SIMULATION OF DROPLET MOTION ON LOW-ENERGY AND HETEROGENEOUS SURFACES

A method of calculation is introduced that allows the simulation of th e time-dependent three-dimensional motion of liquid droplets on solid substrates for systems exhibiting finite equilibrium contact angles. T he contact angle is a prescribed function of position on the substrate . An evolution equation is presented, using the lubrication approximat ion, that includes viscous, capillary, disjoining, and gravitational f orces. Motion to and from dry substrate regions is made possible by us e of a thin energetically stable wetting layer. Axisymmetric spreading on a uniform substrate is calculated, and it is found, in agreement w ith reported experiments, that spreading rates are independent of the contact angle until the drop has almost stabilized. We simulate motion on a heterogeneous substrate composed of two different materials havi ng widely different contact angles. Motion proceeds in an almost disco ntinuous fashion as the initial droplet breaks up into smaller pieces through the action of the wetting forces. Various forms of the disjoin ing energy functional are employed; the particular choice is found to have only a limited quantitative effect of the drop dynamics. Experime ntal observations confirm the basic features of the simulation, althou gh a time-scale correction needs to be applied. (C) 1998 Academic Pres s.