Design and Operational Characteristics of a Robotic Wilhelmy Balance

A robotic Wilhelmy balance (plate method) that automates data acquistion from a multiplicity of liquid samples is described. The design employed a commercial XYZ translation stage to locate a force transducer over any one of an XY array of liquid-sample positions. Z motion performed the immersion/withdrawal cycles involved in measurement of contact-angle hysteresis curves. Data files corresponding to the individual hysteresis curves were analyzed batch-mode by computer according to algorithms specifically developed for this task. Utility of the robotic balance as an extension of available surface-science analytical tools was demonstrated in four general wetting applications: (i) measurement of liquid-vapor interfacial tensions (γ_(lv)) of water and organic fluids, (ii) determination of contact angles formed by these fluids on silane-treated glass and silicon-metal substrata, (iii) measurement of concentration-dependent γ_(1v) for three surfactants and a protein (nonionic, Tween-80; anionic, Aerosol-OT; and cationic, cetyl dimethylethylammonium bromide; human serum albumin, HSA), and (iv) monitoring Tween-80 and HSA adsorption kinetics to a silane-treated glass slide. These studies verified that a robotic balance approach could provide accurate contact angle and interfacial tension measurements of both organic liquids and aqueous surfactant solutions.