Wetting Hysteresis and Instability of Hydrophobized Glass and Mica Surfaces

Abstract

Wetting of methylated glass and of Langmuir-Blodgett monolayer coated mica by water and ethanol-water mixtures was investigated by applying the Wilhelmy plate technique. A detailed characterization of the hydrophobized solid surfaces is achieved by a high-resolution continuous record of the wetting tension as a function of the position of the partly immersed plate, the speed of the immersion and retraction, and the time immersed. Among several nonequilibrium and irreversible phenomena studied are (i) static and dynamic wetting tension hysteresis, (ii) wetting tension relaxation and the attainment of equilibrium tension, (iii) desorption and/or reorientation effects at the three-phase contact line and at the solid-liquid interface, and (iv) electrification of solid surfaces on retraction from liquid. The layers produced by dimethyldichlorosilane on glass are stable in both water and ethanol. Dynamic and static wetting tensions are increased, and the equilibrium values are attained at shorter times with increasing alcohol content. The superimposed effect of electrostatic charging also diminishes on the transition from pure water to pure ethanol. Monolayers on mica are generally unstable and undergo degradation both at the three-phase contact line and at the interface with water (similar changes can be traced with composite silane layers on glass before extraction with ethanol). Wetting scanning reveals the changes of wetting tension along the plate arising from the instability of the hydrophobic layer. A rapid desorption of deposited Langmuir-Blodgett layers is observed on immersion in ethanol.