Nanoparticle deposition from nanofluid droplets during Leidenfrost phenomenon and consequent rise in transition temperature

Abstract

Termination of film boiling at an elevated Leidenfrost temperature can have important implications in applications which require rapid cooling. Here we report the Leidenfrost phenomenon of nanofluid (nano-TiO2 dispersed in water) droplets exposed to a substrate heated to 300 °C and evaporation lifetime of water droplets dispensed over a copper substrate heated to 300 °C as a benchmark for comparison. Unlike water or sparsely dispersed nanofluid droplets, the abundantly dispersed nanofluid droplets exhibit deposition and boiling at 300 °C accompanied by violent agitation and fragmentation followed by continuous ejection of small droplets. As a result, the Leidenfrost phenomenon ceases to occur and the droplet evaporation lifetime is reduced by an order of magnitude in comparison to water. We anticipate that the vapor generated beneath the droplet for a higher concentration of nanoparticles is unable to levitate the droplet (mass) as deposition of a nano-particles generating a random pattern on the hot substrate disrupts the Leidenfrost state. We confirm the evidence of nanostructured porous coating from microscopy and topography analysis which reveals micro-pits and protuberances as high as several micrometers formed by clustered nanoparticles. In the presence of such random nanometric surface features, the interplay between the capillary pressure and vapor pressure dictates the existence of Leidenfrost condition. At a substrate temperature of 300 °C, the capillary pressure dominates the vapor pressure for the highly concentrated nanofluid droplets thereby, causing deposition of nanoparticles and ejection of small droplets. However, the same droplets exhibit levitation over the vapor layer when the substrate temperature is 400 °C. Thus, we conclude that the rise in Leidenfrost temperature for nanofluid droplets is governed by the concentration of dispersed nanoparticles, whose deposition generates a unique porous structure on the substrate and causes capillary wicking.