Reverse roll coating flow

The Galerkin finite element method of solution formulated by Coyle et al, is applied to analyse the coating flow field between two reverse rotating rolls. A wide range of operating conditions that are of practical importance were studied. Many quantitative results that can benefit a practising engineer are presented and physical insight is gained by the analysis of the data. It is shown that the reverse roll coating process is affected by various factors, among which the roll speed ratio is the most significant. Regardless of other parameters chosen, a critical speed ratio always exists at which the metered film thickness experiences a minimum. At this critical speed ratio, the dynamic contact line is exactly located at the centre of the gap between the two rolls. The significance of the Reynolds and the Capillary numbers increases with increasing speed ratio. For speed ratios beyond the critical value, a thinner metered film can be achieved by means of either increasing the film thickness of the entering layer or by decreasing the principal roll radius. Furthermore, the computational predictions indicate that changing the roll radius ratio has no obvious effect on reverse roll coating and that gravity effects, quantified by the Stokes number, can be ignored under normal operating conditions. Copyright (C) 1999 John Wiley & Sons, Ltd.