  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 reveal that the surface
tension is indeed non-monotonic (~0.3 mN/m). This work provides valuable
insight for applications like inkjet printing or spray cooling.
