Evaporation dynamics and deposition patterns of ethanol-based Al2O3 nanofluid droplets: Effects of particle sizes and substrate temperatures

Abstract

The evaporation of nanofluid droplets holds significant potential for applications in thermal management. Nonetheless, the ways in which nanoparticles affect the evaporation dynamics and deposition properties of nanofluid droplets are still not well understood. Here, we investigate the effects of Al₂O₃ nanoparticle sizes and substrate temperatures on the wetting characteristics, evaporation rate, and particle deposition behavior of ethanol-Al₂O₃ nanofluid droplets. Additionally, the features and transitions of internal convective instabilities are observed and discussed. The results reveal that the addition of nanoparticles has a minor impact on the changes in wetting patterns during droplet evaporation, and all droplets evaporate in a mixed mode. Nanoparticle size plays a pivotal role in regulating deposition patterns and wetted area, which in turn indirectly affects the evaporation rate. Specifically, the heat flux at the solid-liquid interface and the evaporation rate are enhanced compared to the base fluid, with these differences becoming more pronounced at higher substrate temperatures and with larger nanoparticle sizes. The internal convection within the droplets is primarily driven by Marangoni flow, and the presence of nanoparticles has negligible influence on the flow behavior. Smaller nanoparticles tend to deposit near the center of the droplet, resulting in a reduced coffee-ring effect, whereas larger nanoparticles predominantly deposit at the contact line, forming a marked coffee ring. Therefore, by selecting nanoparticles of appropriate sizes, it is possible to enhance evaporation rates or mitigate the coffee-ring effect to meet the requirements of various industrial applications.