Is “Complete” Spreading Really Complete?

Abstract

The kinetics and mechanisms of complete spreading of polydimethylsiloxane (PDMS) droplets on a solid substrate were examined to discern the limits of the hydrodynamic regime and to identify the nature of the subsequent transport processes. Experiments were conducted on PDMS-1000 in comparison with glycerin on soda-lime glass. The kinetic data were analyzed within the context of a thermodynamic framework which accounts for the driving force of droplet spreading as the difference between Laplace pressure and the solid/liquid interfacial energy. As Laplace pressure within the droplet was depleted to a minimum the glycerin droplet ceased to spread and the PDMS droplet was caused to transform into a 100 micrometer thin film spreading in a Fickian-type behavior. Contrary to the monolayer notion, the diffusive transport of the liquid film ended up with fragmented island domains hopping over the solid substrate, probably driven by short range forces. Based on the evidence gathered in this study what is generally accepted as “complete” spreading falls into three distinct regimes: Hydrodynamic regime, diffusive regime and fragmented islands regime; each is driven by a different mechanism. In analogy with other liquids, PDMS is suggested to exhibit partial wetting with small, but finite, equilibrial contact angle of about 8 degrees. Diffusive and fragmented island regimes are likely a manifestation of the unusual molecular mobility of the PDMS molecules.