| The instability of thin liquid films on partially wetting substrates, i.e. the appearance and growth of dry spots and their evolution into droplet patterns has been observed in many systems, ranging from polymer films (Reiter, PRL 1992) to submicrometer aqueous layers. We investigate the rupture of liquid films subjected to impinging laminar air-jets both with systematic experiments and numerical simulations. We quantitatively study the film height profiles and rupture characteristics for stationary as well as moving air-jets. Measurements of rupture site density and distributions of residual droplets show a power law dependence on the liquid film profile and air flow parameters. Our numerical model, which is based on the lubrication approximation, accounts for the shear stress and stagnation pressure profiles of the impinging jet as well as an empirical disjoining pressure potential. In this fashion we obtain an accurate match with the time of rupture in our experiments. For jets of low Reynolds number, we found a high sensitivity of the shape of the liquid-air interface and the location of film rupture to the presence of minute concentrations of surface-active impurities. |
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