Numerical Study To The Spontaneous Jumping Of Droplets On Wettable Surface

Super-hydrophobic materials are widely used in HVAC(Heating,Ventilation and Air Conditioning),aviation industry,metal protection,textile industry,hydraulic engineering and other industries,which have an important influence on the development of products in various related fields.Studying the self-bounce phenomenon of droplets on wettable surfaces has a very prominent role in the fields of condensate droplet self-cleaning,enhanced condensation heat transfer,and anti-icing.The droplet bounce phenomenon on the wettable surface occurs in a very small size range about 20?m?200?m,and it is difficult to accurately control the physical parameters such as droplet radius,viscosity and surface tension in experimental studies,so we use numerical simulation method to study the spontaneous jumping phenomenon of droplets on the wettable surface.Most of the papers study the phenomenon of jumping induced by merged droplets on super-hydrophobic surface.We not only study the effect of different physical parameters on the jumping of merged droplets,but also study the shedding phenomenon of droplets on different wettability surfaces.Using the spherical cap model can better restore the morphology of droplets on different wettability surfaces,and introducing the concept of wettability gradient will further accelerate the drop off of droplets from hydrophobic surface.In this work,coalescence-induced jumping phenomena of droplets on superhydrophobic surfaces was investigated theoretically and numerically,and it was found that contact angle,droplet radius,radius ratio,Bond number,Ohnesorge number and surface tension did affect the process of coalescence and the spontaneous jumping.Our simulations showed the contact angles had positive effect on the spontaneous jumping,which can also increase the range of jumping of non-equal diameter droplets.The coalescence-induced jumping on hydrophobic surface occurred in a certain size range,and the droplet radius was so small that the droplet can't jump off the surface of the material.The role of gravity in the droplet merging process is so small that the merged droplets can jump at the similar speed,but the larger Bo number,the greater the gravitational acceleration and the smaller the maximum bounce height.The larger the Oh number,the greater the wall resistance of the droplet,and the lower the jumping speed of merged droplets,which in turn leads to a smaller bounce height.With radius ratio decreasing,the contact time increased first and then decreased.This paper presented the pseudo-potential Lattice Boltzmann method multiphase flow model,and it was proved by using Power-law,Young-Laplace equation and experiment data.Based on the study of the drop phenomenon of droplet polymerization on homogeneous surface,we discussed the fluence of contact angle,droplet radius and radius ratio on wettability gradient surface.Our study showed that the contact angle had positive effect on the spontaneous jumping.The larger Ohnesorge number,the smaller shedding time;The smaller Bond number,the larger shedding time;With decreasing Bond number drastically,the droplet can't jump off the surface of the material.Our simulations revealed that the critical shedding radius of droplets is 60 on wettability gradient surface.With a wettability gradient of 4°,the droplet undergone two acceleration processes and two deceleration processes;With wettability gradient more than 4°,the droplet only undergone one acceleration processes and deceleration processes.The surface adhesion work and surface energy showed a trend of decreasing with the increase of the radius ratio,while the gravitational potential energy and viscous dissipation just experienced a small fluctuation.Our simulations also revealed that the larger radius ratio,the larger differential pressure inside the droplet,the larger droplet merge speed,and the samller the shedding time.