Evaporating Water Droplets on Hydrophobic Surfaces: Influence of Insoluble Surfactants*

When a water droplet is deposited on a hydrophobic, thermally conductive substrate, the evaporation rate is maximum at the apex, resulting in lower temperatures at the top. Consequently, density and surface tension gradients emerge within the droplet and at its surface, giving rise to flows from the apex towards the contact line and vice versa, respectively. In small droplets, capillary forces (thermal Marangoni effects) are expected to dominate over buoyancy forces ("Rayleigh effects''). However, contrary to theoretical predictions, in recent experiments dominance of the circulation from the apex to the contact line was observed, indicating prevailing Rayleigh convection. Furthermore, the same experiments showed an unexpected asymmetric flow that persisted for several minutes. We hypothesize that a small amount of insoluble surfactants, which may arise from dust particles or experimental imperfections, reduces the capillary effects, thereby promoting the dominance of Rayleigh convection. Our Finite Element numerical simulations demonstrate that, under the experiment's conditions, a mere 0.5% reduction in the initial surface tension caused by surfactants leads to a reversal in the flow direction compared to the theoretical prediction without surfactants. Additionally, we investigate the stability of the solutions obtained under azimuthal perturbation at a specific mode, revealing that the presence of surfactants also affects the axisymmetry of the flow.