Droplet impact characteristics on sinusoidal wavy micro-structured hydrophobic surfaces

The dynamics of an impacting droplet can be controlled by microstructuring the surface. This work presents a numerical investigation of droplet impact characteristics on a hydrophobic surface microstructured with sinusoidal wavy patterns. The effects of amplitude and wavelength of the wavy surfaces on droplet deformation are analyzed for different impact velocities. A dynamic contact angle model is incorporated into the numerical method to track the three-phase contact line accurately. The influence of the Weber number on the wettability transition is analyzed, and the flow characteristics inside the droplet at different flow regimes are explained. A regime map is prepared to show the transition between different regimes for different surface attributes and Weber numbers. The initial contact of the impacting droplet with the surface is influenced by the amplitude and wavelength of the roughness element, leading to different wettability states. The Wenzel (wetting), Cassie (non-wetting), and mixed wetting states affect the droplet's spreading, recoiling, and rebound characteristics. The Cassie state of a plane surface is transformed into the Wenzel state due to microstructuring the surface with small amplitude and wavelength. A further increase in amplitude and wavelength leads to a transition from the Wenzel state to a mixed state, and finally from the mixed state to the Cassie state. The amplitude-controlled mixed state results in partial rebound of the droplet, whereas the wavelength-controlled state results in partial rebound and rebound with droplet breakup. The study may aid in designing droplet retention surfaces required for practical applications.