Sessile drops: Do they really stand on solid surfaces?

Abstract

Bangham and Razouk [1] have described the decrease in free energy at a solid-vapor interface on adsorption of vapor to the solid from zero pressure (vacuum) to any pressure up to p^o, the saturation vapor pressure. Derjaguin and Zorin (DZ) [2] have proposed adsorption isotherms of vapor to solid surface which cross the p^o line and then return to p^o to form sessile drops. Adamson and coworkers [3] have presented a theory to generate a DZ type isotherm and point out that the isotherm implies formation of an autophobic layer on all solid surfaces that support sessile drops. Schrader and Weiss [4] have proposed an adsorption isotherm for sessile drop formation which crosses the p^o line but does not return to p^o on formation of the sessile drop. This last approach does not require ubiquitous formation of autophobic layers on solid surfaces in sessile drop systems. Free energy isotherms based on the last approach are presented to describe film formation (Type 1 free energy isotherm), sessile drop formation on low-energy surfaces (Type 2), and sessile drop formation on autophobic layers (Type 3). The approach also enablcs derivation of a new equation for the vapor pressure of sessile drops subject to contact angle hysteresis.

Keywords:

• Sessile drops
• adsorption isotherms
• free energy isotherms
• autophobic layers
• oriented layers
• spreading pressure
• vapor pressure
• contact angle hysteresis