DESIGN AND OPERATIONAL CHARACTERISTICS OF A ROBOTIC WILHELMY BALANCE

A robotic Wilhelmy balance (plate method) that automates data acquisti on from a multiplicity of liquid samples is described. The design empl oyed a commercial XYZ translation stage to locate a force transducer o ver any one of an XY array of liquid-sample positions. Z motion perfor med the immersion/withdrawal cycles involved in measurement of contact -angle hysteresis curves. Data files corresponding to the individual h ysteresis curves were analyzed batch-mode by computer according to alg orithms specifically developed for this task. Utility of the robotic b alance as an extension of available surface-science analytical tools w as demonstrated in four general wetting applications: (i) measurement of liquid-vapor interfacial tensions (gamma(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 o f concentration-dependent gamma(1v) for three surfactants and a protei n (nonionic, Tween-80; anionic, Aerosol-OT; and cationic, cetyl dimeth ylethyl-ammonium bromide; human serum albumin, HSA), and (iv) monitori ng Tween-80 and HSA adsorption kinetics to a silane-treated glass slid e. These studies verified that a robotic balance approach could provid e accurate contact angle and interfacial tension measurements of both organic liquids and aqueous surfactant solutions.