Marangoni instability triggered by selective evaporation of a binary liquid inside a Hele-Shaw cell

Interfacial stability is important for many processes involving heat and mass transfer across two immiscible phases. For the evaporation of a binary solution with one component more volatile than the other, gradients in surface tension can arise. These gradients can ultimately destabilize the liquid-gas interface. We study the evaporation of an ethanol-water solution, for which ethanol is more volatile. The solution is contained in a horizontal Hele-Shaw cell which is open from one end to allow for evaporation into air. A Marangoni instability is then triggered at the liquid-air interface. We study the temporal evolution of the instability through its effects on the bulk of the liquid. More specifically, the growth of convective cells is measured with confocal microscopy and the velocity field with microparticle image velocimetry. The results of numerical simulations based on quasi-two-dimensional equations satisfactorily compare with the experimental observations, even without consideration of evaporative cooling, although this cooling can play an extra role in experiments. Furthermore, a linear stability analysis applied to a simplified version of the quasi-two-dimensional equations showed reasonably good agreement with the results from simulations at early times, when the instability has just been triggered and before coarsening. In particular, we find a critical Marangoni number below which a regime of stability is predicted.