Dynamics of the apparent contact angle on porous and dense cylindrical surfaces

An experimental study of the dynamics of the apparent contact angle, motivated by various applications, including dropwise condensation, heat and mass transfer, and flow separation, is presented. The study investigates the dynamics of the apparent contact angle and wetting behavior of droplets on both dense and porous hydrophobic tubular surfaces. Test liquids include distilled water, ethyl alcohol, Glycerol, and 35 g/L NaCl solution. Diagnostics consisted of high-speed backlight imaging to track the droplet geometry and contact angle evolution over time. A theoretical model was developed to investigate the temporal effect on the apparent contact angle due to evaporation. The study identifies three distinct stages of droplet base dynamics: expanding, constant, and shrinking. On porous polypropylene membrane, the apparent contact angle initially decreased exponentially, indicating the dominance of absorption, followed by a linear decrease signifying evaporation. In contrast, dense surface exhibited a linear decrease in the apparent contact angle, dominated by pure evaporation with no absorption. Additionally, the influence of gravity on the apparent contact angle of the droplet was investigated for different Bond numbers. The results indicated that an increase in the Bond number led to a decrease in the apparent contact angle. A modified Shapiro theory incorporating droplet height was developed to predict equilibrium contact angles for different Bond numbers. The present study explored the dynamics of apparent contact angles on downward-facing tubular and flat surfaces, focusing on droplet stability before detachment. A critical Bond number, found to be 3.5, characterized the event of droplet detachment. Finally, the study examined the impact of centrifugal forces on droplet contact angles for different fiber geometry and hydrophobicity. It was observed that larger droplets required lower rotational speeds for ejection, with hydrophobic surfaces facilitating easier droplet detachment. The findings of this study highlight the potential for controlling droplet behavior in various industrial and scientific applications, using dynamic effects.