Transient Growth Process of Precursor Film at Early Stage of Droplet Spreading

We conduct an experimental study focusing on the spreading droplet in the vicinity of the boundary line of solid-liquid-gas interface, which is called macroscopic contact line (M-CL). When the droplet spreads on the solid surface completely, a very thin film whose thickness is of a few nm is formed ahead of the M-CL. This thin film is so-called precursor film. We pay our attention to the spatio-temporal growth of the precursor film in terms of its thickness and length at an early stage of the droplet spreading. The target system is a tiny droplet of 2-cSt and 5-cSt silicone oil spreading on the glass substrate. We apply confocal laser displacement sensor to measure the temporal variation of the precursor film thickness at a designated point, and the Brewster angle microscope to detect the precursor film ahead of the M-CL and to evaluate its existing length. We show the effects of the liquid viscosity on the development of precursor film through the results of both its length and thickness.