We focus on a formation process of a quite thin liquid film, known as ‘precursor film,’ ahead a droplet spreading on a smooth solid substrate. The spreading droplet on the solid substrate is accompanied with a movement of a visible boundary line so-called ‘macroscopic contact line.’ Existing studies have indicated there exist two major regions of the precursor film, that is, a region dominated by the fluid dynamics, and a region dominated by the molecular diffusion. Our group has dedicated our special effort to detect the formation process of the precursor film by applying a convectional laser interferometry and a high-speed camera, and to evaluate the spreading rate of the precursor film. In the present study, the existing length of the precursor film at a very early stage of the droplet spreading is evaluated by applying a Brewster-angle microscopy as well as the interferometer. We extend our attention to the advancing process of the precursor film on inclined substrate.

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