Spreading and Evaporation Dynamics of Nanofluid Droplets on Heated Hydrophobic and Soluble Solid Surfaces

The impingement and evaporation of the H₂O/n-Al nanofluid droplet with ammonium perchlorate (AP) particles is the fundamental process in the preparation of core-shell structure particles via a fluidized bed. In this work, the spreading and evaporation dynamics of the H₂O/n-Al nanofluid droplet impact on the heated aluminum (Al) and ammonium perchlorate (AP) surfaces have been investigated experimentally using high-speed photography. It has been demonstrated that the addition of a high concentration of n-Al particles improved the spreading diameter of droplets impacting solid surfaces, increased the pinning time, and decreased the evaporation rate. Shell formation and collapse phenomena were observed for nanofluid droplet evaporation on the Al surface and inhibited on the AP surface. The evaporation time of an impacting droplet is shorter at a higher Weber number because of the higher heat transfer area due to a larger contact area. The evaporation process of the nanofluid droplets was found to be more complex on AP surfaces due to the hygroscopic nature of AP, which enhances liquid immersion and affects the heat transfer characteristics. All the liquids were immersed in the ammonium perchlorate within 2 s, which is approximately 100 times shorter than on the Al surface at the same surface temperature and Weber number. Nonlinear droplet volume change during evaporation is caused by the simultaneous evaporation and immersion on the AP surface. The current findings provide new insights into the understanding of the formation process for novel core-shell structures such as AP@Al.