Physiochemical hydrodynamics of the phase segregation in an evaporating binary microdroplet

TL;DR

This study investigates the complex physiochemical hydrodynamics during phase segregation in an evaporating binary droplet, revealing three stages of flow behavior driven by Marangoni effects and segregation.

Contribution

It combines experiments and theoretical analysis to elucidate the flow dynamics and stages of phase segregation in evaporating binary droplets, specifically the 1,2-hexanediol-water system.

Findings

Identified three stages of evaporation with distinct flow behaviors.

Demonstrated segregation-driven flow reversal during evaporation.

Showed Marangoni effects dominate flow dynamics at different stages.

Abstract

Phase segregation triggered by selective evaporation can emerge in multicomponent systems, leading to complex physiochemical hydrodynamics. Recently, Li et al. (Phys. Rev. Lett., vol. 120, 2018, 224501) and Kim & Stone (J. Fluid Mech., vol. 850, 2018, pp. 769-783) reported a segregative behavior (i.e., demixing) in an evaporating binary droplet. In this work, by means of experiments and theoretical analysis, we investigate the flow dynamics after the occurrence of the phase segregation. As example, we take the 1,2-hexanediol-water binary droplet system. First, we experimentally reveal the overall physiochemical hydrodynamics of the evaporation process, including the segregative behavior and the resulting flow structure close to the substrate. By quantifying the evolution of the radial flow, we identify three successive life stages of the evaporation process. At Stage I, a radially outward flow is observed. It is driven by the Marangoni effect. At the transition to Stage II, the radial flow partially reverses, starting from the contact line. This flow breaks the axial symmetry and remarkably is driven by the segregation itself. Finally at Stage III, the flow decays as the evaporation gradually ceases. At this stage the segregation has grown to the entire droplet, and the flow is again controlled by the Marangoni effect. The resulting Marangoni flow homogenizes the distribution of the entrapped volatile water over the whole droplet.