Evaporation of Nanofluid Sessile Droplets Under Marangoni and Buoyancy Effects: Internal Convection and Instability.

Previous research has studied the evolution of patterns during the evaporation of sessile droplets of pure liquid, although there is a lack of reports focusing on the transition of flow regimes and flow stability of nanofluids. In this study, we investigate the evaporation of sessile droplets of AlO-ethanol nanofluid to elucidate the dynamic characteristics of the evaporation process from the perspective of internal convection. As the temperature increases, internal convection intensifies, significantly accelerating the evaporation rate. Three distinct convection flow patterns are observed under the combined influence of the Marangoni effect and buoyancy during evaporation: initially, two macroscopic convection cells form, followed by the periodic generation and propagation of hydrothermal waves (HTWs) near the contact line. Subsequently, Bénard-Marangoni (BM) convection cells gradually emerge and ultimately dominate the flow dynamics. The deposition patterns, which differ in part 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. Furthermore, the critical Marangoni (Ma) and Rayleigh (Ra) numbers for the onset of convection flow instability increase with rising substrate heating temperature.