Influence of viscosity and impingement speed on intense hydrodynamic assist in curtain coating

The onset of air entrainment for curtain coating onto a pre-wetted substrate was studied experimentally in similar parameter regimes to commercial coating [Re = rho Q/mu = O(1), We = rho Q u (c) /sigma = O(10), Ca = mu U/sigma = O(1)]. As with coating onto dry substrates, a large flow-rate dependence was observed; termed hydrodynamic assist. The influence of the impingement speed and viscosity are shown to be critical and three qualitatively different regimes of assist are identified. The interaction of the impinging curtain with the pre-existing film also led to a significant hysteretic effect throughout the flow rate-substrate speed parameter space.