On Computational Modeling of Dynamic Drop-surface Interactions During Post-impact Spreading of Water and Aqueous Surfactant Solution

Book excerpt: The objective of the present work is to develop a computational model to simulate liquid droplet impact and post-impact spread-recoil dynamics on a horizontal, flat, smooth, dry surface. The governing equations of continuity and momentum are solved to simulate the transient flow. Volume of Fluid (VOF) method is applied to capture continuously deforming gas-liquid interface. Simulations are carried out for pure water and aqueous solution of Sodium Dodecyl Sulphate (SDS) droplet impact on a hydrophobic (Teflon) surface. Simulations are carried out for Weber number 20 and 80 with impact velocities of 0.7 m/s and 1.4 m/s and droplet diameters of 2 mm and 3 mm, respectively. The results show increase in maximum spreading factor with increase in Weber number. Computational results predict advancing, recoiling and bouncing behavior of water droplets on the Teflon surface which agrees with the experimental observations available in literature. Aqueous solution of Sodium Dodecyl Sulphate (SDS) at twice the Critical Micelle Concentration (2xCMC) is used and its time and space dependent surface tension behavior is modeled. It is observed that dynamic surface tension plays a primary role in the modification of droplet spread-recoil process. The simulation results for surfactant solution show larger drop spread followed by weaker recoil and no rebound from the surface. These results are validated with experimental measurements reported in the literature. Non-isothermal impact conditions are investigated to study heat transfer phenomenon during droplet impact. The solid surface is maintained at 353°K and corresponding heat flux values and overall heat transfer rates for both pure water and surfactant solution drops are calculated. The results indicate that aqueous surfactant solution improves the liquid-solid wetting area and results in higher heat transfer compared to pure water droplet.