A Lattice Boltzmann Simulation Of Dynamic Behavior Of Droplet On Superhydrophobic Surface With Randomly Distributed Rough Structures

Due to its excellent water repellency,superhydrophobic surface(SHS)has attracted wide attention in energy saving,environmental protection and other fields.The special micro-nano structure of SHS contributes to many kinds of functional properties,including self-cleaning,drag reduction and anti-icing.Understanding the effect of surface morphology on the flow and heat transfer of droplet on SHS is of great significance.It will be helpful to optimize the preparation of SHS and develop energy applications related to SHS.Currently,the dynamic behaviors of droplet on smooth and regularly textured surfaces have been well understood.However,the superhydrophobic surface with randomly distributed rough structure,which is common in nature and engineering,is rarely investigated,especially its micro mechanism.Therefore,in this study,the dynamic behavior of droplet on the SHS with randomly distributed rough structure is investigated by a mesoscale lattice Boltzmann method.It includes droplet collision and rebounding,coalescence-induced droplet jumping and droplet evaporation.Meanwhile,the influence of rough surface structure on droplet dynamics is summarized.The study will provide theoretical guidance for the preparation and optimization of SHS.Firstly,the Fast Fourier Transformation is used to generate randomly di stributed rough surfaces with rough parameters obtained from real surfaces,including the skewness(Sk),kurtosis(K)and the standard deviation(Rq).Those parameters character the rough surface from the amplitude asymmetry,the peakedness and the discrete degree of surface,respectively.Compared to the experimental measurement,the rough parameters are controllable and rough surfaces with arbitrary size and roughness can be generated.Furthermore,the influence of rough parameters on the SHS can be studied independently,which makes a foundation for the study of the SHS with randomly distributed rough surfaces.Secondly,a mesoscale LB simulation method are used to investigate droplets impacting on randomly structured surfaces.The proposed LBM simulations reappear the details of droplet dynamic behavior after impacting the randomly distributed rough surfaces,mainly including spreading,retracting and bouncing.The effects of roughness parameters,surface wettability and impinging velocities on the droplet bouncing abilities are studied.It is found that the surfaces with smaller skewness and a kurtosis around 3.0 show the best bouncing ability.Surfaces with larger CA are beneficial for reducing the impacting velocity for droplet bouncing and shortening the contact time.Compared to the well-textured surface,it is found that the bouncing ability of rough surfaces can be enhanced by decreasing the number of sharp edges and narrowing the spacing between grooves.Thirdly,coalescence-induced droplet jumping on SHS with randomly distributed structures is theoretically analyzed and numerically investigated.The effects of roughness parameters and surface wettability on the abilities to jump of coalesced droplets are completely studied.It shows that based on the topography of rough surfaces,coalesced droplet can exist in three states namely Wenzel state,composite state and Cassie state.The rough surfaces with smaller Sk,larger Rq and K at 3.0 are beneficial to coalescence-induced droplet jumping.Rough structures with larger CA can increase the jumping velocities and shorten the contact time of coalesced droplet.The states of coalesced droplet are determined by a combination of rough structures and hydrophobicity.Fourthly,pinning-depinning mechanism of the contact line during evaporation of micro-droplets on rough surfaces is investigated by LBM.It is found that the mode of droplet evaporation is related to the local structure in the vicinity of the contact line,such a s the height and the slope of the groove.When a droplet evaporates on rough surfaces,the pinning of the contact line at the sharp edge is a slow motion,which is achieved by the self-adjustment of the contact line according to the sharp edges.Even though the modes of droplet evaporation on randomly distributed rough surfaces are random,droplets are likely to evaporate under a specific mode on a properly designed surface.At last,the origin of pinning-depinning of contact line is analysed based on the local force balance.Above work discloses the effect of rough structures on droplet dynamic behavior on the SHS.This basic data is the tools for the preparation and optimization of SHS.