Contact Angle Hysteresis on Aquagels

Micrographic studies of drops of methylene iodide on agar aqua gel surfaces reveal substantial contact - angle hysteresis, which is virtually independent of agar concentration. Maximum advancing and minimum receding angles obey the relation cos theta A + cos theta R = 2cos theta E where theta E is the equilibrium contact angle made by methylene iodide on a plane liquid water surface. These observations, and consideration of the microstructure of gels, are at variance with theories of hysteresis based on surface contamination, surface roughness. and surface heterogeneity. A new theory of contact angle hysteresis on solids is presented which, by analysis of molecular energetics, ascribes the phenomenon solely to the lack of free lateral mobility of the molecules in solid surfaces.

When a drop of liquid is brought into contact with the surface of a solid (or of another liquid with which it is immiscible), it will either spread without limit on that surface, or retain Its identity as a drop, attaining an apparent equilibrium configuration on the surface such that a characteristic angle is formed between the tangent to the liquid surface at its point of contact with the substrate, and a line parallel to the substrate surface at that point. This so-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.g., liquid-liquid-gas), the three interfacial free energies manifest themselves as contractile tensions acting tangentially to the surfaces at all points. Under these circumstances, a single two-dimensional force-balance is adequate to establish an equilibrium value for the contact angle in terms of these surface tensions.