The Dynamic Behaviors And Contact Time Of Droplets Impacting Superhydrophobic Surfaces

Impacting of droplets on superhydrophobic surfaces is ubiquitous in many practical applications involving anti-icing,self-cleaning,anti-corrosion,and so forth.Impacting of droplets on solid surface is always accompanied by momentum,energy and mass transfer,which are strongly depended on the contact time between droplet and surface.Therefore,it is of great significance to reveal the mechanism of impacting process,so as to reduce the contact time.The main factors which affect the contact time includes the physical properties of the droplet(density,surface tension,and viscosity,etc),the impact velocity,and the physical as well chemical characteristics of surfaces(such as the wettability,roughness,microstructures,and angle of inclination of surface,etc).For impacting of multiple droplets,the contact time is also relevant to the interaction between the droplets.Recently,the method by modifying the surface characteristics has been proposed aiming to reduce the contact time.However,significant efforts are still necessary to develop an effective and reliable method.In addition,the motheds are proposed based on single droplet.Whereas,for impacting of multiple droplets,the influence of the interaction between droplets on the contact time is unrevealed and become an urgent factor to be analyzed.Because of several distinct advantages such as the simple processing of boundary conditions,the automatic tracking with two-phase interface,and high efficiency of parallel computing,lattice Boltzmann method has been widely applied to investigate the impacting of droplets.In this article,a pseudopotential-based lattice Boltzmann method using multiple-relaxation-time(MRT)collision operator is utilized to realize a large density ratio around the interface,which makes the simulation results more closed to real fluid.The impact dynamics of droplets on superhydrophobic surfaces and the contact time are investigated through four sections:(1)impacting of singe droplet on surface decorated by a macro-rectangular ridge;(2)impacting of single droplet on smooth inclined surface decorated with parallel macro-ridges;(3)impacting of two droplets on inclined surface;(4)impacting of two droplets on surface decorated with three different macrotextures.The purpose of this article is to reveal the influence mechanism of characteristics of solid surface and the interaction between droplets on the contact time for impacting of droplets on the superhydrophobic surface.Subsequently,potential methods are proposed to reduce the contact time,which provides a theoretical basis for the development of anti-icing and other technologies.The results show that the presence of macro-rectangular ridge can induce an asymmetric spreading and retraction dynamics of droplets.Once the droplet is split into two fragments by the ridge,the contact time will be reduced significantly.Through analyzing the dynamic behavior of droplets impacting on macro-rectangular ridge,three different retraction ways,which refer to retracting only once before rebounding,retracting twice before rebounding,and re-coalesce after retraction once,are observed for the split fragments depending on the impact Weber number as well as the height and width of ridge.The retraction time is affected by the retraction way,so as to change the contact time.A minimum contact time is reached by the way that the split fragments retract only once before rebounding,which is 52%of that for droplets impacting flat surface.When a droplet impacting on an inclined superhydrophobic surface decorated with parallel macroridges,depending on the normal velocity,center-to-center spacing between ridges,ridge height,and inclination angle of the surface,there are two types of impacting dynamics including splitting or non-splitting of the impacting droplet.Increasing the impact velocity,center-to-center spacing,ridge height,and angle of inclination of surface,the impacting droplet is more likely to be split.In non-splitting regime,the asymmetric spreading and retraction plays a critical role for the contact time.A stronger asymmetry is benificial to reduce the contact time.The asymmetry is influenced by the geometrical parameters of macro-ridges,inclination angle of the surface,and the impact normal Weber number.In the splitting regime,Once the fragments touch the ridges,they adhere to the ridges and slide along the ridges,which means that the adhesion effect becomes the key point to alter the contact timeOn an inclined surface,the contact time in the splitting regime is higher than that in the non-splitting regime.When two droplets(the leading droplet and the trailing droplet)impact an inclined surface successively,increasing the velocity of trailing droplet leads to a stronger interaction between the two droplets.If the velocity of trailing droplet exceeds a threshold,the coalesced droplet will retouch the surface,resulting in a longer contact time.It is found that the coalesced droplet may not rebound from the surface when the retouching occurs on a hydrophobic surface.A longer length between impacting points is beneficial for coalesced droplet rebounding,obtaining a lower contact time.To reduce the contact time of two droplets simultaneously impacting superhydrophobic surfaces,the influences of surface decorated bathtub-like groove,vertical wall,and rectangular ridge on the contact time are investigated.Because of the effect of macrotextures,a strong asymmetry is exhibited in spreading and retracting processes.The coalesced droplet detaches from the surface in a wavy rebound regime,and the contact time is basically lower than that of flat surface.The geometrical parameters of the macrotextures as well as the center-to-center distance of the two droplets are the key parameters for influencing the contact time.The contact time of droplets impacting bathtub-like grooves is minimum,and can be decreased up to 41%comparing with flat surface,while the rebounding direction could be adjusted when the surface is decorated with vertical wall and rectangular ridge.