A contaminant-concentration-dependent surface tension does not explain the absence of solutal Marangoni flow in evaporating droplets
Theoretical models of evaporating droplets predict Marangoni flows orders of magnitude faster than those observed experimentally. While this discrepancy is often attributed to surface contamination, the underlying mechanism by which contaminants weaken Marangoni stresses remains unclear. In this study, we compare particle image velocimetry (PIV) experiments with a coupled hydrodynamic and solute transport model to investigate the internal flow of evaporating aqueous droplets containing salt, glycerol, or ethanol. By analyzing both sessile and pendant droplets, we demonstrate that the flow is driven entirely by natural convection, in contrast to theoretical predictions that use surface-tension gradients. Remarkably, in some cases, the experimental surface velocity is found to be directed against the predicted surface-tension gradient. We further prove that commonly used contamination models---whether based on surfactants lowering the surface tension or on surface rheology---cannot account for this flow reversal, as they would require the surfactant to be transported against the flow when the surface velocity is opposed to the surface-tension gradient. Our results show that a reconsideration of these models is required to explain the absence of solutal Marangoni in evaporating droplets.