CONTACT ANGLE HYSTERESIS ON AQUAGELS
A. S. Michaels and R. C. Lummis
MassachuseHs Institute of Technology, Cambridge, MassachuseHs
Presented at the
A.I.Ch.E.-S.P.E. JOINT SYMPOSIUM ON
WETTING AND CAPILLARITY IN
FLUID DISPLACEMENT PROCESSES
at the Fortieth National Meeting
in Kansas City, May 17-20, 7959
I. Summary
Micrographic studies of drops of methylene iodide on agar
aquagel surfaces reveal substantial contact-angle hysteresis,
which is virtually independent of agar concentration. Maximum
advancing and minimum receding angles obey the relation
coseA + cose
made by methylene iodide on
=
2cose where
the equilibrium contact angle
E
R
a
plane liquid water surface. These
observations, and consideration of the microstructure of gels,
are at variance with theories of hysteresis based on surface con•
tamination, surface roughness, and surface heterogeneity.
A
new theory of contact angle hysteresis on solids is presented which,
by analysis of molecular energetics, ascribes the phenoluenon
solely to the lack of free lateral mobility of the molecules in solid
surfaces.
I!. Introduction
When
a
drop of liquid is brought into contact with the surface
solid (or of another liquid with which it is immiscible), it will
either spread without limit on that surface, or retain its identity as
drop, attaining an apparent' equilibrium configuration on the
surface such that characteristic angle is formed between the
tangent to the liquid surface at its point of contact with the substrate
and line parallel to the substrate surface at that point. This so•
of
a
a
a
I
a
called contact angle is usually considered to be determined solely
by the magnitudes of the free energies of the three interfaces
defining the line of contact at which the angle is measured.
When the three phases under consideration are fluid (e.
liquid-liquid-gas) the three interfacial free energies manifest
themselves as contractile tensions acting tangentially to the
surfaces at all points. Under these circumstances, single two•
g ••
I
a
dimensional force-balance is adequate to establish an equilibrium
value for the contact angle in terms of these surface tensions. The
relation for the case where one fluid boundary is plane constitutes
the well-known Young equation: