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Abstract

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.