Molecular Dynamics Study On Nanodroplet Impacting Solid Surface Under Electric Field

The impact of droplets on solid surfaces is the most common phenomenon in nature and industrial and agricultural production,such as rain falling,spraying pesticides,surface cleaning,3D printing,and other fields.Affected by gravity,inertial force,surface tension,and viscous force,the process of droplets impacting the solid surface is always accompanied by the exchange of energy and mass.Applying an electric field during droplet impact has important application prospects in industrial production.The electric field changes the type and quantity of force in the process of droplet impact,and the physical parameters of the impact process such as spreading radius and restitution coefficient are also varied.The research on the impact of nanodroplets on a solid surface under the electric field is still in its infancy and is a problem that needs to be solved urgently.Molecular dynamics methods can reproduce experimental processes at the nanoscale and are considered to be an ideal method to solve the time and space constraints in micro-nano systems.At present,molecular dynamics has become an effective method to study the phenomenon and mechanism of droplet impact on the microscopic scale.In this paper,molecular dynamics simulation methods are used to study the behavior and mechanism of micro-scale droplets impacting on a solid surface under the applying of an electric field,to broaden and provide theoretical guidance and technical supporting.The main work and results of this paper are as follows:The spreading behavior of water nanodroplets on the hydrophobic surface is affected by the electric field force.The electric field enhances the maximum spreading factor βmax,especially for strong electric fields greater than 0.1 V (?)-1.When the field strength remains constant,the direction of the electric field does not influence the spreading radius.When the aspect ratio of the nanodroplet before impact is less than 1,the maximum spreading radius remains constant.Since the electric field will change the surface tension of the liquid,thereby changing the capillary force,a new factor Φconsidering the influence of the dielectric constant and conductivity,the initial droplet shape and the field strength is proposed to modify the surface tension.An analytical model is derived to predict the maximum spreading factor that affects nanodroplets under the vertical electric field.The model shows good agreement with molecular dynamics simulations in the low Reynolds number range.Nanodroplet may bounce after impacting a superhydrophobic surface at a certain velocity,and the applied vertical electric field also makes this phenomenon obvious.Using electric field strength as a parameter,two bouncing regimes are determined:inertial force bouncing(IFR)regime(E<0.08 V (?)-1)and electric field force bouncing(EFFR)regime(E>0.08 V (?)-1).In the IFR bounce regime,the recovery coefficient εb is constant as no electric field is applied,and the contact time τc is shorter than when there is no electric field.In the EFFR regime,εb is proportional to the electric field strength,and τc decreases as the field strength increases.At the boundary of the two regimes(approximately 0.08 V (?)-1),both εb and τc increase sharply due to the deformation of the droplet shape.The droplet bounces in the shape of a sphere in the IFR regime,and bounces in a long stripe shape in the EFFR regime.A new bouncing criterion based on εb,Weber number,characteristic length,and new factor Φ is proposed,which determines the bouncing of nanodroplets under a vertical electric field.The criterion indicates that when a nanodroplet impacts the surface at a low velocity,the electric field may cause the droplet to bounce.In addition to the strength,the directions of the electric field also affect the impact of nanodroplets on the solid surface to a certain extent.The electric fields with inclined angles α=0°,30°,45°,60°,and 90° with the solid surface change the bouncing dynamics of nanodroplets,especially when the electric field strength is higher than 0.08 V (?)-1.When the direction of the electric field is not parallel to the surface and the intensity is greater than 0.08 V (?)-1,the restitution coefficient εb increases significantly.Applying an electric field of α=60° and>0.08 V (?)-1 will cause the maximum bouncing velocity of the nanodroplets.When E>0.08 V (?)-1,the contact time is elongated by the vertical electric field.When the impact velocity is not sufficient to make the droplet bounce,electric fields in all directions can promote the bounce of the droplet.Established regime maps of nanodroplet impact solid surface.Impact dynamics are affected by surface wettability,impact velocity,and electric field strength.Under different conditions,the outcomes of droplet impact are different,and the effect of the electric field also changes the outcomes of nanodroplet impact on the solid surface.Four new outcomes of nanodroplet impact have been discovered under a strong electric field,including stretch-spreading,stretch rebound,stretch splash and stretch breakup.A strong electric field increases the surface energy of the impacting nanodroplet,making it easier to spread,bounce,deform,and evaporate.The electric field does not influence the splash/breakup boundary,while the bouncing and deposition velocity boundary is reduced by the electric field.Regardless of the impact velocity,the nanodroplets are prone to bouncing under a strong electric field on the hydrophobic surface.