Molecular Dynamics Investigation On Dynamics And Mechanism Of Droplet Movement On Superhydrophobic Surface

Condensation and icing are common phenomena in nature.They are also widely used in industrial fields such as energy and power,cryogenic and refrigeration,electronic technology,and aerospace.As an efficient heat exchange method,condensation is expected to improve its condensation efficiency and has been extensively studied.Many detriments in industry caused by icing make anti-icing receive much attention.The phenomenon of coalescence induced droplet jumping promotes the rapid removal of droplets from the surface with a smaller size,which greatly improves the efficiency of condensation heat transfer.In recent years,it has attracted more and more attention.In addition,the rebound behavior of droplets after impacting the superhydrobic surface has also become a reliable method in anti-icing measures,which has attracted widespread attention.On the other hand,with the rapid development of nanotechnology,the droplet size decreased,and the energy conversion mechanism for nanoscale droplets differ from those for macroscale droplets significantly.As a result,theories and methods constructed based on macroscale droplets are found to be inapplicable to nanoscale droplets.Molecular dynamics methods have become effective means to explore nanoscale phenomena and mechanisms.Therefore,the motion behavior and mechanism of condensed droplets on superhydrophobic surfaces at the micro-scale is studied by using molecular dynamics simulation methods,aiming to deeply understand the self-jumping and impact behaviors of droplets on superhydrophobic surfaces and further reveal its physical mechanism at the nanoscale.On this basis,in order to enhance the efficiency of condensation heat transfer,it seeks to improve the coalescence induced self-jumping velocity.To provide theoretical support for the realization of anti-icing and other engineering problems,it needs to grasp the law of the contact time of droplets impacting the super-hydrophobic wall.Coalescence-induced jumping phenomena of two unequal sized droplets on superhydrophobic surfaces are investigated theoretically and numerically.Firstly,by introducing modified inertial-capillary velocity(uci*)and Ohnesorge number(Oh*)with consideration of radius ratio(r*)of two coalescing droplets,we proposed a generalized inertial-capillary scaling law for the jumping velocity of coalesced droplets,which is expected applicable for both two identical droplets and two unequal sized droplets coalescing on superhydrophobic surfaces.Subsequently,we employed molecular dynamics simulations to investigate the coalescence-induced jumping process of two unequal sized nanodroplets.Our simulations showed that the dimensionless jumping velocity(vj/uic*)well follows the generalized inertial-capillary scaling law with vj/uic*?0.127 in a specific Oh*range;however,it rapidly reduces and finally vanishes when the radius ratio of large droplet to small droplet larger than a certain threshold value.Our simulations also revealed that non-jumping for two unequal sized droplets with a very large radius ratio is due to that the larger droplet swallows the small one,so that the liquid bridge has no chance to impact the solid surface and hence the "liquid bridge impacting substrate"mechanism fails in this circumstance.The specific superhydrophobic surfaces by patterning a single groove,ridge,or more hydrophobic strip,whose size is comparable with the radius of coalescing droplets,are designed by molecular dynamics simulation method,and the bounce phenomenon of two equal radius droplets on these surfaces is studied.It is found that a maximum vj=0.23uic is achieved on the surface with a 1.6 nm high and 5.9 nm wide ridge,which is 1.81 times higher than the nanoscale velocity limit.It is also demonstrated that the presence of groove,ridge,and strip alters coalescence dynamics of droplets,leading to a significantly shortened coalescence time which remarkably reduces viscous dissipation during coalescenceCoalescence-induced jumping on textured surfaces with pillar structure has been studied.s.Although the jumping velocity with different Young's angle and solid fraction was different,the velocity trend was consistent in each Young's angle according to the droplet equilibrium state of Cassie or Wenzel.The main cause of the variation in velocity was attributed to the different equilibrium states of the droplets,which is reflected in the distinct apparent angles.In addition,the condensation process was performed to demonstrate the wetting states with different solid fraction,and further indicated that the Cassie state droplet was favorable to heat transfer.The bouncing dynamics of nanodroplets on superhydrophobic surfaces is studied.It is shown that there are three velocity regimes with different scaling of the contact time,?.Although ? remains constant in a wide velocity range,like as macroscale bouncing,we demonstrate that viscosity plays an essential role in nanodroplet bouncing even for low-viscosity fluids.A new scaling??(??R04/?2)1/3=(R0/v0)We2/3Re-1/3 to characterize the viscosity effect is proposed,which well agrees with the simulated results for water and argon nanodroplets with various radii and hydrophobicities.It is also found a pancake bouncing of nanodroplets,which is responsible for an abruptly reduced ? in a high velocity regime.