| The phenomenon of droplet self-jumping on superhydrophobic surfaces has attracted much attention because of its potential applications in condensation enhancement and self-cleaning.However,the droplet jumping efficiency,reliability and directionality on planar superhydrophobic surfaces are very low,which greatly limits their practical use.In this paper,we design and fabricate a class of honeycomb bionic superhydrophobic surfaces using 3D printing technology.We investigate the droplet jumping phenomenon and the mechanism of surface-droplet interaction on the honeycomb bionic superhydrophobic surface by combining experiments and numerical simulations.The main work and results of the paper are as follows:In this study,we investigated the growth and motion of condensed droplets on a honeycomb bionic superhydrophobic surface by conducting simple condensation experiments.We found that the honeycomb bionic superhydrophobic surface with a smaller top angle(?)can form uniformly sized droplets by condensation and position them by gravity.We also found that the prismatic cratered cells can regulate the droplet size and position better than the prismatic cone cratered cells,and that the hexagonal honeycomb bionic cells with hexagonal distribution have a better regulation effect than those with quadrilateral and triangular distribution.These results suggest that the honeycomb bionic superhydrophobic surface can control the size and position of condensed droplets well under the effect of gravity,which lays the foundation for large-scale efficient droplet jumping and has the potential to realize large-scale efficient droplet self-jumping.The experimental results of droplet fusion jumping on a flat superhydrophobic surface and a honeycomb bionic superhydrophobic surface were compared.It was found that the droplet self-jumping efficiency on the HF30 surface was more than45%,while it was only about 4% on the flat superhydrophobic surface.The mechanism of droplet jumping enhancement by the honeycomb bionic superhydrophobic surface and the effect of geometry on droplet jumping efficiency were analyzed from the energy perspective,based on the simulation results.It was concluded that the conversion efficiency of droplet surface energy to kinetic energy in a certain direction could be improved by simplifying the energy conversion process in the droplet merging process as much as possible,by promoting more direct conversion of surface energy to kinetic energy in that direction,and by inhibiting the conversion of kinetic energy in that direction to kinetic energy or surface energy in other directions as much as possible.It was shown by experimental results that the efficiency of droplet self-jumping on honeycomb bionic superhydrophobic surfaces increased significantly as ?decreased.For the same ?,the highest efficiency of droplet jumping was achieved by the hexagonal distribution of honeycomb bionic superhydrophobic surface and the lowest by the triangular one.The efficiency of droplet hopping on the prismatic cratered honeycomb bionic superhydrophobic surface was higher than that on the prismatic conical cratered surface when ? was small,and the opposite situation occurred as ? increased.The droplet size mismatch experiments of honeycomb bionic superhydrophobic surface also showed that a higher efficiency of droplet jumping could be achieved by the honeycomb bionic superhydrophobic surface with a smaller droplet radius ratio.In conclusion,both experimental results and numerical simulations showed that the droplet self-jumping efficiency was improved excellently by the honeycomb bionic superhydrophobic surface.Simulations and partial experiments were conducted to systematically investigate the effects of various asymmetries on the droplet jumping direction.It was found that on honeycomb bionic superhydrophobic surfaces,the symmetry of droplet hopping was broken by droplet size mismatch and the jumping droplets were caused to rotate,but the horizontal velocity achieved by this broken symmetry was extremely limited.Moreover,a rapid decrease in droplet jumping efficiency was caused by the decrease in radius ratio and it was therefore not suitable as a means to regulate droplet jumping direction on a large scale.Two typical XOZ cross-sectional asymmetric structures of honeycomb-like surfaces were investigated in this paper.One was that a smaller droplet jumping angle and a smaller horizontal direction velocity were achieved by changing the top angle ? of two neighboring honeycomb bionic cells where droplet fusion occurred.However,this structure had too little influence on the droplet jumping direction and was difficult to realize large-scale arrays,so it was not suitable as a means to achieve efficient and controlled jumping of large-scale droplets.Another was to adjust the side wall surfaces of the honeycomb bionic structure so that different inclinations were given to the two side walls of the cell,but this change had almost no effect on the direction of droplet jumping.The symmetry of the surface structure in the YOZ cross section was broken in this paper by changing the tilt angle of one side of the ridge structure between two adjacent honeycomb cells.It was shown by the results that the droplet jumping direction could be effectively controlled and a higher level of velocity could be achieved by changing the tilt angle of the inclined ridge.The structure could also maintain high droplet jumping efficiency and achieve efficient droplet jumping while changing the jumping direction.Moreover,the structure was easy to carry out large-scale arrays and was expected to be a means to realize large-scale and efficient controlled droplet jumping. |
