Thermocapillary fingering in surfactant-laden water droplets

The drying of sessile droplets represents an intriguing problem, being a simple experiment to perform but displaying complexities that are archetypical for many free surface and coating flows. Drying can leave behind distinct deposits of initially well dispersed colloidal matter. For example, in the case of the coffee ring effect, particles are left in a well-defined macroscopic pattern with particles accumulating at the edge, controlled by the internal flow in the droplet. Recent studies indicate that the addition of surfactants strongly influences this internal flow field, even reversing it and suppressing the coffee ring effect. In this work, we explore the behavior of droplets at high surfactant loadings and observe unexpected outward fingering instabilities. The experiments start out with droplets with a pinned contact line, and fast confocal microscopy is used to quantify a radially outward surfactant-driven Marangoni flow, in line with earlier observations. However, the Marangoni flows are observed to become unstable, and local vortex cells are now observed in a direction along the contact line. The occurrence of these vortices cannot be explained on the basis of the effects of surfactants alone. Thermal imaging shows that thermocapillary effects are superimposed on the surfactant-driven flows. These local vortex cells acts as little pumps and push the fluid outward in a fingering instability, rather than an expected inward retraction of the drying droplet. This leads to a deposition of colloids in a macroscopical flower-shaped pattern. A scaling analysis is used to rationalize the observed wavelengths and velocities, and practical implications are briefly discussed.