaristotelisresembles cormorants, except it is smaller and slightly
slimmer. As its name implies, the European shag is distributed throughout western Europe,
usually found along rocky, marine coastlines and islands, rarely traveling far from its breeding
area. Like cormorants, European shags feed mainly on fish, although it preferentially consumes
different types of fish than the great cormorant, even when they co-occur. Shags forage deeper
than great cormorants and notably exhibit the characteristic "leap" out of the water before
plunging for prey [21].
2.2.2
Anhinga rufa: African darter
Like the Phalacrocoracidaefamily, the Anhingidae family has been classified under the
same broad order of Pelecaniformes. Similar in appearance and comparable in size to
cormorants, the Anhinga rufa possesses a distinctly long neck and largely black plumage with
white streaks. Distributed throughout sub-Saharan Africa near large bodies of water, these
aquatic birds are largely sedentary but are given over to infrequent, opportunistic, local travels in
response to environmental conditions.
The darter prefers shallow and inland bodies of water and usually avoids marine regions.
It primarily consumes fish and is notorious for its low buoyancy, which aids it in its foraging of
fish, mostly done in shallow water. In fact, the African darter is colloquially referred to as
"snakebird," reflecting how it looks in water, where only its neck protrudes above the water.
They are specialist shallow-water divers, and researchers have observed their dives in various
ecological conditions, with typical water depths <0.5 m [16]. Notably, as Rijke points out, they
can emerge from water "dripping wet" and are able to immediately take up flight. This bird is
also known to exhibit spread-wing postures [22].
20
2.2.3
Tadorna tadorna: common shelduck
Tadorna tadorna, or common shelduck, belongs in the Anatidae family, which also
includes geese and swans. It is likened to a short-necked goose in appearance and has a long,
broad body; it is a mainly white duck with chestnut brown patches, a prominent red bill, blackgreen head, long legs and pink feet, and chestnut and white upper parts.
Shelducks are found mainly in coastal areas, although they can also be found around
inland waters, and favor saline, muddy habitats. They breed in temperate Eurasia and are
seasonally migratory, with the exception of some sedentary, European population [23]. The
common shelduck is particularly common around the greater part of Great Britain's and Ireland's
coastlines. They feed predominantly on salt-water mollusks and other aquatic invertebrates [23].
The shelduck is better classified as a dabbling duck than a diving duck, meaning it obtains its
food on land or just on the water's surface, either by surface dipping or upending (immersing the
entire front half of its body in the water so that its hind parts are thrust in the air and its tail
remains above the surface). The young dive freely, especially when faced with danger, but the
adults only do so when frightened or injured.
2.2.4
Anas platyrhynchos: mallard
Also in the Anatidae family, the mallard (Anasplatyrhynchos) shares some qualities with
the common shelduck. With regards to appearance, the male is also quite noticeable, possessing
an iridescent green head, rusty chest, and green body. In size, however, it is smaller than the
common shelduck. This very familiar duck can be seen throughout North America and Eurasia
in wetland habitats of all kinds. They migrate southward to warmer regions following the
breeding season. It feeds primarily on vegetable matter, insects, worms, and other aquatic
invertebrates [24].
21
Like the shelduck, the mallard is also classified as a dabbling duck. It filter-feeds on the
surface of the water and upends in shallow water. Occasionally, though, it will also dive in
deeper water [24,25]. Neither the common shelduck nor the mallard display spread-wing
postures.
2.3
Effective microscopy to characterize surfaces in terms of D*
To elucidate the underlying reasons for these bird behaviors, particularly for bird-water
interactions, the details of feather structure serve as this study's starting point. The focus was on
quantitatively characterizing feather structure using "effective D*" analysis and the robustness
parameter, A *, which we successfully show capture topographical features otherwise lost when
characterized directly, visually, through microscopy.
With a focus on superhydrophilic, superhydrophobic, and oleophobic surfaces, the Cohen
group in collaboration with the McKinley group (Course II) and the Air Force Research
Laboratory has developed a design chart framework to predict the wettability by a liquid of a
textured surface. In doing so, they address among other problems the question of how stably a
textured surface can resist wetting by a contacting liquid.
D* and A * analysis requires contact angle measurements not just for water, but for a series
of liquids, which include several lower surface tension liquids. However, these liquids more
easily wet bird feathers, precluding contact angle measurements. Dip-coating the feathers
overcomes that hurdle by essentially making the feathers oleophobic, or oil-repellent, with the
additional benefit of ensuring chemically identical surfaces across feather samples, thus
excluding chemical variation as an explanation for differences in wettability and robustness. A
collaboration with the Air Force Research Lab has granted access to the fluorinated polyhedral
oligomeric silsesquioxane (fluoro-POSS or F-POSS) compound. As a hydrophobic crystalline
22
solid with one of the lowest solid-surface energy values reported to-date, F-POSS enables
completion of the analysis with a whole array of liquids thanks to the low surface energy of
fluoro-POSS molecules and the surface texture formed by the feather's barbs. Fluorodecyl
POSS is currently the molecule of choice for designing super-nonwetting surfaces and has
proven suitable for the simple dip-coating process, which has been used by the Cohen group to
confer flexible, conformal coatings to an array of surface textures [2,26,27].
(b) mesh 50, R = 114 pm
(a)
0
dv (0)
'
I
0
60
90
120
'
180
1.0 1-
U mesh 50
iAA
mesh 100
0.5 1-
A mesh 325
60
cos0* =-1+--[sin OE+(-OE)Cos
D
U)
(c) mesh 100, R = 57
pm
E]
0.0
0
0
D-
-0.5
2
D =2245
120
0.20
(d)
-1.0'
-1I
.0
-0.5
0.0
mesh 325, R = 18 pm
180
1 .0
0.5
cos 0adv
Figure 6. Generalized non-wetting diagram for textured surfaces: dip-coated, woven meshes.
(a) Cosine of the advancing contact angle on the textured, dip-coated meshes (6*adv) is plotted
versus cosine of the advancing contact angle on flat silicon wafers (OE) spin-coated with the same
solution as the dip-coated meshes. Diagram contains data for three meshes (b - d) of different
length scales, R, and constant spacing ratio, D* = 2.2, each probed with an extensive set of polar
and non-polar liquids. The effective spacing ratio, D*, inset in (a) is computed from regression
to the advancing contact angle data on the textured (O*adv) and flat (OE) surfaces according to the
equation also inset in (a). [28]
23
The Cohen group has applied the "effective D*" approach to fabrics [2,26], wire meshes
[28], and other re-entrant texture surfaces including electrospun fiber mats and fabricated microhoodoo arrays [29,30]. Figure 6 highlights the role of surface topography in textured surfaces,
for which this single, effective spacing parameter, or effective D*, can be computed and used to
quantify the surface's "openness." Distinct from past approaches, this "openness of the weave"
is quantified by an effective D*, which is obtained by measuring contact angle measurements on
the textured feather surface and on a chemically identical, flat surface. An effective D* is
computed for each texture from regression to the advancing contact angle data on the textured
surface
(O*adv)
and on the flat surface (OE) according to the modified Cassie-Baxter relation
(Equation 1). The contact angle quantities 0* and OE are explicitly defined in Section 2.4, and the
Cassie-Baxter relation is explained in Section 2.4.1. Figure 6 shows three dip-coated, woven
meshes that despite having varying length scales, R, share the same effective spacing ratio, D*=
2.2. Their shared openness of the weave as characterized by the effective D* approach is
evidenced by their excellent agreement with the Cassie-Baxter model, as shown in the
generalized non-wetting diagram for textured surfaces, which plots the cosine 6*adv against the
cosine of OE (Figure 6a). This whole procedure was also carried out for bird feathers to study
their structure with a particular interest in wettability.
2.4
Wettability, composite interfaces, and textured surfaces
As with the structure or roughness of surfaces in general, the structure of feathers can
reveal much about their wettability. Wettability is commonly quantified by placing a liquid on a
surface and, after it attains thermodynamic equilibrium, measuring this equilibrium contact
angle, OE, established between the liquid and the flat, solid surface. Figure 7a illustrates precisely
this case of a flat, smooth surface. The equilibrium contact angle is governed by the balance
24
between interfacial tension components and given by Young's relation: cos OE
s(yv - V)
/v
The interfacial tension, y, between solid, liquid, and vapor phases is accounted for and denoted
by subscripts s, 1, and v, respectively. Wettability of a material has been shown to depend on two
main factors: surface chemistry and topography [31-35].
2.4.1
Cassie-Baxter (CB) relation and D*
The concept of wettability is not limited to flat surfaces; hence its dependence on
topography and application to surfaces such as bird feathers. When a liquid droplet interacts
with a textured solid surface, it either fully wets the solid, resulting in a Wenzel state [31], or
forms a composite solid-liquid-air interface due to air trapped between surface asperities
supporting the contacting liquid, producing a "Cassie-Baxter" interface (Figure 7b) [32]. Multivalued surface topography for which a vector normal to the x-y plane (plane running parallel
with the surface) intersects the texture at multiple points, also known as re-entrant curvature, has
been identified in previous work as essential for supporting a composite interface with lowsurface-tension liquids [29,30].
(a)
(b)
]- cylinders
2I)
2R
Figure 7. Schematic diagrams of the interface formed by a water droplet on (a) a flat, smooth
surface with y indicating the surface tension resulting from interactions between the solid (s),
liquid (1), and vapor (v) phases and OE indicating the equilibrium contact angle measured using
contact-angle goniometry; and on (b) a rough or textured surface.
25
No longer the equilibrium contact angle (OE) considered for a flat, smooth surface (Figure
7a), the apparent contact angle (0*) formed by the liquid droplet at the composite interface can be
calculated using the classical Cassie-Baxter (CB) relation: cos 0* = r#, cos
represents roughness of the wetted area and
E
s
-1 , where ro
#, the fraction of the projected area of the solid
surface in contact with the liquid [34]. The literature as well as previous research in Prof.
Cohen's group catalogs many approaches to manipulating the wettability of surfaces, resulting in
superhydrophilic (apparent contact angle 0*~ 00), superhydrophobic (0'> 1500), and oleophobic
surfaces [29]. Many textured surfaces, including all the wing feather samples considered for this
project, can be geometrically modeled as cylindrical arrays, which in turn can be described with
radius R and inter-cylinder spacing 2D (Figure 8). This geometrical idealization results in a
more conveniently expressed CB relation [2,28,30,36]
1
D
cos 0* =1-+ -
where r = (z - OE) / sin0,
[(r -OE)COSEsinE
(1)
= R sin OE /(R +D), and D* =(R + D) / R . These equalities show
the dependence of the surface texture variables, r, and
#,
on the equilibrium contact angle (OE),
whereas the dimensionless spacing ratio (D*) is independent of the properties of the contacting
liquid and, as a purely geometric parameter, quantifies the openness of the surface. The
generalized non-wetting diagram in Figure 8 presents various D* regimes for a textured surface
modeled as an array of parallel cylinders. Given a uniform R among cylinders, D* =1 describes
touching, parallel cylinders, whereas D* = 271 refers to cylinders farther apart.
As applied to the bird feathers in this study, length scale, R, is reasonably approximated
as constant along the barbule length, and differences in the D* parameter then only reflect
variations in spacing between barbules. As a dimensionless parameter, D* is independent of
26
length scale. However, changes in length scale, R, are important for wettability, and its effect on
resistance against liquid penetration is discussed in Section 2.4.2 (particularly detailed in Figure
9), which takes robustness of the interface into account.
120
180
D*=I
90
0
60
=x/2
---
0.5
0 (")
60
-- d= ,
0.0
90
8
-
0 (")
-0.512
-0.5
0.0
0.5
COSO
180
1.0
*
-1.0
-1.0
Figure 8. Schematic diagram of representative D* regimes for a parallel array of cylinders.
Supposing negligible variation for R, a dimensionless spacing ratio of unity (D* = 1) refers to the
case when the cylinders are touching each other; D* = 2a represents a case when the parallel
cylinders are farther apart.
2.4.2 Robustness parameter, A *, and breakthroughpressure, Pb
The CB relation only applies when a solid-liquid-air composite interface is assured, a
condition threatened by the sagging of the liquid-air interface formed by the trapped air, as the
derivation of the CB relation assumes a flat liquid-air interface. If sufficiently severe, the
sagging of the liquid-air interface will touch the lower level of solid surface, thus transitioning to
a fully wetted state. Robustness against wetting, or the ability to withstand sagging of the liquidair interface, can be quantified by the dimensionless robustness factor, A* [29,30]. The
robustness factor is essentially the ratio of the threshold pressure difference that triggers the
transition ("breakthrough pressure," Pb) to the characteristic reference pressure, Prf = 2Y7, /
where fcap is the capillary length (,,p
=,
Y
cap,
/ pg ), ylv is the liquid surface tension, and pg is the
27
specific weight. For an arrangement of cylinders with radius R and inter-cylinder spacing 2D,
the robustness factor can be calculated from the following expression [2,28,30]:
A
b
(COSOE)
cap
-
Prf
R(D*-l)
L(D*-1+2sinOE)_
(2)
Thus, A * accounts for the equilibrium contact angle (OE), the effective spacing ratio (D*), and the
ratio of capillary length (f,) to the feature length scale composing the solid's surface texture (R),
f, / R. Values of A * - 1 identify surface-liquid combinations for which a drop will
spontaneously transition to a fully-wetted state.
All these parameters can be combined in a "design chart" for liquid wettability, in which
OE is plotted versus D*, and contours of the apparent contact angle (0*) can be generated from
Equation 1 to identify a priorithe combinations of OE and D* that will yield a desired 0* (Figure
9). The developed design chart framework allows for the design of robust interfaces. For
example, to design a super-nonwetting, textured surface, 6E and D*values for coordinates above
the 0* = 1500 contour would be sought, but robustness must also be considered. Importantly,
incorporating the robustness factor, A
,
into the design chart then predicts the available parameter
space for textured surfaces that will maintain robust solid-liquid-air interfaces for a designated
threshold A * (commonly A * = 1) below which the surface will fully wet. This region of
instability is shown as the grey area in the modified design charts in Figure 9. The shaded grey
area diminishes and the unshaded, white area expands from Figure 9a to Figure 9b, highlighting
the relevance of R for optimizing the openness of the surface: all other things being equal, having
a small feature length scale, R, allows for a high breakthrough pressure, Pb.
28
(b)
(a)
R
cap
120
-
A*> I=-> ro
(=
=1000
100
- - =120"
-=1500
80
C"p
R
120
0
-
posite interface
100
Sca = 2mm, R =2pm,
""a =10
fcap = 2mm, R=20pm,
80
160
_,60
40
A*<1 = composite interface is
20
unstable against any pressure
40
20
perturbation
0/
1
0
2
4
3
5
6
1
4
3
2
5
6
D
D
Figure 9. Effect of length scale, R, on the robustness of a textured surface. The modified
version of the non-wetting design chart, which predicts the range of allowed D* and OE for
forming a robust composite interface, in this case for two values of the ratio of the capillary
length (f,) to the feature length scale, fe, / R. Typical surfaces of (a) commercial textile, with R
~ 200 tm, and (b) electrospun mat, R~ 2m. For both surfaces, f, ~ 2 mm. Smaller R, and
therefore maximum openness of surface, is desirable to achieve non-wetting, robust interfaces
(A* b 1). [28]
2.5
Impact of openness of the weave on surface wettability
Probing a surface with various liquids is not just a part of the technique for extracting the
D* parameter and quantifying the openness of the weave. Varying the surface tension of a liquid
interacting with a textured surface is the equivalent of varying the Laplace pressure or an
externally applied pressure. By substituting pressure with surface tension, keeping surface
structure constant throughout, Equation 2, Pf = 2y,v / e,
and
a,,p
= 4,
/ pg show how varying
the surface tension, Ylv, alone changes f, and Pef values, resulting in different Pb and A * values.
Determined by the liquid used, surface tension, then, can be varied in place of pressure: probing
a textured surface with one liquid and then with another of lower surface tension is analogous to
29
submerging that surface-a mesh or feather, for example-in the first liquid, and then lowering
the sample deeper to a certain depth still within the same liquid.
Under normal conditions, bird feathers normally interact with just water. In this project,
bird feathers were exposed to various liquids with different surface tensions to properly
characterize the surfaces through D* and A * and to find the expected breakthrough pressures,
which should affect bird/water interactions. This concept is well illustrated by subjecting wire
meshes to the same testing, as done in previous work in the Cohen group (Figure 10) [28]. As
the non-wetting design chart shows, for wire meshes with constant coating chemistry (OE) and
feature length scale (R), apparent contact angle increases monotonically with D*. For the
parameter space in which a robust composite interface is ensured (A*> 1, white area), a constant
R and increasing D* means larger spacing between wires (D), a lower wetted fraction of the
solid, and thus higher apparent contact angles. In contrast, when a robust interface is not ensured
(A * < 1, shaded grey area), as D* increases, inter-wire spacing D increases, except in this case the
air-liquid interface more severely sags and easily transitions to the fully wetted state (Figure 10).
In the example in Figure 10 of wire meshes dip-coated with 50% POSS-50% Tecnoflon, the D*
spacing of 2.2 preserves a composite interface against the lowered surface tension from
contacting water (yiv = 72.1 mN/m) to rapeseed oil (yiv = 35.5 mN/m), but another mesh with D*
= 5.1 has a weave that is actually too open and cannot withstand the decreased surface tension
when probed with a droplet of rapeseed oil. The air-liquid interface sags to the point of fully
wetting (open, right-pointing triangle enclosed in a red circle within the shaded, grey area in
Figure 10). The open symbols represent droplets in the fully wetted state. These results convey
the connection between wettability and robustness. If a bird feather had been designed like the
30
mesh of smallest D* value, it would more robustly resist wetting, even against the oils that
typically wet bird feathers.
D*= 2.2, R = 83 gm
Water
120
100
Rapeseed oil
80
-exadecane
D*= 3.9, R = 83jm
ALA
0
60
Heptane
40
Pentane
D*= 5.1,R = 83 tm
20
0
1
2
5
4
3
6
D
Figure 10. Model of bird feather wetted by oils. Example of design chart for liquid wettability
demonstrating effect of varying surface tension. Contact angle data for nine different liquids on
three meshes with different D* values and dip-coated with 50% POSS-50% Tecnoflon. Filled
symbol indicates a robust composite interface formed by a liquid droplet, whereas open symbol
indicates a fully wetted interface formed by a droplet. Left inset image shows non-wetting water
(blue, A* = 14.5) and rapeseed oil (red, A* = 5.8) droplets with robust composite interfaces on a
mesh with D* = 2.2. Right inset image shows water (A * = 2.1) in a robust composite interface
and rapeseed oil, with a lowered robustness factor (A * = 0.9), wetting a textured surface with a
higher spacing ratio D* = 5.1. SEM micrographs of the three meshes are shown on the right. [5].
2.6
Impact of openness of the weave on breakthrough pressure, Pb
A surface can display high water repellency, and therefore high apparent contact angles,
without necessarily forming a highly robust interface. As with wettability, openness of weave
and its dimensionless parameter, D*, also affect robustness, A *, and the breakthrough pressure,
Pb. In this study, then, feather structure is primarily characterized by the effective D* approach,
31
which is presented as more adequate than microscopy for studying feather-water interactions.
Surface texture was quantified in terms of an effective spacing ratio (D* -- Deff), and robustness
factor values for the various species were generated.
The D* and A* parameters connect the structure of bird feathers to ecological behavior,
specifically related to bird interactions with water. In a laboratory setting, the connection
between D* and A * and diving depth can be practically demonstrated by submerging a wire,
woven mesh into water, which is analogous to the submersion of bird feathers during water
contact (Figure 11). As a mesh is submerged increasingly deeply in liquid, the pressure
difference across the liquid-air interface (P) increases, and the solid-liquid-air composite
interface increasingly sags and presses against the air pockets (Figure 1 lb-d). Once it reaches
the breakthrough pressure, (P = Pb), the composite interface is driven to a fully-wetted state.
Trapped air is displaced by penetrating water, as seen in the bottom segment of the meshes in
Figure 11 a, which contrasts two meshes, one with low and one with high D*. The former has a
high Pb and thus maintains the robust, composite interface against higher pressures at lower
depths: the latter can only withstand low pressures (low Pb), and thus, submersion below a
shallow level of water easily fully wets the mesh.
The same can occur to bird feathers as birds dive. and remain in water for extended
periods of time, which may also dictate the time birds will spend submerged and on land in
wing-spreading posture to dry. As explained in the previous section, surface tension is directly
varied instead of pressure to generate the D* and A* values required to characterize feather
topography and to compare to diving depth trends.
Thus, the mathematical framework applied to meshes, fabrics, and micro-fabricated
surfaces will be applied to these species, yielding effective D* values using known variables
32
*
including contact angle measurements, not visual inference as done in past research efforts. A
and Pb values from these feathers allow for the estimation of a maximum diving depth, which at
least ranks the species in relative order of diving depth practiced when it not also provides
quantitative values comparable to those observed.
(b)
(a)
Pressure
(Pa)
rO
"100
(c)
n200
*300
. 400
(d)
. 500
low D
- high Pb
high D
-> low Pb
Figure 11. Model of bird feather submerged in water. Evolution of the solid-liquid-air
composite interface formed between a wire, woven mesh and the water in which it is submerged.
(a) A snapshot of two woven meshes with same R submerged in a cylindrical tank of de-ionized
water: the left with D* = 2.2 and the right with D* = 5.1. Pressure is indicated in Pascals (Pa) for
the corresponding depths of the tank, and every 100 Pa corresponds approximately to 1 cm in
height of the water column. (b) - (d) Progression of the solid-liquid-air composite interface
corresponding to the same woven mesh under increasing pressure differential values of 50, 100,
and 300 Pa, respectively, as simulated using Surface Evolver@ FEM software. Blue, light blue,
and red color represent wet solid, liquid on air, and dry wire surface, respectively. Inset in (b)
shows the structure of the dry wire surface. Adapted from [5].
33
3
3.1
Experimental Procedures
Bird Feather Specimens
Through collaboration with world-renowned biomimeticist and expert zoologist Prof.
Andrew Parker of the Natural History Museum, London and Oxford University, UK, access to
study the set of carefully-selected feather specimens was obtained. Feather samples from twelve
birds from six species were furnished by the Natural History Museum (NHM), London, UK and
are cataloged in Table 2. No birds were sacrificed specifically for this study.
Table 2. A catalog of the twelve bird feather specimen supplied by Dr. Andrew Parker (BMNH,
London), containing scientific names, common names, and identification codes. The wing
feather of each listed specimen was studi
Number Common name
Scientific name
1
Reed cormorant
Phalacrocoraxafricanus BMNH 1900.1.20.84
2
Great cormorant
Phalacrocoraxcarbo
BMNH 1895.6.20.121
3
Great cormorant
Phalacrocoraxcarbo
BMNH 1894.6.20.137
4
European shag
Phalacrocoraxaristotelis BMNH 1897.4.19.2
5
European shag
Phalacrocoraxaristotelis BMNH 1941.5.30.3280
6
European shag
Phalacrocoraxaristotelis BMNH 2007.64.c
7
African darter
Anhinga rufa
BMNH 1904.11.19.57
8
African darter
Anhinga rufa
BMNH 1955.6.N.17.5
9
Common shelduck
Tadorna tadorna
BMNH 1955.3.10
10
Common shelduck
Tadorna tadorna
BMNH 1992.9
11
Mallard
Anas platyrhynchos
BMNH 1980.14.43
12
Mallard
Anas platyrhynchos
BMNH 1980.14.61
Identification code
For this project, wing, breast, and belly feathers of these birds were available, and the
wing feathers were selected for characterization. Figure 12 displays photographs of the wing
feathers studied and of the breast and belly feathers, as well, that are representative of the actual
plumage of other individual birds of the same species.
34
(a) Reed or long-tailed cormorant
(Phalacrocoraxafricanus)
(b) Great cormorant (Phalacrocoraxcarbo)
(c) European shag (Phalacrocoraxaristotelis)
(d) African darter (Anhinga rufa)
(c) Common shelduck (Tadorna tadorna)
(b) Mallard / wild duck (Anas platyrhynchos)
Figure 12. Photographs of wing, breast, and belly feathers for the six bird species. (a) reed
cormorant, (b) great cormorant, (c) European shag, (d) African darter, (e) common shelduck, and
(f) mallard.
3.2
3.2.1
Coating Methodology
Fluorodecyl POSS (polyhedraloligomeric silsesquioxane) and Tecnoflon@ solution
Fluorodecyl POSS (polyhedral oligomeric silsesquioxane) molecules consist of
silsesquioxane cages surrounded by eight IH,lH,2H,2H-heptadecafluorodecyl groups [27]. Due
to the high density of perfluorinated carbon atoms present in the eight alkyl chains surrounding
the silsesquioxane cages, a smooth fluorodecyl POSS surface has one of the lowest solid-surface
energy values reported to date (ys, ~ 10 mN/m) [30]. To confer thin, uniform, flexible, and
35
conformal coatings of fluorodecyl POSS to textures of interest, the commercially available
Tecnoflon@ fluoroelastomer (BR 9151, Solvay Solexis) (ysv ~ 18 mN/Nm) was employed as the
continuous polymeric matrix, and common solvent Asahiklin AK225 (Asahi Glass Company)
was used as the solvent for the Tecnoflon polymer and fluorodecyl POSS. Figure 13 shows the
chemical structures of fluoro-POSS molecules and the Tecnoflon polymer.
(a)
Rf\
0
Rf
S'
Rf.s S\O'S'
/\
-
RO
0 R
Si
(b)
0
.
H2
/
F2
c-c-
CF
F2
F2
F2
c-
I
F----
SiH-2
SR
f
Figure 13. Chemical structure of fluorodecyl POSS and Tecnoflon@ fluoroelastomer. (a)
General chemical structure of fluoro-POSS molecules, containing alkyl chains (Rf) according to
the general molecular formula: Rf = -CH 2-CH 2- (CF2)n-CF 3, where n = 0, 3, 5, or 7. For
fluorodecyl POSS, n = 7, and thus Rf = -CH 2-CH 2-(CF 2)7-CF 3 (ysv ~10 mN/m). (b) Molecular
structure of Tecnoflon (ys,~ 18 mN/m). Fluorodecyl POSS and Tecnoflon are dissolved in
common solvent Asahiklin to form the solution used for dip-coating the feathers.
3.2.2
Coating smooth and textured surfaces
Silicon wafer substrates were spin-coated with a POSS-Tecnoflon solution (50%-50%
by weight, total solids 10 mg/mL) (ys, = 10.7 mN/m) at a rotation speed of 900 rpm for 30 s. The
bird feathers were dip-coated in the POSS (50%)-Tecnoflon (50%) solution (10 mg/mL). After
immersion for 5 min, the samples were removed and dried in air to ensure complete evaporation
of the Asahiklin solvent.
36
3.3
3.3.1
Surface Characterization
Contact angle measurements
Contact angle measurements and sliding angle measurements were obtained with a ramd-
hart 590-Fl goniometer. Advancing and receding contact angles were measured using -5 [tL
droplets of various liquids (purchased from Aldrich and used as received).
The advancing contact angle represents the wetting of liquid droplets on previously dry
surfaces. The receding angle represents the interaction of the liquid with the surface after liquid
has already come into contact with the surface. The advancing angle has been the angle of
interest for computing effective D* in previous work [26]. Given the main motivation of this
work-to study how a bird begins to interact with water during its diving activities-the
advancing angle is the more relevant quantity of the two and thus was selected for D*
calculations over the receding angle. Both advancing and receding contact angles were recorded
and are both are reported for completeness in Table 3 (Section 4.1).
Initially, a distinction was drawn between "inner" and "outer" contact angles, or contact
angles measured on the inner region of a feather, near the main shaft, and those measured near
the edge of a feather's barbs, respectively. Despite knowledge of a regular, highly waterrepellent central region and irregular, wettable distal region documented for the body feathers of
great cormorants [7], the inner-outer construct was abandoned for this study after preliminary
experiments revealed no statistically significant differences between inner and outer contact
angles across species.
3.3.2
Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) was conducted for wing feathers from two of the
species using a JSM-JEOL 6060 microscope (Institute for Soldier Nanotechnologies at MIT) at
37
an accelerating voltage of 5 kV. Samples were cleaned in Asahiklin solvent and gold-coated
with a thickness of- 5nm in preparation for microscopy. Micrographs were obtained capturing
each level of structure on the surface of the feathers. The feature length scale, Rgeometry, was
visually measured from the micrographs and taken the as input for the simple 1 -D model of
parallel cylinders. Apart from those values used for this purpose, values for Rgeometry were not
further considered in this thesis, and dimensionless D* is independent of length scale, regardless.
4
4.1
Results
Apparent contact angles for bird feathers from each species
Each feather was probed by various liquids, and the apparent advancing and receding
contact angles measured (0*adv and 0 rec, respectively) are reported in Table 3. The probing
liquids are listed in order of decreasing surface tension: water (yIv = 72.1 mN/m), diiodomethane
(ylv = 50.8 mN/m), ethylene glycol (ylv = 47.7 mN/m), dimethyl sulfoxide (yIv= 44 mN/m),
rapeseed oil (yIv = 35.5 mN/m), hexadecane (ylv = 27.5 mN/m), and dodecane (yiv = 23.8 mN/m).
The equilibrium contact angles (OE) on a flat, fluorodecyl-POSS-coated surface are also provided
for comparison as a reference point. Water droplets displayed the highest apparent contact
angles, and as surface tension decreased with each of the other liquids, corresponding 0*
decreased. The behavior is exemplified by what happens to a feather in water versus what
happens to it in oil: water, with higher surface tension, will not wet the droplet as easily as oil,
which has the lower surface tension. Equilibrium contact angles and apparent contact angles
(*adv
or 0*rec) inherently do not provide any information on surface structure; information about
structure is obtained using D* [26].
38
Table 3. Average, apparent advancing and receding contact-angle data (O*adv and 0*rec,
respectively) for wing feathers of each species and for chemically identical, flat, fluorodecyl
POSS surface (OE) with various liquids. For each species, liquids are listed in order of
decreasing surface tension.
Bird species
Reed cormorant
Liquid
Water
Diiodomethane
Ethylene glycol
Dimethyl sulfoxide
Rapeseed oil
European shag
(0)
107
5
90
96
5
101
89
1
13
78
0
Water
Diiodomethane
Ethylene glycol
Dimethyl sulfoxide
Rapeseed oil
Hexadecane
Dodecane
142
124
134
120 :
108
0
0
4
2
5
5
10
133
109
120
104 +
90+
Water
Diiodomethane
Rapeseed oil
Hexadecane
Dodecane
140
137
137
112
114
0
0
6
7
4
8
9
Water
137
Ethylene glycol
Ethylene glycol
Dimethyl sulfoxide
African darter
O*rec
85
0
Hexadecane
Dodecane
Great cormorant
0*ad& (0)
125 6
103 4
121 + 9
115 3
113 + 8
7
15
12
5
6
6
5
1
0
0
130
115
120
114
100
0
6
5
6
6
6
0
+
2
126
2
3
2
104+ 6
103 8
6
3
7
132 + 3
115 4
107 + 7
Dimethyl sulfoxide
120
112
Mallard
Water
Ethylene glycol
Dimethyl sulfoxide
141
133
125
Common shelduck
Water
Ethylene glycol
Dimethyl sulfoxide
149 10
140 + 5
142
124
6
131
122
124
116
5
*Flat fluorodecyl POSS surface
Water
Diiodomethane
Ethylene glycol
Dimethyl sulfoxide
Rapeseed oil
Hexadecane
Dodecane
6
100
2
2
111
98
88
80
75
2
2
3
1
1
9
79
87
80
66
61
60
* Equilibrium contact angles (6 adv and 0rec) for same liquids on a flat fluorodecyl POSS surface provided for comparison
39
4.2
"Effective microscopy" of feathers with D*
The 6*adv values along with the equilibrium contact angles (OE) measured on a flat,
chemically identical, fluorodecyl POSS surface (Table 3) served primarily as input data in the
modified Cassie-Baxter (CB) relation (Equation 1, Section 2.4.1), which yielded effective D*
values for the wing feather of each bird species shown in Figure 14 and Table 4.
Figure 14 presents the generalized non-wetting diagrams for all six bird species. The
effective spacing ratio, D*, for the wing feather of each species was first found by regression of
the CB relation to the advancing, apparent contact angle data for each species. Once D* was
obtained, it served as the fitting parameter to plot the CB relation as a fit (solid line) for the
%
contact angle data (large data points in Figure 14). The dashed lines represent the 95
confidence intervals for the D* fit. Good agreement of contact angle data with its CB fit
indicates the 1 -D model can adequately characterize the textured surface. The MATLAB@ code
used for the fitting procedure, courtesy of Shreerang Chhatre, is attached in Appendix A (Section
9.1).
All these feathers form composite interfaces for an array of liquids with a range of
surface tensions. By contrast, a natural, uncoated feather lacks the low-surface-energy coating of
fluorodecyl POSS and is only aided by its re-entrant surface topography: its barbs and barbules.
To verify the expected behavior of these same probing liquids on a natural feather, the apparent
contact angles of the probing liquids on uncoated feathers from two of the species, the one with
lowest and the one with highest, effective D*, were also measured (Table 5). Those tests
confirmed the same monotonic trend of decreasing contact angles with decreasing surface
tension of the probing liquids. The African darter had the lower effective spacing ratio
(D* = 1.23
0.21) and, therefore, also had lower contact angles but resisted wetting by
hexadecane (yiv = 27.5 mN/m). By contrast, the common shelduck had the highest D*,
40
(a)
180
90
120
180
1.0[
1X20
0
60
90
0
0.5-
60
,
0.5-
(b)
0
-0
60
1.0
60
90
0.0
(0)
(c)
12
-0.5
0.0
cos9
0.5
1.0
120
90
60
0
wV
0
-0.5
0
-1.0
-1.0
-0.5
180
120
D =1.36
(d)
0.0
cos9.
|180
0.5
1 .0
60
0
0 C)
90
0
1.0 F
0
120
0.39
/
/
180
1.0
0.21
-
-1.0
-1.0
= 1.23
-
-0.5 -
0.5
-
60
0.0
p
-1
D
-1.0
.0
-0.5
0.0
cos9
120
90
=
1 61
0 25
-0.5
1120
1.018 0
0.5
-0.5
120
0
180
1.0-
60
0.5
(I)
0
60
0
-1.0
(*)
180
.0-
d0 .)
7..
';.d0
-0.5
(e)
90
0.0
900
0
60
0.0
'9
1, 77 1
0.34
-120
0.5
1.0
60
0
80
(0)
90
0
A
7
0 .-
0.0
90
1-/
0
d.dO
60
90
'
0
20I
-0.5
-
-1.0
-1.0
D =1.78
-0.5
0.0
0.5
-0.5 -
120
0.18
'
180
10
-1.
-1.0
1.0
'
=2.28
0.22
1
180
-0.5
0.0
0.5
1.0
cosO.
cosO
Figure 14. Generalized non-wetting diagram for dip-coated bird feathers from each species.
Cosine of the apparent, advancing contact angle (6 adv) is plotted against the cosine of the
advancing contact angle on a smooth silicon wafers (OE) spin-coated with the same solution as
the dip-coated feathers. The effective spacing ratio, D*, for each specie's wing feather is first
found by regression of the Cassie-Baxter relation to the contact angle data for each species
(Equation 1, Section 2.4.1). The solid lines correspond to the Cassie-Baxter (CB) equation
plotted for each D* value inset in the diagram; the dashed lines indicate the 95 % confidence
intervals for the respective CB plots. The data plotted on each non-wetting diagram corresponds
to the wing feathers tested for each bird species, in order of increasing effective D*: (a) African
darter, (b) reed cormorant, (c) great cormorant, (d) mallard, (e) European shag, and (f) common
shelduck. Inset in (b) shows a typical water droplet on a wing feather of reed cormorant.
41
Table 4. Effective spacing ratio, D*, values for each bird species
Bird species
D*,ff
African darter
Reed cormorant
Great cormorant
Mallard
1.23 + 0.21
European shag
Common shelduck
1.36
1.61
1.77
1.78
2.28
0.39
0.25
0.34
0.18
0.22
2.28 h 0.22, which agrees with its lower wettability-higher contact angles-but lower
robustness, as evidenced by it being fully wetted by hexadecane when the other species of lower
D* is not (see Section 2.5 and 2.6 for concept). The data also showcases the effect of the
fluorodecyl POSS coating, which increases liquid repellency, and thus 6, and generally renders
oleophobic surfaces that would otherwise not repel oils.
Table 5. Average, apparent advancing and receding contact-angle data (O*adv and 6*rec,
respectively) with various liquids for wing feathers of the species with highest and lowest D*,
African darter and common shelduck, respectively. For each species, liquids are listed in order
of decreasing surface tension.
Bird species
African darter
Common shelduck
Liquid
Water
0*adv (0)
128
2
Ethylene glycol
104
5
0*rec (0)
116+ 5
93
3
Dimethyl sulfoxide
90 + 8
77 + 7
Hexadecane
45
3
31
3
134 1
125 + 3
129
109
2
4
Water
Ethylene glycol
Dimethyl sulfoxide
Hexadecane
114
7
0
86
5
0
As an additional, dynamic test for wettability, sliding angle measurements were taken for
a few of the liquids on selected feathers. In those experiments, the contact angle becomes a
sliding angle: the incipient angle at which the droplet begins to slide off the surface as the
surface is gradually tilted. Sliding angle data is included as Appendix B (Section 9.2).
42
4.3
Scanning electron microscopy (SEM) of selected feathers
Past studies have relied on photographic and microscopic techniques in attempts to
*
characterize feather structure and wettability [6,8-10,22], but this study presents the D* and A
frameworks as a more adequate alternative for the characterization of complex surface
topography. Scanning electron micrographs obtained for the African darter (D* = 1.23
and common shelduck (D* = 2.28
0.21)
0.22) showcase the complexity and diversity of feather
topography (Figure 15). The micrographs start with the main shaft and barb structure of the
feather and, with increasing magnification, reveal underlying structure in a hierarchical manner:
-
from barbs (Figure 15a, i), to barbules (c, d; k, 1), to smaller scale rods on the African darter (e
h) and branch-like structures on the common shelduck (m - p) on the order of 10 pm.
Additionally, the coverage of different areas on the feather surfaces highlights other differences
in structure between the more central areas and distal areas (edges) of the feathers. Micrographs
(c) and (k) can be contrasted with (g) and (o), for instance, to show differences in the order and
density of barbules between central areas and distal areas; (d) and (1) and (h) and (p) can likewise
be contrasted as barbule tips on the feather edges.
Finally, the difference in D* between the two bird species is noticeable. The African
darter apparently has a tighter weave, and hence a lower D*eff, whereas the common shelduck has
a more open weave and higher D*eff. Also apparent in such ramage is the difficulty of manually
measuring R and D to find D* values, which regardless of accuracy nevertheless fails to fully
assess how topography contributes to wettability. Indeed, if manually measured at all, R and D
should be measured with respect to barbules, because barbules contribute most to the surface
area of the feather with which a liquid would come into contact. Unfortunately, barbules are also
finer, can pack differently from barbs, and can possess even finer features, as in the case of the
43
barbules of the common shelduck, which branch out with even smaller-scale structures that
resemble the split ends in human hair (Figure 15m, n). This complexity, however, is not
unconquerable, and topography and wettability can still be approached by generating D*eff values
and combining them with "Rgeometry" values, taken from the barbules of these micrographs, as
approximations of the effective length scale, Reff. With this Reff, D* values, and the contact angle
measurements, the robustness of the surface against wetting (A *) and the breakthrough pressure
(Pb) can be readily computed.
44
Common shelduck
African darter
(a)
(i)
(b)l
(j)
(c)
(k)
(d)
(1)
45
Common shelduck
A frican darter
(e
(n)
(m)
(o)
(p)
46
Figure 15. Scanning electron micrographs for topography of wing feathers of African darter and
common shelduck. Left column (a - h) corresponds to African darter, and right (i - p) to
common shelduck. Top-to-bottom order corresponds with increasing magnification for (a - f)
and (i - n) of central feather area and for (g, h) and (o, p) of distal (edge) feather area. Colored
boxes indicate dimensions of representative areas that were successively magnified and match
with micrographs of the magnified areas. Arrow indicates when successivley magnified area is
beyond visible area. Accelerating voltage, 5 kV. Spot size of 50 at a working distance of
8 to 12 mm.
4.4
Critical diving depth from robustness parameter, A*, and breakthrough pressure, Pb
Robustness parameter, A *, and breakthrough pressure, Pb, can be obtained for each
species from Equation 2 (Section 2.4.2). Instead of finding a feature length scale, R, for each
species, Reff is approximated by the Rgeometry values measured from scanning electron
micrographs like those in Figure 15. Both Rgeometry and Dgeometry were measured for the barbs and
barbules of the African darter and common shelduck. From these values, D *geometry was
computed, and all three quantities are reported in Table 6. African darter had a D*eff Of
1.23
0.21 and the common shelduck a D*effof 2.28
0.22, both of which are approximately the
same as the D*geometry values computed from the R and D of barbules, but far from those
computed from the R and D of barbs. Agreement of D*eff (Table 4) with D*geometry computed
from barbule measurements and disagreement with the same quantity computed from barb
measurements supports the expectation that barbules will play a greater role than barbs (and
rachis) in liquid-feather interactions due to their contribution to surface area. Therefore,
calculations for A* and Pb proceeded with Reff taken as the overall average of Rgeometry values
based on barbule measurements on the two birds (Rgeometry = 5.4
1 pim).
Table 6. Rgeometry and Dgeometry as measured from scanning electron micrographs and
corresponding, calculated D*geometry for barbs and barbules of African darter and common
shelduck feathers
Bird species
Structural feature
African darter
Barbules
Common shelduck
Rgeometry (pm)
Deometry (pm)
D *eometry
6.5
0.6
2
0.8
1.4
0.1
Barbs
37.6
2.4
109.9
5.3
3.9
0.2
Barbules
Barbs
4.3
11.1
0.4
0.6
5.4
134.9
1.0
3.4
2.3
0.2
13.2
0.9
47
Using the 1 -D model, the robustness factor, A *, and breakthrough pressure, Pb, were
estimated for each species with water as the contacting liquid. Based on estimated Pb values, the
equivalent swimming velocity, v, due to dynamic pressure (P=
pv2 , where p is fluid density)
and the expected "breakthrough depth" were also calculated. The breakthrough depth, hb,
represents the critical depth at which the breakthrough pressure of the feather structure is met by
the bird in the course of diving. As expected from the A* equation (Equation 2), for a fixed
liquid and feature length scale, R, a higher D* is accompanied by a lower robustness factor and
lower breakthrough pressure. These variables, collected in Table 7, are those quantities that may
help connect feather structure to bird behavior.
Table 7. Compilation of fitted D*, calculated A*, expected Pb, equivalent breakthrough
swimming velocity, and breakthrough depth for each bird species.
African darter
Effective
spacig ratio,
D ,f(kPa)
1.23 0.21
Reed cormorant
Great cormorant
1.36
1.61
Mallard
European shag
Common shelduck
Bird species
5
Robustness
factor, A*
Breakthrough
Pressure, Pb
Equivalent1 2velocity,
v = (2PP) / (M s_')
13.6
Breakthrough
depth, hb (M)
9.4
1727
92
0.39
0.25
1033
544
55
29
10.5
7.6
5.6
3.0
1.77
1.78
0.34
0.18
403
396
22
21
6.6
6.5
2.2
2.2
2.28
0.22
201
11
4.6
1.1
Discussion
5.1
Apparent contact angles used to assess wettability of bird feathers
From Table 5 of O*adv and O*rec values for uncoated feathers and Table 3 for coated
feathers, contact angles are statistically different. Feathers previously wetted by low surface
tension liquids become liquid-repellent after coating. The wing feathers of the common
shelduck, mallard, and great cormorant achieved the highest apparent contact angles with water
(O*adv
in the range of 140 - 150'), whereas they saw lower O*adv for dimethyl sulfoxide droplets
48
(-
120 - 130'); the great cormorant, which was also probed by rapeseed oil, had even lower
contact angles for that liquid, with 6*adv
=
108
100. With a lower-surface-tension liquid,
hexadecane, the reed cormorant feather resists wetting and supports the droplet at a composite
interface, whereas the feathers of great cormorant and European shag do not and are fully wetted
by the liquid. Ideally, the contact angle data can be refined into one parameter that characterizes
texture, namely D*.
5.2
Effective microscopy (1-D model) characterizes feather texture
The "effective microscopy" by which the structural details of each feather were captured
in a single parameter yielded D* values for the wing feather of each species (Table 4). This
effective D* analysis employed throughout the present work managed to characterize the
complex, delicate feather surfaces about as well as it has highly ordered, mesh surfaces in past
work [28]. For comparison, the average error in D* for all bird feathers was ~ 0.27, close to the
0.2 reported for dip-coated, woven meshes and smaller than the 0.6 error recorded for dip-coated,
carbon paper composed of randomly oriented, cylindrically textured microfibers [28].
To date, then, including in this study, a 1 -D model of D*eff has proven sufficient for
characterizing diverse surface textures, and need for a higher-dimensional model for contrast
against the 1 -D model has not led to any new understanding of feather wettability (see Section
7). The species whose contact angle data had the greatest disagreement with the Cassie-Baxter
(CB) fit, the reed cormorant, is difficult to address because of the small data set: it is the only
species for which only one feather was tested due to availability.
49
5.3
Effective D* is better indicator of feather wettability than D* from geometry
In evaluating our effective microscopy technique as applied to bird feathers, a
comparison with literature values of D* = (R + D) / R obtained geometrically (henceforth,
"D *geometry), namely those reported by Rijke and in related work, is useful. Effective D* values
(D*eff) as obtained from the CB fits (Figure 14) are reproduced alongside D*geometry values
obtained by visual measurements in Table 8. In this regard, literature unfortunately provides
limited information on D* values, which are only cataloged for some species and for varying
feather categories. Table 8 indicates when wing feather values for the particular species were not
available and another value was used and when other values were also included based on
relevance.
Table 8. Effective D* values (D*eff) from this work and D* et = (R + D) / R values from the
literature
Bird species
African darter
Reed cormorant
Great cormorant
Mallard
European shag
Common shelduck
* from breast feathers in
D*eff
1.23 +0.21
(R + D) /R
Reference
~ 2-3, 4.5
[10,11,22]
1.36 0.39
1.61 + 0.25
3-4,4.3*
4.8*
[10]
[6]
1.77 + 0.34
1.78 0.18
~ 6-7, 5.9*
[10]
--
--
~ 9-0,
[10]
2.28 0.22
Rijke's study [6]
As D*eff increases, the (R + D) / R values increase, as well. A comparison between
calculated D *eff values and the D *geometry values shows they both follow the same general order;
however, there is no quantitative agreement. Admittedly, a few of the only available previous
measurements in the literature were on breast feathers, not on wing feathers, so some of the
mismatch is not surprising, but lack of literature data aside, the D* values do follow a general
trend.
50
The objective of this work was to study liquid-feather interactions, which is the reason
for characterizing feather texture. D*eff is a macroscopic measurement employing a wide range
of liquids over many parts of a feather. Depending on surface tension, different liquids wet the
same textured surface to different extents. This sampling better mimics water-feather
interactions under different hydrostatic pressures. For example, probing a feather with a
rapeseed oil droplet compared to a water droplet is roughly equivalent to subjecting it to twice
the pressure under water (see Section 2.6). By using a set of contact angles measured for
different liquids, the effect of hydrostatic pressure on wettability is captured, as the technique is
based on actual liquid-feather interactions, just like the water-related behavior of birds.
On the contrary, D* measurements based on microscopy (D *geometry) explore only a small
portion of a surface a time. Moreover, D*geometry characterizes the physical appearance in a top
view, and not the wetting behavior. The re-entrant portion of the texture, (i.e. the portion below
the top half of the structural features) is not visualized, and therefore is missed, by microscopic
or visual techniques. The scanning electron micrographs displayed in Figure 15 attest to the
complexity and diversity of feather structure and even show signs of hierarchical structure,
qualities not easily characterized in a quantitative sense by data obtained from visual methods.
Finally, bird feathers are fragile, and electron microscopy is relatively slow and expensive. An
effective D* is directly relevant to wettability and captures structural details otherwise missed.
5.4
Role of wettability (O*adv) and feather texture (D*) on ecological behavior
As the Cassie-Baxter (CB) relation shows (Equation 1), everything else being the same,
an increase in D* means an increase in 6*, and thus, improved non-wetting. For example, the
common shelduck displayed the highest 9* and had the highest D*, as shown in Table 3 and
Table 4, respectively. Evident directly from the CB relation for a 1 -D cylindrical array, this
51
trend also generally held for the feathers of the six species with distinct D* and is graphically
conveyed in Figure 14. Those graphs emphasize that a simple comparison of contact angle data
is not the objective, but rather, the connection between topography and wettability behavior that
D* helps explore. A revisited version of Table 1, Table 9 presents the values in this study that
were initially posited and now confirmed as relevant for analyzing bird-water interactions.
Table 9. Catalog for the six species of metrics of interest for this study for analyzing bird-water
interactions. Species are arranged in order of increasing D*, decreasing A*, and decreasing,
predicted hb.
B
Bird species
African darter
Diving depth Diving speed
(m s-1)
<5
0.19
Wingspreading
0.161
D*ef
Robustness
factor, A*
Y
1.23
0.21
1727
Breakthrough
depth, hb (M)
9.4
Reed cormorant
5 - 6*
0.7-0.85
Y
1.36
0.39
1033
5.6
Great cormorant
4.7, < 101
1.1-2.1
Y
1.61
0.25
544
3.0
403
2.2
N
1.77 0.34
Dabbling
Mallard
Dabbling
396
2.2
Y
1.78 0.18
1.7-1.9
33 -35
European shag
201
1.1
N
2.28 0.22
Dabbling
Common shelduck
Dabbling
* from neutral buoyancy experiments, not natural observation; T horizontal traveling speed, not diving speed;
I usually < 10 m, but can dive to depths of 35 m [17]; Wing-spreading: Y = predictably, N = never;
Dabbling = dabbling species, not primarily divers
The D* framework as applied to birds reveals a correlation between feather structure and
diving behavior that microscopy and photography alone cannot reveal. In discussing diving
trends, it is taken for granted that the birds are not limited by lack of strength to propel
themselves to desired depths. Feathers are expected to play a significant role in diving behavior,
as diving for birds such as cormorants is believed to be quite an energetic investment relative to
other divers due to poor insulation and less-efficient foot propulsion [37]. Effective openness of
the weave as captured by D*eff for the birds of Phalacrocoraxspecies (the cormorants and shag)
serves as a starting point for discussing their behavior. From reed cormorant, to great cormorant,
to European shag, as the diving depths increase, so does D*. As for the ambiguity of which dives
less deeply, whether the reed or great cormorant based on actually documented depths, the great
52
cormorant, while recorded in literature as typically diving up to 4.7 m and < 10 m, actually does
show more range than the reed cormorant. In fact, the great cormorant is credited as having
feather structure that allows for partial plumage wettability, meaning it is wettable, but not
completely, a feature thought to enable it to be less positively buoyant [7,38]. In light of such
studies and observation, then, it is likely great cormorants routinely dive deeper than reed
cormorants, and their differences in D* predict the same. Remarkably, the highest D* value
among the cormorants and shag studied belongs to the European shag, which without doubt
exhibits the deepest diving. Shags are also estimated to have lower energetic costs to dive than
great cormorants, which is attributed to the streamlined body shape-hinting at the idea that
individual feather structure is not the only factor dictating diving ability-and their thick layer of
trapped air [37]. In general, birds in the Phalacrocoracidaeare known for spending extended
periods underwater, perhaps contributing to their need to assume spread-wing postures to dry.
Closely related to cormorants (Phalacrocoracidae),the African darter displays
significantly different behavior that agrees with its greater wettability. The species with the
lowest D* of the set (D* = 1.23
0.21), Anhinga rufa is known for its low buoyancy, causing
them to look like snakes, with only their neck protruding from underwater as they forage. Unlike
the cormorant and shag, the darter and others in the Anhingidae family are considered shallow
divers. Similar to its low D* value relative to the other species, the darter's plumage is assigned
the label of "fully wettable" by past studies. In addition to that, they possess thinner, less spongy
skin, and denser bones, and smaller air sacs [16]. These features, and the fact that its feathers are
the most wettable of the species characterized by D* and 0*, make the bird suitable for its
shallow foraging, which is characterized as slow and stealthy compared to other diving birds.
They would also serve as good reasons to practice wing-spreading in-between dives.
53
The extension of the D* framework for aquatic birds is not limited to divers, however.
Even behavior already seen as somewhat self-explanatory, such as the absence of spread-wing
postures in the lives of the two dabbling duck species in this set can be revisited and now more
directly attributable to feather topography. Dabbling ducks usually feed on the surface, a trait
that does not demand much, if any, diving from them. In fact, the liquid-repellency of mallard
and shelduck feathers as characterized by 6 *adv and D* reflect just how their feather structure
suits their practices: their O* values with water are two of the three highest, well in the range of
1400 to 1500, and D* for their wing feathers are likewise in the top three highest, contributing to
a greater fraction liquid/air interface that drives water droplets to easily bead up and roll off a
bird's coat. This is important during their surface dipping or upending and a general benefit for
such birds that spend much of their time in water.
Documented ecological characteristics such as the great cormorant's partially wettable
plumage, the African darter's notoriously low buoyancy, and the dappling ducks' high waterrepellency warrant a discussion on the robustness parameter, A*, and how in addition to
wettability, robustness at the water-feather interface correlates with behavior.
5.5
Role of robustness factor, A*, and breakthrough pressure, Pb, on ecological behavior
For the deep-diving birds, the expected breakthrough depths are significantly smaller than
actual diving depths. This simply parametric analysis indicates feathers are wetted below this
particular depth, and beyond an equivalent threshold velocity of swimming, both tabulated in
Table 7. The reed cormorant and the dabbling birds are the only ones for which the expected
breakthrough depth quantitatively agreed with observed behavior. On the other hand, the
behavior metrics for the European shag, for example, which appears to have an good balance of
both water-repellency and robustness, were captured at least in concept if not quantitatively by
54
its relatively high D* and low A*: after its deep and long dives [15], its feathers will undoubtedly
need drying, according to the low breakthrough depth for the shag (hb = 2.2), which is in the low
range of the dabbling birds studied. Also, the A* may be representative of more than trends, and
may also link specific behavior with structure. For example, the high robustness of the darter,
while not following the expected trend with diving depth, may be key for explaining Rijke's
observation that although the darter emerges dripping wet from diving, remarkably, it is able to
immediately become airborne.
The analysis does show agreement in ranking order of diving depths and capabilities, and
where there is disagreement in magnitude between breakthrough depths and actual diving depths,
it is possibly attributable to other unexplored factors, such as the packing of whole arrays of
feathers, or more likely, a limitation of the 1 -D, cylindrical model. As for the former, there may
be reason to expect higher-order structures to operate on the same physico-chemical principles as
its feather subcomponents, suggestion that explanation may be less probable but also difficult to
test.
While the A * values obtained by the simple 1 -D, cylinder model do not follow the exact
trend expected for diving depths, they do correlate with other water-bird phenomena, namely
wing-spreading and length of time spent submerged or on water. The observation that
cormorants and shags spend much longer times submerged, whereas the African darter spends
much less time does not necessarily agree with its high calculated robustness factor alone, but
combined with its D* parameter (see Section 5.4), the calculated parameters together paint the
more complete picture that describes its behavior.
Unlike deep or shallow diving birds, the mallard and common shelduck don't require
feathers that resist water penetration at great depths or against high diving speeds or fast
55
underwater swimming. However, their calculated A * values and breakthrough depths indicate
their feathers do adequately equip them for spending extended periods of time on water and are
generally robust; the expected Pb is a sufficient threshold for them to carry on their water
activities as dabbling ducks unrestrained above a predicted 1 - 2 m of water. Young shelducks,
for instance, reportedly speedily dive just below the surface when in danger or approached while
the adult attendants take flight. As dabbling birds that do not face great hydrostatic or dynamic
pressures, these ducks already don't seem to have any need for the spread-wing postures the
other aquatic birds exhibit, and the robustness of their feathers more so suggests they would not
need to use such techniques, even if they were to partake in more demanding water activities of
their own.
Theoretically, like for many textured materials, the longer a bird remains in water, the
more vulnerable its feathers may become to impinging water [1]. Also like textured surfaces in
general, previously wet feathers that do not fully dry are predicted to be more susceptible to
wetting. To guard against these two threats, even dabbling birds appear to require robust
feathers, while the other birds combine the wing-spreading, drying technique with whatever
robustness their feathers offer.
6
Conclusions
It is demonstrated that the 1 -D, effective-D* paradigm can effectively be used to
characterize the wettability of complicated textures such as bird feathers. Using the D* analysis
in conjunction with the robustness parameter framework allows us to a prioridetermine
breakthrough pressures of bird feathers. This allows us to compare the diving depth up to which
a bird's feathers will theoretically not wet. Alternatively, it also predicts a bird's resistance to
56
wetting when it comes into contact with water contaminated with oil or other lower-surfacetension liquids. In addition to diving, other water-related behavior of aquatic birds such as wingspreading, shallow foraging, and dabbling were correlated to the calculated metrics, namely the
effective spacing ratio, robustness factor, and breakthrough depth, with notable agreement for
several of the aquatic bird species.
7
Future work
As this project has demonstrated, while D* and A * offer useful insights for the behavior of
aquatic birds studied, not all behavior is captured by the model of a 1 -D cylindrical array, and
future work is warranted. Complementary wettability experiments with higher-order structures
of feathers, such as feather groups, feather coats, or whole birds would help more clearly draw
connections between the parameters quantified in this work. Also, a better model or surface
evolver simulation should reveal further details of bird-feather wettability, especially given that
the breakthrough depths predicted by the 1 -D model are of the same order as the observed diving
depths.
Other areas of interest for follow-up study encompass various ecological and biological
variables. Molting intensity, which can affect the integrity of feathers and their structure, for
example, would be one variable to track for a set of species. Similar to molting, seasonal
variations might be studied to reveal possible differences in feather structure throughout a year.
Feather diversity for a single bird based on different locations on its body could also come into
play, and studying breast and belly feathers in addition to wing feathers, for example could
contribute to the picture of bird/water interactions, as they may show distinct effective spacing
ratios, such as significantly different D* for feathers from relatively fluffier areas of the body.
57
As more information on these birds enters the body of literature, there will be more ways
to frame bird behavior questions. One such example is the average times a species might spend
in the spread-wing posture, which may or may not significantly differ from species to species.
Such new data could be seen in light of effective D* and A * from this work.
58
8
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
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60
9
Appendices
Appendix A: MATLAB® code for D* fitting procedure
9.1
function estimateDstarfeathers
This function reads the contact angle data from an excel spreadsheet and
fits the cylindrical cassie baxter equation to estimate D
written by Shreerang Chhatre
global costheta;
from ACU fa-bric- Dstar calculation.xls
read the- data
first
for Bird Feathers v2.xls',
xlsread. 'Graphs
contactangle :
'graph',
'C5:Cll'. ;
'graph',
'U5:Ull'. ;
cos. contact-angle pi.180.;
cos theta :
contact angle
xlsread. 'Graphs
costhetastar
cos.contact_angle pi.180.;
for Bird Feathers
initial
-
Vstar0 ' 2;
v2.xls',
guess
actual function
. Dstar,r,J,SIGMA,mse.
:
fun,Dstar0.
nlinfit. costheta,costhetastar,
Dstar
mse
ci :
nlparci. Dstar,r,'covar',SIGMA.;
ci. 1... 2
.ci. 2.
delta :
Now plotting- the datapoints along with the fitted CB relation
100;
n
xx :
1,1,n: ;
linspace.
figure; plot. xx-,- fun. Dstar,xx ,
plot. costheta,costhetastar,
fun. Dstar delta,xx, xx, fun. Dstar delta,xx.. ; hold on;
1 1. . ;
1 1
axis. .
xx,
'o'. ; hold on;
from regression', 'D^
legend. 'D
delta'. ; xlabel.
delta', 'D
cos\thetaE'. ; ylabel. 'cos\theta'
hold off;
return;
function
y
:
1
y
fun. D,x.
.1.
D.
. sin. acos. x. .
pi acos. x. ..
x.;
the function
return;
Figure 16. MATLAB@ code used to estimate the dimensionless spacing ratio, D*, for each bird
species. Takes contact angle data as input to fit the Cassie-Baxter equation for parallel cylinders
and output D* as the fitting parameter. Courtesy of Shreerang Chhatre.
61
9.2
Appendix B: Sliding angles measured for selected birds and liquids
Sliding angles were measured using 15 pL droplets deposited on the feather surface such
that upon tilting the droplets slid parallel to the main shaft (rachis) of the feather and
perpendicular to the barbs. A tilt angle range of 0 to 900 was spanned. Due to the inconsistency
of measurements obtained from preliminary sliding-angle experiments and the ambiguous
meaning of the dynamic sliding angle as related to bird-water interactions, the sliding angle
experiments were not conducted more extensively, more static contact angle measurements were
conducted instead.
Table 10. Sliding angle (co) measurements for water and ethylene glycol on wing feathers from
species in the Phalacrocoracidaefamily.
Liquid
Bird species
(Owarer (0)
Cwethylene glycol (0)
Reed cormorant
90*
Great cormorant
<5
30 7
27 5
40 20
European shag
* did not slide at 900, but exhausted tiling range
without sliding off.
62