Wetting Characteristics of Aqueous Surfactant-Laden Drops

Abstract

Strategies to promote or control droplet spreading on solid surfaces is important to many industrial and biological processes. The manipulation of surface tension and contact angle by the addition of surface-active compounds to the bulk liquid of a drop offers one strategy for altering the adhesion tension maintaining drop integrity on a solid surface. Here, we present results of an experimental study of surfactant-laden drop behavior on solid surfaces. Wetting experiments were performed to determine how contact angles and contact angle hysteresis change with varying soluble surfactant concentration in the bulk liquid phase. Static, advancing, and receding contact angles of aqueous solutions of two different surfactants were measured over a wide range of surfactant concentrations on three different solids using an inclining surface. In general, adhesion tension increased and then plateaus as the surfactant concentration increased from very dilute values to higher concentrations. At surfactant concentrations above the critical micelle concentration, a continued decrease in contact angle was observed, leading to an increase in the adhesion tension. Subsequent experiments confirmed adsorption of surfactants to the solid substrate. Contact angle hysteresis was calculated from the experimental data. The behavior of the contact angle hysteresis was substrate and surfactant concentration dependent, but only at lower surfactant concentrations. Material combinations that led to partial wetting could be converted to complete wetting by the addition of sufficient surfactant in the bulk phase, but at the cost of increasing adsorption onto the solid, which restricts drop motion.