Experimental Investigation of Droplet Impact Dynamics on Solid Surfaces

Book excerpt: A study of the normal impact of liquid droplets on a dry horizontal substrate is presented in this thesis. The impact dynamics, spreading and recoil behavior are captured using a high-speed digital video camera at 2000 frames per second. A digital image processing software was used to determine the drop spread and height of the liquid on the surface from each frame. To ascertain the effects of liquid viscosity and surface tension, experiments were conducted with four pure liquids (water, ethanol, propylene glycol and glycerin) that have vastly different fluid properties. Three different Weber numbers (20, 40, and 80) were considered by altering the height from which the drop is released. To understand the influence of drop size, experiments were performed in which the drop size was varied for the same fluid. Also, the effect of substrate material was studied by comparing the impact on two different substrates: glass (hydrophilic) and PTFE or Teflon (hydrophobic). The high-speed photographs of impact, spreading and recoil are shown and the temporal variations of dimensionless drop spread and height are provided in the paper. Experiments were performed to study the influence of addition of surface active agents or surfactants in aqueous solution on the droplet impact phenomenon. Three surfactants were used with varying diffusion rates: SDS (anionic), CTAB (cationic) and Triton x 100 (non ionic). The spreading and recoil of the drops of surfactant solutions is studied at concentrations of half the critical micelle concentration (CMC) and twice CMC. To underscore the dynamic effects, comparative experiments for the three surfactants were performed so that all the solutions had the same value of equilibrium surface tension. The role played by impact velocity in the collision of surfactant laden drops was studied by comparing the results for two different impact velocities. The influence of surfactant concentration was studied by performing experiments varying the surfactant concentrations. The results show that changes in liquid viscosity, surface tension, and surfactant concentration significantly affect the spreading and recoil behavior. In the case of pure liquids, for a fixed Weber number, lower surface tension promotes greater spreading and higher viscosity dampens spreading and recoil. Using a simple scale analysis of energy balance, it was found that the maximum spread factor varies as Re 1/5when liquid viscosity is high and viscous effects govern the spreading behavior. The drop size had no binding when the Weber number was maintained constant and the Reynolds numbers were comparable. The nature of the substrate plays a very important role. Impact on hydrophobic substrates can result in dramatic recoils and rebound. For aqueous solutions with surface active agents, it was observed that higher diffusion rate surfactants result in higher spreading factors and weaker oscillations. The spreading and recoil behavior can be correlated to the dynamic surface tension response of the surfactant solutions. With increase in impact velocity, the gain in spreading factor over a pure water drop decreases. Also, lowering of the surfactant concentration results in lower spreading factor and stronger recoil - a behavior closer to that of pure water.