Gravitational effect in evaporating binary microdroplets

Abstract

The flow in an evaporating glycerol-water binary sub-millimeter droplet with Bond number Bo $\ll$ 1 is studied both experimentally and numerically. First, we measure the flow fields near the substrate by micro-PIV for both sessile and pendant droplets during the evaporation process, which surprisingly show opposite radial flow directions -- inward and outward, respectively. This observation clearly reveals that in spite of the small droplet size, gravitational effects play a crucial role in controlling the flow fields in the evaporating droplets. We theoretically analyze that this gravity-driven effect is triggered by the lower volatility of glycerol which leads to a preferential evaporation of water then the local concentration difference of the two components leads to a density gradient that drives the convective flow. We show that the Archimedes number Ar is the nondimensional control parameter for the occurrence of the gravitational effects. We confirm our hypothesis by experimentally comparing two evaporating microdroplet systems, namely a glycerol-water droplet and a 1,2-propanediol-water droplet. We obtain different Ar, larger or smaller than a unit by varying a series of droplet heights, which corresponds to cases with or without gravitational effects, respectively. Finally, we simulate the process numerically, finding good agreement with the experimental results and again confirming our interpretation.

References in corpus (3)

• Evaporation-triggered microdroplet nucleation and the four life phases of an evaporating Ouzo drop
• Evaporating pure, binary and ternary droplets: thermal effects and axial symmetry breaking
• Self-wrapping of an ouzo drop induced by evaporation on a superamphiphobic surface

Cited by in corpus (10)

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• Marangoni instabilities of drops of different viscosities in stratified liquids
• The asymmetric coalescence of two droplets with different surface tensions is caused by capillary waves
• Marangoni instability triggered by selective evaporation of a binary liquid inside a Hele-Shaw cell
• Physiochemical hydrodynamics of the phase segregation in an evaporating binary microdroplet
• Physicochemical Hydrodynamics of Droplets out of Equilibrium: A Perspective Review
• Unexpectedly absence of Marangoni convection in an evaporating water meniscus
• The Inverse-Square Law Force between Vapor-Mediated Droplets
• Evaporation-Induced Crystallization of Surfactants in Sessile Multicomponent Droplets