Evaporating sessile droplets: solutal Marangoni effects overwhelm thermal Marangoni flow

TL;DR

This study reveals that in evaporating water droplets, solutal Marangoni effects caused by contaminants can dominate over thermal Marangoni flows, leading to unexpected flow directions and asymmetries, contrary to theoretical predictions.

Contribution

The paper demonstrates experimentally and numerically that tiny amounts of surfactants can reverse flow directions in evaporating droplets, highlighting the importance of contaminants in flow behavior.

Findings

Thermal Rayleigh convection often dominates over thermal Marangoni flow in small droplets.

Surfactants can reverse flow direction with as little as 0.5% reduction in surface tension.

Presence of surfactants affects flow symmetry and stability.

Abstract

When an evaporating water droplet is deposited on a thermally conductive substrate, the minimum temperature will be at the apex due to evaporative cooling. Consequently, density and surface tension gradients emerge within the droplet and at the droplet-gas interface, giving rise to competing flows from, respectively, the apex towards the contact line (thermal-buoyancy-driven flow) and the other way around (thermal Marangoni flow). In small droplets with a diameter below the capillary length, the thermal Marangoni effects are expected to dominate over thermal buoyancy ("thermal Rayleigh") effects. However, contrary to these theoretical predictions, our experiments mostly show a dominant circulation from the apex towards the contact line, indicating a prevailing of thermal Rayleigh convection. Furthermore, our experiments often show an unexpected asymmetric flow that persisted for several minutes. We hypothesise that a tiny amount of contaminants, commonly encountered in experiments with water/air interfaces, act as surfactants and counteract the thermal surface tension gradients at the interface and thereby promote the dominance of Rayleigh convection. Our finite element numerical simulations demonstrate that, under our specified experimental conditions, a mere 0.5% reduction in the static surface tension caused by surfactants leads to a reversal in the flow direction, compared to the theoretical prediction without contaminants. Additionally, we investigate the linear stability of the axisymmetric solutions, revealing that the presence of surfactants also affects the axial symmetry of the flow.