Evolution of Marangoni Thermo-Hydrodynamics Within Evaporating Sessile Droplets

In the present study, we numerically analyse the temporal evolution of internal advection pattern and corresponding thermal contours of an evaporating sessile droplet. For this purpose, a numerical evaporation model based on Arbitrary Lagrangian–Eulerian (ALE) framework is adopted. The governing differential equations for the transient heat and mass transfer phenomenon are solved in a fully coupled manner. To understand the role of wettability of the underlying substrate, the droplet volume was considered to be constant while the contact angle was varied over a wide range. Results show formation of multi-vortex pattern inside the droplet at initial stages for the hydrophilic surfaces. These findings may be attributed to the local internal thermal imbalance due to evaporative cooling effect that set off buoyancy driven advection and thermal Marangoni flow. On super-hydrophobic surfaces, buoyancy effects play dominant role at initial stages due to large contact angle. With progressing evaporation, the effect of Marangoni convection becomes significant gradually. Also, the overall internal circulation velocity in such cases is greatly enhanced due to large temperature gradient across liquid–vapour interface near the periphery. These findings may have strong implication towards understanding the mixing characteristics inside an evaporating droplet during transient stage.