Forestry 485
Lecture 2-3-2: Wood Surface Properties,
Part II
Surface Properties: Wetting
 Regardless
of adhesion mechanism, optimal adhesion is
dependent upon effective contact of adhesive and
adherend; contact is dependent upon Surface Wetting
phenomenon.
Surface Energy: “Bond Breaking”
Bonds
broken to create surfaces=“excess energy
in the surface” or “surface energy”
Surface does not exist of itself: It must be part of
an interface (two substances or phases)
Liquid-vapor, liquid-solid interactions result from
dispersion forces and hydrogen bonds
Surface Energy vs. Surface Tension
Strictly, surface energy DOES
NOT equal surface
tension
HOWEVER, it is generally impractical to measure
precise values, especially for solids
THUS, the terms “surface energy” and “surface
tension” are often taken as synonymous, except
in theoretical treatments
Surface Energy Magnitude
Organic polymer
γ
surfaces; typical values:
< 100 mJm-2
Hard
(low energy)
inorganic surfaces (metals, ceramics,
etc.)
γ
> 100’s to 1,000’s mJm-2
(high energy)
Slide courtesy Dr. Doug Gardner, University of Maine
Surface Energy Measurement
Sessile Drop
Method
Capillary Rise Method
Wilhelmy Plate Method

http://www.ksvinc.com/wilhelmy_plate.htm
From: Fundamentals of Adhesion, ed. Lee, L.H., p.126, Plenum Press, New York, 1991
Contact Angle Hysteresis
Microscopic “surface inhomogeneities”
(roughness) causes variations in contact angle
Contact angle measurement varies between
advancing and receding liquid-solid interface
Dynamic contact angle measurement (e.g.,
Wilhelmy plate method) helps to account for
these variations
Contact Angle Hysteresis
Another cause of
hysteresis is “heterogeneous
contamination” of surfaces with low-energy
impurities
In the case of wood adherends, such
“contaminants” are typically hydrophobic
extractives
Young’s Equation: Proposed in 1805 (!) to
explain the equilibrium of a drop of liquid on
a solid surface
“When a droplet of liquid, L, with its vapor, V, is at
rest on a solid surface, S, it takes a configuration
which minimizes the energy of the system and
highlights the liquid-solid interactions.”
The equilibrium condition is represented by:
γSV = γSL + γLVcos Ө
- Fourche, 1995
Contact Angle
Vapor
γLV
Liquid
Ө
γSL
γSV
Solid
Young’s Equation:
γSV = γSL + γLVcos Ө
Or
cos Ө = (γSV – γSL)/ γLV
If Ө = 0, Spreading Occurs
If Ө < 90o, Wetting is Favorable
If Ө > 90o, Wetting is not Favorable
Critical Surface Energy
 C =
Critical Surface Energy, is that surface
energy at which complete wetting occurs
Notice (in
the following slide) that if cos θ = 1,
cos-1 θ = 0o
Zisman Plots
cos Ө = 1 + b (C - L)
Ө = measured Contact Angle
b = the slope of the line
C = Critical Surface Energy
L = Liquid Surface Energy
cos theta
Zisman Plot
1.1
0.9
0.7
0.5
10
γC = 18 mN/m
20
30
40
Surface Energy (mN/m)
50
Dupre’s Equation: Work of Adhesion
(“postulated centuries ago”!!)
Consider:
“An elastic material of unit crosssection is subjected to a tensile force. The
material breaks, creating two new surfaces.”
Since “the new surfaces are each made of the
same material, then the total energy expended
must be twice the surface energy of the material.”
Thus, work of COHESION, Wcoh = 2γ
» Pocius, 2002, chapter 4
1
2
Dupre’s Equation: Work of Adhesion
NOW, Consider: “A situation in which two dissimilar materials are in
intimate contact. A tensile force splits the materials into two
dissimilar materials. If the sample is of a unit cross sectional
area, then the energy expended should be the sum of the two
surface energies…”
BUT, “because the two dissimilar materials were in contact there
were intermolecular forces present that are now missing since the
materials were separated. That is, an interfacial energy may have
been present before the materials were split apart. As this energy
is missing after the two surfaces are separated, we must subtract
it from the energy used to create the two new surfaces.”
» Pocius, 2002, chapter 4
Work Of Adhesion
Thus, we have the Dupre’ Equation:
WA = γ1 + γ2 – γ12
Where γ1 =surface energy of
material 1, γ2 =surface energy of
material 2, and γ12 = interfacial
energy between materials 1 and 2.
Definition: Work required
to separate two bonded materials
solely in terms of surface energy.
1
1
2
2
Resin – Wood Furnish Interactions
Goal: Achieve even distribution of resin within adhesive joint, and
promote intimate contact between resin molecules and furnish surface.
Mechanism
•Application – Droplet Formation
•Fluid Motion – Compression/Consolidation
•Molecular Motion – Spreading/Wetting
Acknowledgement
 Illustrations
in slides 2,
11, 13-14 and 16 courtesy
of Carter Johnson.
 Literature cited is from
module 2 optional
readings, except for
Pocius, chapter 4 (copy
available on request)