François Lequeux

We investigate the formation of glassy skins at the surface of drying solutions. We quantitatively predict both the formation time of the skin and its growth, whatever the concentration dependence of the mutual diffusion coefficient.

In microchannels, the stability of a fluid jet injected into another immiscible fluid strongly depends on its degree of geometric confinement. When the width of the jet, w, is larger than the channel height, H, the surface tension driven Rayleigh–Plateau instability is suppressed so that the 2D (bidimensional)-confined jet is absolutely stable and never collapses into bubbles (or drops) in contrast to what occurs when w ≤ H [Dollet et al., “Role of the channel geometry on the bubble pinch-off in flow-focusing,” Phys. Rev. Lett. 100(3), 034504 (2008); Guillot et al., “Stability of a jet in confined pressure-driven biphasic flows at low Reynolds number in various geometries,” Phys. Rev. E 78(1), 016307 (2008)]. We here demonstrate both experimentally and theoretically that this picture is, indeed, no longer valid when Marangoni effects are considered. We experimentally show that the addition of small length alcohol molecules into the liquid phase destabilizes a 2D-confined gas–water microfluidic stream ( w > H), leading to the generation of steady non-linear waves and further to the production of bubbles. Using a simple hydrodynamic model, we show through a linear analysis that the destabilization of the gas stream may result from a Marangoni instability due to the fast adsorption of the alcohol molecules, which occurs on a timescale comparable to that of the microfluidic flow.