Influence of the flow field in curtain coating onto a prewet substrate

Abstract

The onset of air entrainment for curtain coating onto a surface prewetted with the coating fluid was studied. The substrate used was a polished, scraped steel wheel and coating was performed over ranges of dimensionless parameters observed in commercial coating processes (Reynolds number, 0.14<Re=ρQ∕μ<33.02; Capillary number, 0.19<Ca=μU∕σ<25.07). The substrate velocity for the onset of air entrainment was obtained as a function of the curtain flow rate per unit width of curtain (1<Q<9cm2s−1), fluid dynamic viscosity (0.0326<μ<0.878Pas), curtain height (0.035<h<0.095m), and thickness of the prewet film (1×10−7<c<3×10−5m). A remarkable and strong dependence of the onset of air entrainment on curtain flow rate was observed (hydrodynamic assist) and the general features of the hydrodynamics were very similar to those observed for previous works onto dry substrates. However, the presence of the prewet film led to higher maximum substrate velocities at the onset of air entrainment than observed for dry substrates. For high liquid viscosities, the air entrainment curve bifurcates; under these conditions, the maximum substrate velocity is no longer inversely proportional to the fluid viscosity and stable coating is possible at higher substrate velocities than would be predicted by conventional theory. This “intense assist” exhibits a complex relationship with the prewet film thickness. The results presented in this paper demonstrate that hydrodynamic assist is not exclusive to wetting, but is a generic phenomenon of fluid flows.