SPONTANEOUS SPREADING OF VISCOUS-LIQUID DROPS

The spontaneous spreading of viscous liquid drops on solid surfaces, just after deposition, has been studied. Silicone oils (polydimethylsiloxane, PDMS) of two different viscosities were placed on fluorinated silicon wafers, tetrafluoroethylene (Teflon), and glass. The contact angle, height, and contact radius have been measured with the passage of time. The spreading rate depended on the size of the drop, the physical properties (surface tension and viscosity) of the liquid, and the available capillary pressure. Less-viscous or smaller drops spread faster. Later on, as drops approached equilibrium, surface interactions appeared to contribute to the movement of the contact line. A simple model assuming downward flow during the early stages of spreading has been used to interpret the experimental data. © 1993 Academic Press, Inc.