Evaporation of a sessile colloidal water-glycerol droplet: Marangoni ring formation

TL;DR

This study investigates particle transport in evaporating water-glycerol droplets, revealing a novel Marangoni ring formation influenced by solutal Marangoni flow and colloidal crystal assembly, with implications for understanding complex evaporation phenomena.

Contribution

It uncovers the formation of a Marangoni ring near the liquid-air interface in evaporating droplets, highlighting the role of non-hydrodynamic interactions in colloidal assembly.

Findings

Marangoni ring forms near the liquid-air interface.

Particles assemble into colloidal crystals, causing iridescence.

Simulations do not fully capture experimental particle distribution.

Abstract

The transport and aggregation of particles in suspensions is an important process in many physicochemical and industrial processes. In this work, we study the transport of particles in an evaporating binary droplet. Surprisingly, the accumulation of particles occurs not only at the contact line (due to the coffee-stain effect) or at the solid substrate (due to sedimentation), but also at a particular radial position near the liquid-air interface, forming a "ring", which we term as the Marangoni ring. The formation of this ring is primarily attributed to the solutal Marangoni flow triggered by the evaporation dynamics of the water-glycerol droplet. Experiments and simulations show fair agreement in the volume evolution and the general structure of the solutal Marangoni flow, that is, the Marangoni vortex. Experiments show that the location of the Marangoni ring is strongly correlated with the Marangoni vortex. However, finite element numerical simulations fail to describe the particle distribution seen in the experiments. Interestingly, the particles not only accumulate to form the Marangoni ring but also assemble as colloidal crystals close to the liquid-air interface, yielding iridescence. The formation of the colloidal crystals in the experiments is strong evidence that non-hydrodynamic interactions, which are not represented in the simulations, also play a significant role in our system.