(459f) Delaying Wetting Failure In Coating Flows Via Meniscus Confinement

Wetting failure brought about by air entrainment remains one of the biggest obstacles to faster operation of many coating processes. Improving upon the coating speeds used in current technologies requires a better understanding of how system parameters influence wetting failure. Confinement of the meniscus associated with the wetting line is one such parameter commonly found in high-speed coating methods. In this study, we explore the effects of confinement on wetting failure through experiments performed on a laboratory-scale plunge-coating system. Our experimental apparatus consists of a cylindrical steel roll that plunges into a bath of glycerol. Confinement is imposed by bringing a coating die near the wetting line, and liquid is injected through the die to compensate for liquid being dragged away with the roll. Flow visualization is used to record the critical roll speed at which wetting failure occurs.  The data show a clear increase in the critical speed with increasing confinement.  Predictions from a lubrication-theory-based model are qualitatively consistent with the experimental observations, and results from finite-element simulations shed light on the physical limitations of the lubrication model.