Real-time observation of the precursor film for low viscosity liquids spreading on solid substrates by a customized differential laser interference microscopy.

While static wettability is well treated with Young's equation via its static contact angle, theoretical analyses for wetting dynamics are not yet reaching consensus due to a singularity of the spreading forces worked at the vapor/liquid/solid contact line. One plausible explanation to overcome the singularity problem is that there is a so-called precursor film spreading outside the apparent contact line. After its first finding in 1919, many researchers have attempted to visualize its shape. However, because its length and thickness are as small as micrometer and nanometer-order, respectively, its visualization still remains a challenging issue especially for low-viscosity liquids. In the present study, we developed a differential laser interference microscope, which has a thickness resolution of approximately 2 nm at the best, and applied it to the wetting front of 10 cSt of silicone oil spreading on a silicon wafer with an almost constant spreading velocity. As a result, the precursor film of 14 µm long and 108 nm thick was clearly visualized. While the macro contact line has a finite advancing contact angle of 4.0°, the gradient of the precursor film surface gradually decreased and converged to ~ 0.1° at the micro-contact angle. The shape of the precursor film was independent of the time after the dropping for the range of 600 s ± 10%, which is consistent to theoretical estimation. The present study demonstrated that our interferometer simultaneously achieved nanometer thickness resolutions, micrometer in-plane spatial resolution, and at least a millisecond temporal resolution with a simple optical setup.