Dynamics of Condensing Droplets Driven by Multidirectional Laplace Pressure Gradients on Hierarchical Microstructured Surfaces

The rapid removal of condensate droplets is important for achieving stable dropwise condensation, thus improving the thermal efficiency of condensing equipment. The microstructure on the condensing surface can facilitate the departure of droplets. However, most existing microstructures are arrays of a single structure, which can only generate a single directional Laplace pressure gradient. To further improve the droplet-repelling performance of the microstructure, we propose a hierarchical superhydrophobic surface composed of opposed wedge bumps and diverging microgrooves in between, which is capable of generating multidirectional Laplace pressure gradients to trigger the self-jumping and collision-induced jumping behaviors to improve the jumping ability of condensate droplets. Through the application of a three-dimensional multiphase simulation model to condensate droplets on the microstructure, the mechanism of droplet's spontaneous movements is clarified and the optimal surface for departing droplets is determined. Through laser direct writing, chemical etching, and self-assembled monolayers, the optimal microstructure is fabricated on a copper plate. Wet air condensation visualization experiments have shown that the hierarchical superhydrophobic surface is able to realize the rapid removal of large droplets under the multidirectional Laplace pressure gradients. The droplet number density of the hierarchical superhydrophobic surface can reach 9.67 × 108 m-2, which is 106% higher than that of the plain superhydrophobic surface. The surface coverage of the droplet is reduced by 15% compared to the plain superhydrophobic surface, showing excellent potential for enhanced droplet removal.