Non-monotonic surface tension leads to spontaneous symmetry breaking in a binary evaporating drop

TL;DR

This study reveals that non-monotonic surface tension in binary drops causes spontaneous symmetry breaking and spot formation through Marangoni vortex instability, supported by simulations and experimental measurements.

Contribution

It demonstrates that non-monotonic surface tension induces azimuthal instability leading to pattern formation, a novel insight into binary drop evaporation dynamics.

Findings

Non-monotonic surface tension causes Marangoni vortex formation.

Azimuthal instability leads to symmetry breaking and spot formation.

Experimental measurements confirm non-monotonic surface tension.

Abstract

The evaporation of water/1,2-hexanediol binary drops shows remarkable segregation dynamics, with hexanediol-rich spots forming at the rim, thus breaking axisymmetry. While the segregation of hexanediol near the rim can be attributed to the preferential evaporation of water, the symmetry-breaking and spot formation could not yet be successfully explained. With three-dimensional simulations and azimuthal stability analysis of a minimal model, we investigate the flow and composition in the drop. We show that a slightly non-monotonic surface tension causes the emergence of a counter-rotating Marangoni vortex in the hexanediol-rich rim region, which subsequently becomes azimuthally unstable and forms the observed spots. Accurate measurements with several different methods reveal that the surface tension is indeed non-monotonic. This work provides valuable insight for applications like inkjet printing or spray cooling.