Abstract
The evaporation of droplets containing nanoparticles is commonplace in industrial processes, while there is a lack of research on the instability-related characteristics of evaporation convection in nanofluid droplets, which differ from those of pure fluid droplets. Here we investigate the convection instability patterns and transition phenomenon in an Al2O3-ethanol nanofluid sessile droplet evaporation. Three different convection flow patterns are observed under the influence of both Marangoni effect and buoyancy during the evaporation: Two macroscopic convection cells appear at first, followed by the periodic generation and propagation of hydrothermal waves (HTWs) near the contact line. Then, the Bénard-Marangoni (BM) convection cells gradually emerge and eventually assume a dominant role. The deposition patterns, partly different from the classic coffee-ring pattern, are closely related to the flow patterns of HTWs and BM convection cells during the pinning stage of droplet evaporation. The critical Marangoni (Ma) and Rayleigh (Ra) numbers for the onset of convection flow instability increase with the increase of substrate heating temperature.