Fluid flow structures in an evaporating sessile droplet depending on the droplet size and properties of liquid and substrate

TL;DR

This study numerically analyzes internal flow structures in evaporating sessile droplets, revealing how droplet size, liquid properties, and substrate thermal conductivity influence vortex formation and flow patterns.

Contribution

It provides a comprehensive phase diagram of vortex structures in evaporating droplets considering multiple parameters, including droplet size, thermal conductivities, and fluid volatility.

Findings

Vortex structures depend on contact angle and thermal conductivity ratio.

Larger droplets tend to lack certain vortex subregions.

Highly volatile droplets do not exhibit reversed vortex subregions.

Abstract

We investigate numerically quasi-steady internal flows in an axially symmetrical evaporating sessile droplet depending on the ratio of substrate to fluid thermal conductivities, fluid volatility, contact angle and droplet size. Temperature distributions and vortex structures are obtained for droplets of 1-hexanol, 1-butanol and ethanol. To this purpose, the hydrodynamics of an evaporating sessile drop, effects of the thermal conduction in the droplet and substrate and diffusion of vapor in air have been jointly taken into account. The equations have been solved by finite element method using ANSYS Fluent. The phase diagrams demonstrating the number and orientation of the vortices as functions of the contact angle and the ratio of substrate to fluid thermal conductivities, are obtained and analyzed for various values of parameters. In particular, influence of gravity on the droplet shape and the effect of droplet size have been considered. We have found that the phase diagrams of highly volatile droplets do not contain a subregion corresponding to a reversed single vortex, and their single-vortex subregion becomes more complex. The phase diagrams for droplets of larger size do not contian subregions corresponding to a regular single vortex and to three vortices. We demonstrate how the single-vortex subregion disappears with a gradual increase of the droplet size.