Spreading dynamics associated with transient solidification of a paraffin droplet impacting a solid surface

The solidification of a droplet after impacting a solid substrate initiates immediately at the bottom surface owing to rapid heat transfer. Transient splat solidification greatly affects the spreading dynamics, such as flattening and recoil. This study experimentally and numerically studied the spreading and transient solidification of a paraffin droplet impacting transparent solid substrates. The impact phenomena, including flattening, recoil, final splat geometry, and solidification at the bottom surface, were observed using a high-speed camera from the side and bottom. The effects of the substrate material, substrate temperatures, and droplet temperatures were thoroughly discussed. The results suggest that there are two patterns of splat solidification, depending on the impact conditions. The impacted droplet (with a temperature of 123 degrees C) started to solidify at the periphery and gradually progressed toward the splat center on low-thermal-conductivity polycarbonate substrates. For lower initial droplet temperatures or substrates with higher thermal conductivity, solidification occurred immediately after the impact across the entire bottom surface of the splat owing to the faster cooling. Numerical simulations were performed to reproduce the impact phenomena and good agreement was obtained for the spreading factor, recoil height, final splat geometry, and solidification at the bottom surface of the splats. An in-depth comparison between the experimental and simulation results revealed that the transient solidification immediately after impact significantly affected the spreading of the impacted droplets, especially when the substrate had a high thermal conductivity. The results demonstrate that the proposed experimental and numerical methods provide accurate evaluation and prediction of droplet impact, concerning transient solidification and its influence on the spreading dynamics of the droplets.