Droplet impact outcomes of emulsions on smooth and microstructured surfaces

This study explores the impact dynamics of emulsion droplets, with a dispersed phase of either silicone oil, toluene, or heptane, and water as the continuous phase, on both smooth and microstructured surfaces fabricated via photolithography. By preparing emulsions without surfactants, we isolated the effects of surface morphology and liquid properties on droplet behavior. We characterized the rheology of the emulsions and their droplet size distributions. The impact dynamics were recorded using a high-speed camera in a shadowgraph configuration, with analysis performed through image processing techniques. Our results indicate that at higher impact velocities, water exhibits the largest spreading diameter (dmax) on smooth surfaces, while emulsions with higher dispersed phase concentrations show reduced spreading due to increased energy dissipation. On microstructured surfaces, denser structures enhance resistance to spreading and trigger complex phenomena such as Worthington jets and secondary droplets, which are not observed on smooth surfaces. Additionally, we observed a transition in bouncing behavior for the silicone oil 50 cSt 20 v/v% emulsion on Glaco-coated surfaces, attributed to the infiltration of silicone oil into the Glaco microstructure, creating a suction force that prevents bouncing. These findings offer valuable insights for optimizing industrial processes like inkjet printing and pesticide application, where precise control of droplet behavior is crucial.