The effects of thin and ultrathin liquid films on dynamic wetting

Abstract

We examine the effects of thick (micron scale) fluid films and thin molecular scale (10–100 Å) films on the hydrodynamics near advancing contact lines by measuring the liquid–vapor interface shape of a meniscus and comparing the measurements to three models. Using flow visualization, we directly observe the fluid flow field near the moving contact line and give a qualitative description of the stagnation point and dividing streamline emanating from the contact line region. For thick films, when the capillary number satisfies Ca(a/d)3/2⩽O(1) (where d is the film thickness and a is the macroscopic length scale of the system), the liquid–vapor interface is bent only slightly by the viscous flow and the effective dynamic contact angle is close to zero. As Ca approaches O(1/ln(a/d)), a modulated wedge-like region appears at some distance from the film and expands both away from and toward the film as Ca increases. The dynamic contact angle approaches the classic power law behavior as this region expands. For molecularly thin films, the liquid–vapor interface shape within microns of the moving contact line is correctly described by theoretical models based purely on hydrodynamics and without disjoining pressure effects.