Heterogeneous substrates with moderate and extreme wettability contrasts were fabricated by comprising of superhydrophobic/hydrophilic and superhydrophobic/extremely hydrophilic surfaces, respectively. The interactions of water droplets impinging on the surfaces with sharp wettability contrasts were investigated experimentally. The impinging droplets that slightly touch the hydrophilic or extremely hydrophilic areas on each substrate exhibit a directional rebounding towards the more wetting surfaces, i.e., hydrophilic or extremely hydrophilic surface. The trajectory and landing distance of the rebounded droplets were tailored by controlling the releasing height of the droplet, wetting contrast across the border, and portion of the droplet touching the more wetting surface of the substrates with wettability contrasts. The landing distance of the droplet increases with the increased releasing height and higher wettability contrast across the border. Increasing the portion of the impinging droplet touching the more wetting surface of the heterogeneous substrates leads to the shorter landing distance of rebounded droplets.
Keywords: directional rebounding; droplet impingement; heterogeneous; hydrophilic; superhydrophobic; wettability contrast.