Modelling of the dip-coating process

Abstract

Dipcoating, wherein an object is withdrawn from a liquid bath, retaining a thin layer of liquid which then dries, is perhaps the simplest of coating techniques. SOLA Optical asked the MISG to investigate this process as a means of applying a protective coating to spectacle lenses. The theory, as proposed by the group, involves the 'lubrication' form of the equations of slow viscous flow. This yields an estimate of the wet coating thickness variation, which in turn depends on the liquid properties, the pull rate, and the lens curvature. When this thickness is reduced by the volatile fraction of the coating, predicted thicknesses are in rough agreement with reported values. Once applied, the liquid film drains downward and this effect is modelled numerically. An outline of a more complete model, allowing evaporation and drying, is also presented.