Stephen DavisNorthwestern University, Engineering Sciences and Mathematics
Before 1970 no researchers understood that the mutuaI displacement of a liquid by another liquid or gas poses a severe modeling problem. it was not unti[ Hu & Scriven (1971) and Dussan V & Davis (L974) that it became clear that such mutual displacement requires the loss of the no-slip boundary condition at the substrate. This evolution seemed complete in describing the macroscopic behavior of drops until the experiments of Saiz & Tomasia (2005) on liquid metallic drops at elevated temperatures spreading on Mb substrates which showed definitively that the data departed from theory and instead followed the molecular-kinetic theory (Blake 1969) in which fluid flow is absent and molecules of liquid hop from the drop onto the substrate ahead of the contact [ine effectively advancing it. This led to the further consideration of the later theory. Later Davis & Davis (2000) examined both theories in conjunction with metallic macroscopic contact angle verses contact-line speed cross and argued that the spreading initially at high speed follows the hydrodynamic theory until the crossing point and then switches to the molecular theory, and one might expect this to be the case in every spreading experiment. Recently, Karim, Davis, and Kavehpour (2016) show that spreading drops follow the theories above but that forced contact lines diverge and follow the molecular kinetic theory. Thus, the early “simplicity” has been replaced by a complex array of behaviors which must be understood in properly interpreting experimentaI observations.
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