The fluid mechanics of slide coating

Abstract

Slide coating is a means of rapidly depositing multilayered liquid films of precise thickness and uniformity, as in manufacture of photographic products. Liquid is metered through one or more slots onto the inclined surface of the coating die, flows down that face and across a gap onto a fast-moving smooth surface. In this paper the steady, two-dimensional slide coating flow of a Newtonian liquid is analysed by solving the full Navier–Stokes system with the Galerkin/finite-element technique, spine parametrization of free surfaces and full Newton iteration. The lower meniscus in the gap is assumed to remain pinned at the die edge and the wetting-line singularity on the surface being coated is relieved by introducing dynamic-slip and contact-angle parameters. Results include the effects of several design and operating parameters on free-surface profiles and details of the flow field; these are presented by means of contours of kinematic and dynamic variables and local force balances over subdomains. The profiles show standing waves on the slide, rapid film thinning just before the gap, and exponential approach to the final film thickness on the web. As Reynolds number is raised and/or web speed is lowered several recirculation regions are predicted, deleterious features that have also been detected in experiments.