Influence of surfactant upon air entrainment hysteresis 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)). Impingement speed and viscosity were previously shown to be critical parameters in correlating air entrainment data with three qualitatively different regimes of hydrodynamic assist identified (Marston et al. in Exp Fluids 42(3):483-488, 2007a). 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. For the first time, results considering the influence of surfactants are presented in attempt to elucidate the relative importance of surface tension in this inertia-dominated system. The results show quantitative and qualitative differences to previous results with much more complex hysteretic behaviour which has only been reported previously for rough surfaces.