The Flow Characteristics of an Evaporating Ethanol Water Mixture Droplet on a Glass Substrate

Abstract

Droplet evaporation has attracted much interest recently, being relevant to a wide range of biological and technological applications. The underlying mechanisms for this phenomenon are still poorly understood. We report on experimental results, from micro-Particle Image Velocimetry (µPIV), of the spatial and temporal velocity field within pure water and ethanol-water mixture droplets evaporating on a glass substrate. The drop profile, evaporation rate and surface temperature were also measured. For pure water droplets, the redial velocity is found to exhibit a maximum spatially towards the three-phase contact line and to increase dramatically towards the end of the drop lifetime. For ethanol-water droplets, three flow phases of (I) vortical flow, (II) transient flow and (III) radial flow were observed. Phase I has vortices, driven, we believe, by concentration differences arising during the preferential evaporation of ethanol. Phase II sees an exponential decay in vorticity with remaining vortices migrating towards the contact line, accompanied by the formation of one large toroidal vortex, possibly due to ethanol depletion at the apex of the drop leading to a surface tension instability. Phase III is characterized by radial flow towards the contact line, matching the evaporative flux and identical to the flow measured for pure water.

Control of the Particle Distribution in Inkjet Printing through an Evaporation-Driven Sol–Gel Transition

“…These "Marangoni effects" produce a recirculating closed cell. [32][33][34] The cell transports particles away from the contact line and counteracts the build-up of a ring stain. Thermal Marangoni flows may arise on heated/cooled substrates, 27,28 or through evaporative cooling.…”

Numerical Simulation of Evaporation of Ethanol–Water Mixture Droplets on Isothermal and Heated Substrates

“…These local concentration gradients have a significant effect in producing local surface tension gradients and as a result local vortices. The presence of these multiple vortices has been observed by Kim et al and Hamamoto et al As the droplet evaporates, the concentration of ethanol near the contact line becomes lower than that of the droplet apex due to nonuniform evaporative flux. As a result, we have a strong solutal Marangoni stress acting on the interface toward the contact line, which creates an outward velocity near the droplet surface.…”

“…Here, we use the following diffusivity value of ethanol in water D l,e = 1.5 × 10 –9 [m 2 .s –1 ]. 38 The boundary conditions for eq 14 are …”

“…Convection of the solute in eq will be significant since the Peclet number for mass transfer, defined as Pe D = Ua / D l,e , will be much greater than 1. Here, we use the following diffusivity value of ethanol in water D l,e = 1.5 × 10 –9 [m 2 .s –1 ] . The boundary conditions for eq are …”

“…Hamamoto et al 38 conducted experiments using microparticle image velocimetry ( μ PIV) to measure the spatial and temporal velocity variations inside both pure water and ethanol–water droplets. Their results showed three different types of internal flows for the ethanol–water droplets: Vortical flow: a number of vortices are present inside the drop, which exhibit a random rotational direction.…”