Diffusion effects in the wetting of a contaminated surface

Industrial wetting situations often involve the presence of a third compone nt near an interface, potentially modifying the local effective capillary b alance. We here consider a simplified situation in which a drop is placed o n a substrate supporting a thin layer of "contamination." As spreading of t he drop to equilibrium occurs, the contaminant diffuses into the drop, modi fying the effective substrate/drop interfacial free energy (increasing it, it is assumed). Thus the kinetics of spreading are altered. The essential e ffect when the final equilibrium contact angle is zero is that of accelerat ing the process. However, when the final value of contact angle is finite, "overshoot" may occur. The drop spreads beyond its equilibrium state and th en retracts to attain its equilibrium state asymptotically at long times, ( C) 2000 Academic Press.