Penetration Behavior of a Water Droplet into a Cylindrical Hydrophobic Pore

Understanding the dynamics with which a water droplet penetrates a pore is important because of its relationship with transfer phenomena in plants and animals. Using a high-speed camera, we observe the penetration processes of a water droplet into a cylindrical pore on a silicone substrate. The force on the water droplet is generated by dropping the substrate plus water droplet from a height of several centimeters onto an acrylic resin substrate. The penetration characteristics depend on pore size Dp, height of release of a drop h, and the viscosity of the droplet liquid and are classified into the following patterns: spreading, penetration, and breaking. During the process of relaxation to the steady state, various interesting deformation or oscillation phenomena occur. Based on high-speed images, we estimate the interfacial energy ΔG during the intermediate states and find an energy barrier ΔG = 1 × 10(-7) J when Dp = 1.0 mm and h = 15 mm for the spreading pattern and ΔG = 0.7 × 10(-7) J when Dp = 1.0 mm and h = 10 mm for the penetration pattern. Finally, based on a theoretical model considering the driving and suppression factors, we explain the experimentally obtained phase diagram including the separation, penetration, and breaking patterns.