Effects of protuberant structure on coalescence-induced jumping of droplets on superhydrophobic surfaces

Abstract

The coalescence-induced jumping of droplets on superhydrophobic surfaces is useful in engineering-related applications to enhance condensation-based heat transfer, self-cleaning, and anti-icing and, thus, has attracted extensive attention in research. Some researchers have claimed that superhydrophobic surfaces with protuberant structures can yield droplets with a higher jumping velocity. While the structure of the surface influences droplet dynamics, the concomitant energy transition also needs to be considered. The effects of the geometry on the mechanism of jumping and the energy transition need to be investigated. In this paper, an improved volume-of-fluid method is verified based on experiments and then applied to simulate the jumping behaviors of droplets on superhydrophobic surfaces with cuboid protuberant structures. The effects of repulsion caused by the contributions of the surface tension and the superhydrophobicity of the protuberance are crucial to enhancing the jumping of the droplets. The forces due to them provide a thrust oriented in the direction of jumping of the droplets to increase the value of the positive energy term, while reducing the area and duration of contact between the droplet and the substrate to reduce the negative dissipation term and enhance the efficiency of energy conversion. Surprisingly, an excessively tall structure leads to a sustainable increase in the velocity of jumping of the droplets under the effects of repulsion and the Laplace pressure after piercing the liquid bridge. The work here provides guidance to optimally combine a superhydrophobic substrate with special structures to enhance the jumping of droplets.