Surface Wettability Effects on Droplet Dynamics and Heat Transfer on Heated Stainless-Steel Foils

Abstract

We report the dynamics and heat transfer characteristics of water droplets impacting a thin stainless-steel foil maintained at different temperatures. The hydrophobic characteristics are imparted to the surface through polysiloxane coating, and water droplets impact the uncoated and coated heated surfaces at different velocities. High-speed videography is utilized to capture the dynamics of the droplet upon impact, while the temperature field of the substrate, during the phenomenon, is simultaneously recorded using high-speed infrared thermography. Heat transfer to the droplet over different surfaces is determined through energy balance on the foil using the captured thermographs. The results reveal that the spreading phase duration is independent of droplet impact velocity, irrespective of surface wettability, whereas surface wettability primarily affects the receding phase. The coated hydrophobic surfaces exhibited lower resistance to motion at the three-phase contact line, resulting in reduced spread ratios during the receding phase. It is noted that the majority of heat transfer occurred during the initial spreading and receding phases, driven primarily by forced convection. The maximum heat fluxes were observed along the three-phase contact line, particularly at the onset of the receding phase. The coated surface demonstrated lower overall heat transfer rates compared to non-coated surfaces, with the difference increasing at higher surface temperatures. Additionally, an increase in surface temperature to 75 °C enhanced the hydrophobicity of the coated surface, leading to prolonged receding phases and extended time to reach the sessile state.