Contact angle hysteresis: A molecular interpretation

Abstract

Intrinsic contact angle hysteresis is defined as hysteresis that cannot be ascribed to roughness, heterogeneity, or penetrability of the solid surface. It can be explained if we postulate that the layer of liquid immediately adjacent to the solid surface has an ordered structure similar to that of a liquid monocrystal. This structure is fluid (zero yield point in shear) in the plane of the solid surface and presents no obstacle to an increase of the solidliquid interfacial area (advancing of the three-phase boundary line). In planes normal to the solid surface the structure has a positive yield point in shear, which prevents decrease of the solidliquid interfacial area (receding of the three-phase line) until the yield point is exceeded by the surface pressure πSL. Mechanical stability of the system at all values of the contact angle between the advancing and receding angles θA and θR is ascribed to a continuously changing value of πSL and of the corresponding specific interfacial free energy γSL in this interval. This change reflects the elastic response in shear of the solidliquid interfacial film in planes normal to the solid surface in this interval.

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