Numerical Study On Manipulating The Dynamics Of Splitting And Rebound Of Droplets Impacting A Solid Surface

The dynamic wetting of droplets impacting a solid surface has the advantages of high transfer for the mass,momentum and heat,and hence has been widely applied by many industrial fields and high-tech industries,such as microfluidic,anti-icing,heat transfer enhancement and so on.However,the droplet impact is an extremely complex dynamic wetting process.In many applications,it is urgently need to control the droplet impact to realize the process enhancement and improve process efficiency.Therefore,the research on the dynamics of droplet impacting a solid surface has become a very active and emerging field.With the development of new materials and micro-and nano-fabrication technologies,various intelligent heterogeneous surface,such as hybrid-wettability surface and macro-structure surface,are fabricated to manipulate spread,retraction,rebound,movement,coalescence and split of the impacting droplet.In addition,in the dynamic process of multiple-droplet impact,control the interaction among droplets can also effectively manipulate the above dynamic characteristics.However,the physical mechanisms on manipulating the split and rebound of the impacting droplet have not been revealed,the dependent relationship between droplet split and hybrid-wettability surface is always not clear,and the influence mechanism of droplet coalescence on the multiple-droplet rebound has not been explored.Hence,this paper will focus on the split and rebound of droplet impacting a heterogeneous solid surface.In this work,a three-dimensional numerical model is developed to investigate the splitting behaviors of droplets when they impact a hydrophobic strip decorated on a hydrophilic background surface.The volume of fluid(VOF)method incorporated with mesh-refinement technique and dynamic contact angle model is employed to capture the shape evolution of droplets accurately.The split behaviors of impact droplets are analyzed for various strip widths,strip hydrophobicities,and surface hydrophilicities at three typical Weber numbers.On the basis of the simulated results,complete impact phase diagrams were constructed.Moreover,the dependent relationships between the droplet split behaviors and the hybrid-wettability surface is cleared.The non-axisymmetric spreading and retraction dynamics as well as the reduced viscous dissipation are responsible for the droplet splitting.A simplified theoretical model is developed to predict the splitting or no splitting behavior on the basis of energy conservation principle,which agrees with the present simulations well.When a droplet impacts a single hydrophobic strip,four split regimes are discovered:non-split,single-liquid-bridge forming and rupture,double-liquid-bridge forming and rupture,quadruple-liquid-bridge forming and rupture.When a droplet impacts a cross hydrophobic strip,experiences various split pathways.The split pathway determines four split regimes:non-split ?,split ?,non-split ?,split ?.Finally,some new methods had been proposed to further control volume ratios of split satellite droplets and splitting time.In this work,the rebound dynamics of two equally sized droplets impacting a superhydrophobic surface is investigated via a three-dimensional lattice Boltzmann method(LBM)simulations.To improve the stability of numerical simulations,a multiple-relaxation-time(MRT)collision operator is utilized.Two key parameters including the Weber number and the droplet distance are examined to explore the rebound dynamics of two equally sized droplets simultaneously impacting a superhydrophobic surface.On the basis of the simulation results,rebound phase diagram is constructed.The results showed that there are three rebound regimes:a complete-coalescence-rebound(CCR)regime,a partial-coalescence-rebound(PCR)regime,and a no-coalescence-rebound(NCR)regime.All the rebound regimes are closely associated with dynamic behaviors of the formed liquid ridge or bridge between the two droplets.Intriguingly,compared with CCR regime and NCR regime,the contact time is still dramatically shortened in the PCR regime,which is even smaller than that of single-droplet impact.The shortest contact time in the PCR regime does not arise from the reduced viscous dissipation,but is attributed to the morphology of the rebounding droplet.If the superhydrophobicity of the surface is reduced,the complete-coalescence-rebound two droplets will not even rebound but directly adhere to the surface.When two droplets impact a superhydrophobic surface non-simultaneously,the same three rebound regimes are observed,but the contact time increases.Furthermore,on the basis of the split behavior of impact droplet,this paper studies the splitting rebound behaviors of two droplets simultaneously impacting macro ridges decorated on a superhydrophobic surface.The emphasis is placed on revealing how the Weber number and the feature size of the ridge(ridge height and ridge width)influence the splitting rebound behavior.On the basis of the simulation results,the splitting rebound phase diagram is constructed.The results show that there are four splitting rebound regimes:non-splitting rebound I,non-splitting rebound ?,splitting rebound ?,splitting rebound ?.At high Weber number,two droplets take place splitting rebound regime,where the macro ridges are most effective to reduce the contact time.and the splitting rebound ? is more beneficial for the contact time reduction.Both the ridge height and the ridge width have the critical values,which causes the occurrence of the splitting rebound regime,and thus effectively reduce the contact time.This paper focus on manipulating the split and rebound behaviors of droplets impacting a solid surface,reveals the physical mechanisms of single-droplet split and multiple-droplet rebound,develops the method of manipulating single-droplet split and multiple-droplet rebound.The findings of this paper will not only drive significant innovation for the existing technologies,but also promote the emergence of more new high-technologies in the future.