Solutal Marangoni flow as the cause of ring stains from drying salty colloidal drops

TL;DR

This paper investigates the flow dynamics in evaporating salty droplets, revealing that solutal Marangoni flow causes flow inversion and influences deposit patterns, challenging previous assumptions about the origins of ring stains.

Contribution

It provides unprecedented measurements confirming flow inversion due to Marangoni stresses and clarifies their role in salt crystallization and deposit formation.

Findings

Flow inversion confirmed by new measurements

Flow reversal does not prevent ring stain formation

Particles accumulate at the contact line due to interfacial flow

Abstract

Evaporating salty droplets are ubiquitous in nature, in our home and in the laboratory. Interestingly, the transport processes in such apparently simple systems differ strongly from evaporating "freshwater" droplets since convection is partly inverted due to Marangoni stresses. Such an effect has crucial consequences to the salt crystallization process and to the deposits left behind. In this work we show unprecedented measurements that, not only confirm clearly the patterns of the flow inversion, but also elucidate their impact on the distribution of non-volatile solutes. Contrary to what has been often reported in the literature, such a flow reversal does not prevent the formation of ring-shaped stains: particles accumulate at the contact line driven solely by the interfacial flow. We can therefore conclude that the classical "coffee-stain effect" is not the only mechanism that can generate ring-shaped stains in evaporating droplets.