The interaction between water droplets and superhydrophobic holes: Detachment and penetration

Water droplets of varying volumes exhibit distinct behaviors at the tip of a needle under the influence of gravitational force: larger droplets fall from the needle tip, while smaller droplets adhere to the needle tip. By integrating a superhydrophobic hole, which allows the needle to pass through but hinders droplets' passage, a method to allow smaller droplets to fall from the needle was evaluated. The interaction between water droplets and a superhydrophobic hole was studied experimentally, and two phenomena were observed: either the droplet detached from the needle tip or the needle pulled it through the superhydrophobic hole. A critical detachment volume V cri d 0 , D , dependent on the needle diameter d 0 and the superhydrophobic hole diameter D , determines droplet behavior. When the volume of the droplet V droplet > V cri d 0 , D , the droplet detaches from the needle tip. When the V droplet < V cri d 0 , D , the droplet penetrates through the superhydrophobic hole. The results show that the critical detachment volume V cri d 0 , D increases with an increase in d 0 for a given superhydrophobic hole and with an increase in D for a given needle. To enable a 0.25 μl droplet to fall from a 32G needle, a superhydrophobic hole of 0.48 mm diameter was employed. Furthermore, a mechanical model based on force equilibrium was developed to describe the interaction between water droplets and a superhydrophobic hole.