| The study of liquid-solid interfacial properties plays an important guiding role in many fields,such as interfacial science,materials science,and fundamental science.Wettability is one of the most crucial physical properties to describe interfacial interaction between the liquid and the solid surface,which has attracted extensive interests in recent years.Experiment and simulation results have proved that changing surface microstructure or geometrical morphology can be adopted to regulate the surface wettability,which offers the potential for the design of functional materials.In addition to surface wettability.we should have a better understanding of how the liquid dynamics is affected in the design of functional materials,as well as how to control the liquid dynamics.Currently,there are two significant liquid dynamics that attract more attention,one is the coalescence dynamics,the other is the impact dynamics.Recently,the studies have focused on the designed surfaces modified with micro/nanostructures to regulate drop hydrodynamics to achieve the desired wettability,coalescence,and impact behaviors.Great progress has been made in tuning the wettability and behavior of drops by modifying surfaces with regularly-arranged micro/nanopillared structures.However,the underlying wetting mechanism of these micro/nanostructures in tuning drop behaviors is still unclear,and further research is needed on how to design and arrange these micro/nanostructures and the development of novel micro/nanostructures for obtaining the best control effect under different conditions and meeting the needs of different applications.Besides,there are few studies focusing on the manipulation of the substrate to achieve the wetting transition and the regulation of related liquid behaviors,which also needs to be studied.In recent years,with the development and wide application of precise instruments and to meet the requirements of some advanced production technologies,the control of liquid metal wettability and its dynamic behaviors has become more and more important.At the same time,as a key link in the production process,it plays a decisive role in many applications,such as micro-coating,micro-spraying,droplet casting,3D printing,prevention of heavy metal pollution in pipes,micro-welding,and so on.However,due to the difficulties in experimental operation,there is a lack of research on the behaviors of the metallic droplets,especially at the micro/nanoscale.Therefore,molecular dynamics(MD)simulations had been used to investigate the wetting behavior,coalescence behavior,and impact behavior of metallic nanodrops on the carbon-based surfaces.Because we mainly investigate the influence of the surface structure and morphology of the substrates,the designed substrates are fixed and the possible chemical reactions between the nanodrops and the substrates are ignored during the simulation.This study reveals the morphological evolution law of the nanodrops in the process of wetting transition,coalescence,and impact.From the viewpoint of the substrate manipulation and surface micro/nanostructure modification,variety of new strategies to control the wettability of the nanodrop and its behavior are offered,which provides the theoretical guidance in designing and fabricating the new functional surface materials and improving the advanced technology.The main research contents and results are as follows:(1)The coalescence behaviors between the Al and Pb nanodrops is controlled by the nanostructure of the surface and the confinement conditions.Results show that the nanostructure of the substrates affects the growth of the liquid bridge during the coalescing process via transforming the weak wetting on double graphene(DG)into dewetting on pillared graphene(PG)for the Pb nanodrop.The surface topography of the substrates also affects the coalescence dynamics.When the groove direction is the same as the coalescence direction,the coalescing process is supported;otherwise,the coalescence behavior is inhibited.The confined walls can affect the coalescence dynamics of two adjacent films by restricting the movement of one of the nanodrops,showing an asymmetric effect,which is highly related to the different wettability between Al and Pb nanodrops.The graphene(G)-walls confinement mainly restricts the movement of the Al along the x direction and the movement of the Pb along the z direction.But for the pillared graphene(PG)-walls,the confinement only affects the movement of the Pb both along the x and z directions instead of the Al.With the decrease of the confined space or the increase of the height of the nano-pillar on the PG-walls,the coalescence process becomes slower,and the distribution of atoms inside the nanodrops becomes more heterogeneous.Moreover,two separated films that are located on two sides of the walls can still merge into one bigger nanodrop,in which the movement of the Pb nanodrops plays a predominant role.(2)Coalescence behaviors are controlled by the design of surface nanostructures.The coalescence behaviors of identical metallic nanodrops on the surface decorated with nanopillared and nanostripe structures are studied.The arrangement density of the nanopillared structure can change the wettability of the Pb nanodrop and thus significantly affect the coalescence dynamics.A method for calculating the coalescence time of nanodrops on rough substrates is proposed,namely,"velocity method".A larger density of nanopillared structure would produce Cassie wetting state for Pb nanodrops,which is beneficial to coalescing.As the density decreases,the nanodrops present Wenzel wetting state.At this time,the coalescence process will be limited due to the restriction effect of nano-pillars.However,with the density decreasing to a very small value,the restriction effect is weakened and the coalescence process speeds up again.The coalescence behaviors,such as coalescence speed,shape evolution of the nanodrops,final position of the coalesced nanodrop and its adhesion degree with the substrate,can be effectively controlled by the proper design of nano-pillars with a density gradient.Compared to the smooth surface,the stripe-induced restriction effect causes great inhibition to the growth of the liquid bridge,in which the downward movement of the Ga nanodrop resulting from the penetration of atoms plays a predominant role.And the larger the intervals are,the heavier the restriction effect becomes.On the other hand,however,the stripe-like surface is favorable for the uniform of the atoms in the coalesced nanodrops.The width,spacing,and arrangement mode of the stripe structure will observably affect the coalescence process of nanodrops.Based on the facts,the stripe surfaces with spacing gradient are also designed to control the coalescence behaviors(3)Wettability and coalescence behaviors of the nanodrops are controlled by applying the mechanical vibration on the substrate.Results show that with the increase of the vibration frequency,the wettable Al nanodrop can realize the transformation from a strong wetting state to a weak wetting state,and finally to a dewetting state in sequence.A mathematical model based on the force condition of the nanodrop and its wetting state is established,providing the relationship between vibration conditions and wetting state,which can not only directly calculate the critical vibration frequency between wetting and non-wetting,but also easily obtain the desired wetting state by setting a specific value of the vibration frequency.Moreover,results also show that the vibration-induced wetting transition can also affect the coalescence behavior of the metallic nanodrops.With the increase of the vibration frequency,the coalescence time of the nanodrops becomes shorter and the coalescence process is significantly accelerated.With increasing the vibration frequencies,three different coalescence patterns could be found,namely,restricted coalescence,semi-free coalescence,and free coalescence.In order to compare their effects on the coalescence behaviors,we also propose a calculation method of coalescence time in these three patterns,called as“shape method".Also,the coalescence speed and the transformation of these three patterns can be precisely controlled by adjusting the vibration frequency.(4)Impact behaviors of metallic nanodrops on substrates with different surface structures and morphologies are studied,including shape evolution after deposition and rapid rebound.On one hand,Pb nanodrops are deposited on wettable surfaces after impact.Results show that the impacting metallic nanodrops also form the"crater" shape on the smooth surface during the shape evolution,accompanied by isotropic spreading and retraction behaviors.And they would like to spread along the groove direction on the rough substrates constructed by carbon nanotubes,but could be restricted in its vertical direction,causing an elongated shape and a longer time of spreading and retraction.On the PG substrates,due to the weak wettability of the nanodrops and their distinguishing feature of penetrating the valleys between surface protrusions,the spreading area and time are both reduced.Interestingly,impacting Pb nanodrops would exhibit anisotropic spreading and retraction behaviors on the non-flat(curved)surfaces,especially in the radial and axial directions,and gradually become an elongated shape.Additionally,we define a length ratio LRmax/LAmax to describe the asymmetrical impact behaviors of the nanodrops,and it is found that the increase of curvature diameters,the decrease of nanodrop diameters,the decrease of impact velocities,or the increase of the surface tension can weaken the anisotropic behavior.On the other hand,the impacting Ag nanodrop can rebound on the surface modified with nano-ridge structure.Results show that the ridge structure can significantly reduce the contact time,which is attributed to the combination effect of"center-drawing effect" and larger retraction speed along the ridge peak.And the most suitable angle of this ridge structure in reducing the contact time is below 30°with a suitable height.Furthermore,it is found that there is a "stair-stepping" decrease of the contact time with the increase of the impact velocity.More importantly,two-ridge structure with gaps and multi-ridge structure is advantageous to further minimize the contact time due to the increase of center atoms that execute the "center-drawing effect".Generally,the study of wettability,coalescence,and impact behaviors will be unified in the behavior regulation of droplets on the surfaces.Knowing the effect of surface micro/nanostructures modification and substrate manipulation on the wettability,coalescence,and impact behaviors of droplets,and thereafter providing some effective measures to precisely control the droplet behaviors on the surfaces plays an important role in enhancing the quality of the products,improving the processing technology of materials,designing and fabricating functional surface materials,and enriching relevant basic theories. |
