Experimental And Numerical Investigations Of Droplets Jumping On Superhydrophobic Surfaces

The superhydrophobic surface has great potential in the fields of anti-wetting,antiicing,self-cleaning,heat transfer enhancement and so on.Promoting the rapid jumping of droplets on superhydrophobic surfaces is important for realizing the optimal design of superhydrophobic surfaces.In this study,the dynamics of droplets jumping on textured superhydrophobic surfaces is systematically studied via high-speed photography,numerical simulation,and theoretical analysis.The main research contents are as follows:The dynamic characteristics of droplets impact on textured superhydrophobic surfaces are investigated by high-speed photography.The results show that,when the textured surface is evenly superhydrophobic and the spacing between the pillars is small,pancake bouncing occurs at the maximum spreading under which droplets impinge the bottom surfaces.Further theoretical analysis shows that the maximum spreading time and liquid retracting time do not change with the Weber number(We).When the two time scales are similar,pancake bouncing occurs,which is in good agreement with the experimental results.Based on the above results,numerical simulations based on the lattice Boltzmann method is carried out.Four possible types of bouncing are identified in the simulations with varied impact angles,e.g.,conventional retracting bouncing,whole bouncing,impaled retracting bouncing,and tumbling bouncing.The impact obliqueness can widen the range of We number for whole bouncing at low Ohnesorge numbers(Oh)and therefore reduce the contact time due to the asymmetry spreading and retracting.Under high Oh numbers,the droplet is still able to quickly bounce off the surface after the oblique impact due to occurrence of tumbling bouncing.Considering the great potential of self-propelled jumping induced by the released surface energy during droplet coalescence on superhydrophobic surfaces,the effect of surface properties on the jumping velocity is investigated using numerical simulation,and the critical conditions of jumping and the optimal condition of maximum jumping velocity are theoretically derived.The results show that,the nonmonotonic emergence of “non-jumping”-“jumping”-“non-jumping” with decreasing the solid fraction is synergistically controlled by the surface adhesion and the effective impinging pressure.When the solid fraction is large,the transition from “non-jumping”to “jumping” correlates with a dimensionless surface adhesion energy.When the solid fraction is small,the transition from “jumping” to “non-jumping” is dominated by the dimensionless effective impinging pressure.Moreover,the jumping velocity maximizes when the wall wetting critically transits from Cassie-Baxter state to partial-wetting state,and a penetration index is proposed from the wetting theory to predict such critical condition,which shows good agreement with both the present simulations and previous experiments.Recognizing the key of self-propelled jumping is the interaction between the liquid bridge and the wall,the effect of surface structure is further considered as well as the influence of the ladder-like structures on the jumping of droplets on the superhydrophobic surfaces.The results show that,the impinging velocity of liquid bridge is increased by the ladder-like structure and the retraction of droplet bottom areas is accelerated by the sharp edge.The normalized jumping velocity can be increased from 0.28 to 0.54 by adding the ladder-like structures on the superhydrophobic surfaces.