Simulation of transient and three-dimensional coating flows using a volume-of-fluid technique

Slide-coating flow is widely used for the manufacturing of precision film-coating products. Considerable effort is being devoted toward a better understanding of slide-coating processes in hopes of increasing coating speeds and improving the performance of coated film. It has been demonstrated, for example Chen(1), that increasing coating speeds beyond well defined limits can result in a complete breakdown of the coating bead. In this paper we present simulation results of slide-coating flows obtained from a computational method capable of describing arbitrary, three-dimensional and time-dependent deformations of fluid surfaces. The method, which is available in the commercial program(2), uses a fixed grid through which fluid is tracked by a Volume-of-Fluid (VOF) technique(3,4). Surface tension, wall adhesion, fluid momentum, and viscous stresses are fully accounted for in our analysis. The basic method is illustrated through comparisons with dip-coating data(5). Then we present a discussion on how contact lines and dynamic contact angles are implicitly treated in our method. Because we use a VOF technique, we need only sum the forces acting on each control volume containing fluid. The location of contact lines and dynamic contact angles then arise automatically from the computed balance of forces. Our technique is illustrated with examples of startup and bead-breakup phenomena in coating flows. As will be shown, for rapid processes our approach offers efficiency and robustness for the simulation of coating process design and optimization that is difficult to achieve, with conventional analysis methods.