Measurement of dynamic wetting using phase-shifting imaging ellipsometer: comparison of pure solvent and nanoparticle suspension on film thickness profile, apparent contact angle, and precursor film length

Wetting is a fundamental and important phenomenon that is encountered in various engineering processes, and particularly, the understanding of mesoscopic dynamic wetting of not only pure liquids but also suspensions with nanoparticles is required with the recent miniaturization of devices. Therefore, it is important to establish a technique to precisely measure the dynamic behavior of liquid thin films near the three-phase contact line. In the present study, the mesoscopic wetting behavior of nonvolatile polydimethylsiloxane (PDMS, 20 cSt) and a PDMS suspension containing PDMS-modified SiO 2 nanoparticles at a concentration of 1 wt% is measured using a phase-shifting imaging ellipsometer (PSIE) developed in our previous study, although the aggregates of nanoparticles are formed in the suspension. The PSIE can measure the two-dimensional thickness profile of liquid thin films from nanometer to micrometer scales. From the measured thickness profiles, we investigate the effect of the suspended nanoparticles on the apparent contact angle θ a and precursor film length L p . The dependence of θ a and L p on the velocity of contact line U ranging from 10 –8 to 10 –6 m/s is characterized. The contact angles of both fluids reflect the Cox–Voinov law, i.e., the θ a is proportional to U 1/3 . At the same U , the θ a of the suspension is higher than that of pure PDMS. The L p of both fluids is proportional to U −1 and is the same at the same U . Furthermore, the L p and thickness profiles at the nanometer scale are consistent with adiabatic precursor film theory. Graphical Abstract