Dynamic cycling contact angle measurements: Study of advancing and receding contact angles

Dynamic cycling contact angle (DCCA) measurements of six liquids from two h omologous series (i.e., alkanes and alcohols) on FC-732-coated silicon wafe r 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 prov ide more information on the mechanisms of contact angle hysteresis theta (h yst). Both the advancing contact angles theta (a) (except for the one measu red from the first cycle) and the receding contact angles Or obtained from different cycles were found to be time-dependent. By comparing the results between cycles, were obtained theta (a) and theta (r) values at some specif ic drop radii. It was found that both theta (a). and theta (r) decreased wi th increasing number of cycles. Furthermore, both theta (a) and theta (r) v alues 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 t he liquid increases with longer solid/ liquid contact, leading to smaller v alues of theta (a) and theta (r). It was also found that contact angle hyst eresis theta (hyst), the difference between theta (a) and theta (r) at each radius, increased initially and then leveled off with increasing number of cycles. The result suggests that processes which occurred on the polymer s urface during the experiment, such as liquid sorption and evaporation, will eventually approach a steady state and hence lead to constant hysteresis o f the contact angle. This supports the contention that liquid sorption and/ or retention is a likely cause of the time dependence of contact angle hyst eresis (as well as advancing and receding contact angles). All theta (a) da ta obtained beyond the first cycle and all Or data reflect liquid sorption and/or retention by the solid and are therefore not a property of the solid alone. Therefore, only theta (a) obtained in the first cycle (on the dry s olid) should be used in the calculation of the surface energetics of solids . (C) 2001 Academic Press.