K S V INSTRUMENTS LTD
Application Note #106
P.O. Box 128, 00381 Helsinki, Finland
E-mail: info@ksvltd.fi www.ksvltd.fi
Characterization of Fiber Wetting
The wetting behavior of fibers is of interest to researchers in a variety of systems. Fiber wetting
applications include; the spreading of ink on paper, water repellancy of fabrics, treatments on human hair,
fiber reinforced composite production and many others. As the diameter of a fiber decreases the ratio of
surface to volume increases and the effects of the surface on the characteristics of the fiber becomes more
pronounced. In applications involving small diameter fibers the influence of the surface of the fiber may
dominate most interactions.
How is Fiber Wetting Measured?
Characterization of the wetting of fibers presents special research problems. The primary parameter
reported to characterize wetting is contact angle. Typically contact angles are measured using
a
goniometer. This requires that a drop of the test liquid be placed on the surface of the solid tested.
Placement of a drop of liquid on fibers of small diameter may be quite difficult. Strategies used to
overcome this problem have included using very small drops or analyzing the shape of a drop of liquid
surrounding a vertically oriented fiber. Neither of these strategies is entirely satisfactory.
A more reasonable approach for fibers is to use tensiometry to measure contact angle. The Sigma 70
tensiometer produced by KSV Instruments permits you to make rapid and reliable measurement of contact
angles on fibers. This technique, commonly called the Wilhelmy method for contact angles, is described in
more detail in Application Note # 103. Briefly, a sample of your test solid is hung on the balance of the
tensiometer and brought into contact with the test liquid as shown in the figure below.
When the fiber contacts the liquid the change in forces is detected and your Sigma70 registers this
elevation as zero depth of immersion. As the solid is pushed into the liquid the forces on the balance are
recorded. The forces on the balance are
Ftotal = wetting force + weight of probe - buoyancy
Your Sigma70 has tared the weight of the probe and can remove the effects of the buoyancy force by
extrapolating the graph back to zero depth of immersion. The remaining component force is the wetting
force which is defined as:
Wetting force =
lv
P cos
where lv is the liquid surface tension, P is the perimeter of the probe and is the contact angle. Thus at
any depth data is received which can be used to calculate contact angle. This contact angle, which is
obtained from data generated as the probe advances into the liquid, is called the ‘advancing contact angle’.
The sample is immersed to a set depth and the process is reversed. As the probe retreats from the liquid
data collected is used to calculate a ‘receding contact angle’. On almost all surfaces the advancing contact
angle will exceed the receding contact angle. The difference between these two values, know as the
contact angle hysteresis, is a subject of great interest in wetting studies and you are recommended to the
references cited at the end of this note for further details.
An example of the type of graph generated in such an experiment is displayed below.
The graph displays data as wetting force vs depth of immersion. In the experiment shown two cycles of
immersion/emmeresion were performed. The lower slopes of the graph represent the force values as the
fiber is immersed into the liquid and are used to calculate the advancing contact angle. The upper slopes of
the graph represent the force values as the fiber is emmersed from the liquid and are used to calculate the
receding contact angle.
Mounting: KSV Instruments makes sample holders which make fiber mounting simple. If you do not
have these holders the easiest way to mount small fibers may be as follows: make a small throw-away hook
from light gauge wire. Attach a length of your fiber to the end of the hook using a fast drying adhesive.
This will create the type of rigid anchor for your fiber which is necessary for reproducible data. Be careful
not to expose any length of the fiber which will come in contact with your liquid to the adhesive. Cut off
any part of the end of the fiber which has been contaminated by contact during this manipulation.
Length: The fiber sample needs to be able to penetrate the surface of your liquid and maintain a vertical
orientation as it moves through the liquid. For this reason you may have to trim very small and limp fibers
to less than 5mm length. Although it is useful to assay longer lengths of the sample, lengths as short as
2mm can yield good data. If your fiber is not rigid and/or wets poorly with your liquid , you will notice
that it resists penetration of the surface. In such a case you should attempt to shorten the length of the
sample until you achieve penetration.
Fabrics
Analysis of the wetting of fabrics is usually treated differently than the analysis of the fibers of which they
are composed. The most frequently used approach to finding contact angles for fabrics it to treat them as
porous solids and use the Washburn method (see application Note # 104) of analysis. An alternate
approach is to study the fibers which compose the fabric. Studies have shown that the contact angle of the
fabric wetting, as assayed by Washburn method equals the contact angle of the fibers of which it is
composed. This appears to be true regardless of the weave of the fabric(see Textile Res. J.,62(11)677685(1992)).
Adhesion
Interest in the wetting characteristics of fibers has increased greatly in recent years. Much of this new
interest involves the study of fiber reinforced composites. The successful use of fibers in composite
materials depends on the adhesion of the fibers into the matrix material. The quality of the adhesion is in
turn dependent on the wetting of the fibers by the matrix. Characterization of this wetting is done by
contact angle experiments as described above.
Perimeter of fibers
Fibers of a variety of sizes are easily tested for contact angles. It is not unreasonable to expect that you
can get good results for fibers with diameters below 10 micrometers. As in all cases, the accuracy of your
results is directly related to the accuracy of your measurement of the perimeter of the fiber. Measurement
of the perimeter of your fiber can be performed manually or by the following approach which is
recomended for fibers with irregular shapes.
Perform a contact angle experiment on your fiber using a completely wetting liquid such as hexane.
Remembering that:
Wetting force =
lv
and that for completely wetting liquids
Wetting force =
P cos
= 0 or cos = 1 and it follows that
lv
P
Using the measured wetting force and known lv it is simple to solve for perimeter. Please note that the
wetting force used for this technique should be the receding wetting force. This force is chosen rather than
the advancing force as it is more reasonably representing a situation with a zero contact angle. The
software provided with your Sigma 70 performs an automatic calculation of the perimeter of samples
along their entire length.
References:
F.Hoecker, J.Karger-Kocsis, J Applied Polymer Sci.,59,139-153(1996)
Y.Hsieh, B.Yu, Textile Res. J.,62(11)677-685(1992)
B.Miller,L.Penn,S.Hedvat, Colloids & Surfaces, 6,49-61(1983)
J.C.Berg, Wettability, Marcel Dekker,N.Y.(1993)
M.E.Schrader,G.Loeb, Modern Approach to Wettability, Plenum Press
N.Y.(1992)
A.W.Adamson,Physical Chemisrty of Surfaces, Wiley & Sons,(1976)
S.Wu, Polymer Interface & Adhesion, Marcel dekker,N.Y.(1982)
J.D.Andrade in Surface & Interfacial Aspects of Biomedical Polymers,
Vol 1, Plenum Press,N.Y.(1985)