Dynamic Cycling Contact Angle Measurements: Study of Advancing and Receding Contact Angles

Abstract

Dynamic cycling contact angle (DCCA) measurements of six liquids from two homologous series (i.e., alkanes and alcohols) on FC-732-coated silicon wafer surfaces were performed using automated axisymmetric drop shape analysis-profile (ADSA-P). Unlike the previous one-cycle measurements that have been made in a number of studies, these cycling contact angle measurements provide more information on the mechanisms of contact angle hysteresis θhyst. Both the advancing contact angles θa (except for the one measured from the first cycle) and the receding contact angles θr obtained from different cycles were found to be time-dependent. By comparing the results between cycles, were obtained θa and θr values at some specific drop radii. It was found that both θa and θr decreased with increasing number of cycles. Furthermore, both θa and θr values obtained at the larger contact radius were larger than those obtained at the smaller radius. The result is plausible in terms of liquid sorption and/or retention by the solid surface: the solid surface modification by the liquid increases with longer solid/liquid contact, leading to smaller values of θa and θr. It was also found that contact angle hysteresis θhyst, the difference between θa and θr at each radius, increased initially and then leveled off with increasing number of cycles. The result suggests that processes which occurred on the polymer surface during the experiment, such as liquid sorption and evaporation, will eventually approach a steady state and hence lead to constant hysteresis of the contact angle. This supports the contention that liquid sorption and/or retention is a likely cause of the time dependence of contact angle hysteresis (as well as advancing and receding contact angles). All θa data obtained beyond the first cycle and all θr data reflect liquid sorption and/or retention by the solid and are therefore not a property of the solid alone. Therefore, only θa obtained in the first cycle (on the dry solid) should be used in the calculation of the surface energetics of solids.