Local aggregation characteristics of a nanofluid droplet during evaporation

This study experimentally investigates non-uniform particle distributions and evaporation characteristics of nanofluid droplets containing 50 nm average diameter alumina (Al₂O₃) particles, on a hydrophilic glass surface. Using an inverted microscope, the size distribution of aggregated nanoparticles was visualized and analyzed at different sight-of-view locations. From the digital images captured using CMOS cameras and a magnifying lens, the effect of particle concentrations on droplet evaporation rates was examined. In particular, in order to understand the significance of the early stage of droplet evaporation, the dynamics of a corresponding triple line were visualized using a high-speed imaging technique. From the results, it was found that as the volume fraction of nanoparticles in nanofluids increased the total evaporation time and the initial contact angle decreased, while the corresponding perimeter of the droplet increased. Local aggregation was observed when a nanofluid droplet was in contact on the surface, suggesting that the non-homogeneous characteristics should be considered in estimating thermal conductivity of a nanofluid droplet.