Dynamics of droplets impacting hydrophilic surfaces decorated with a hydrophobic strip

Abstract

Mixed-wettability surfaces have been extensively adopted to manipulate impact dynamics of droplets. In this work, a three-dimensional numerical model is developed to investigate the splitting behaviors of droplets when they impact a hydrophobic strip decorated on a hydrophilic background surface. The volume of fluid (VOF) method incorporated with mesh-refinement technique and dynamic contact angle model is employed to capture the shape evolution of droplets accurately. Dynamic behaviors of impact droplets are analyzed for various strip widths, strip hydrophobicities, and surface hydrophilicities at three typical Weber numbers. The results show that the droplet can be successfully split into two symmetric small droplets for all the three Weber numbers, so long as the strip width and wettability contrast between the strip and background surface are properly designed. The non-axisymmetric spreading and retraction of the film triggered by the strip lead to a shorter spreading and a faster retraction on the hydrophobic strip than the hydrophilic surface, which causes the formation of a liquid bridge on the strip and reduces the viscous dissipation on the strip. Thus, the non-axisymmetric spreading and retraction dynamics as well as the reduced viscous dissipation are responsible for the droplet splitting. A simplified theoretical model is developed to predict the splitting or no splitting behavior on the basis of energy conservation principle, which agrees with the present simulations well. Finally, the simulated results are assembled in phase diagrams to obtain the criterion for the droplet splitting from the critical strip width and the critical wettability contrast between the strip and background surface.