Molecular Dynamics Investigation Of Impact Dynamics On Inclined And Decorated Surfaces

Droplet impact processes are widely found in nature and are also fundamental to many engineering applications such as inkjet printing,spray cooling,selfcleaning surface design and anti-icing surface design.Droplet impact dynamics are complex due to the fact that they are not only influenced by many dominant forces,such as inertial,capillary,and viscous forces,but are also significantly affected by surface wettability and Surrounding environmental factors.In contrast to millimeter-sized droplets,recent emerging technologies based on nanoscale droplets,such as nanoscale inkjet printing and molecular brush technology,indicate the importance of studying the dynamic mechanisms of nanoscale droplets impacting on solid surfaces.In particular,viscous force has become very important due to scale effects,wh ich not only make the results of millimeter-scale droplet impact-based studies no longer simple to migrate to nanoscale droplets,but also make nanoscale droplet impact dynamics very complex.Current studies of nanoscale droplet impact dynamics have focused on nanodroplets impacting on ideal smooth flat surfaces vertically,while the oblique impact on surfaces is more common.In addition,modifications to solid surfaces are often expected to manipulate droplet dynamics to meet the needs of various engineering applications.However,the mechanisms of droplet impact dynamics on both inclined impact and modified surfaces are not yet understood.Herein,nanoscale droplet impact dynamics is investigated using molecular dynamics simulations and is divided into the following three parts:(1)nanodroplet oblique impact on superhydrophobic/non-superhydrophobic surfaces;(2)nanodroplet vertical impact on striped immersion surfaces to achieve nanoscale droplet splitting behavior;(3)nanodroplet vertical impact on point texture decorated surfaces to achieve rapid nanoscale droplet bounce off the surface.The main work and results of this paper are as follows:When a nanoscale droplet obliquely impacts a superhydrophobic surface,the droplet produces three outcomes:adhesion,regular bounce and break-up bouncing.Unlike millimeter-scale droplets,significant interfacial sliding can be observed for the impact of nanoscale droplets,showing a weakened adhesion effect between the liquid and surface,which makes the asymmetric behavior of oblique impact nanodroplets disappear during spreading and appear only within the retraction process in the high inclination angle range(α>45°).In addition,the increase of the oblique angle leads to a significant increase of the slip length and a significant decrease of the contact time.When a nanoscale droplet impacts a non-superhydrophobic surface,the outcome of bouncing disappears,and instead,five outcomes of deposition,splash,break-up,separation,and splash are produced.For each outcome,the droplet basically undergoes the basic dynamic process of spreading.Therefore,the feature parameter,the maximum spreading factor(βmax),is modeled,For oblique impacts,there exist two directions of the maximum spreading factor,the tangential maximum spreading factor(βmax.//)and the vertical maximum spreading factor(βmax.⊥).Compared to impacting superhydrophobic surfaces,the asymmetric behavior is very significant on non-superhydrophobic surfaces,noting that βmax.//is always larger than βmax.⊥,due to the fact that the latter is only controlled by the vertical impact velocity,while the former is controlled by both tangential and vertical impact velocities.It is tested that βmax.⊥ depends only on Wenand can be predicted by the scaling relation of βmax.⊥=0.7Wen1/4.However,βmax.//depends not only on Wen,but also on the impact oblique angle.We propose a modified model to predict βmax,//=βmax,⊥+0.001(Wentan2α)3/2，which is in good agreement with the data over a wide range of Wen and α.When a nanoscale droplet impacts the hydrophobic surface modified by hydrophobic stripes,different impact regions were identified based on different impact velocities and stripe widths,including stable non-split region,non-split and split coexistence region,stable split region and mixed region.Due to the strong intermolecular force,the droplet splitting shows a clear scaling effect and the stable non-split region is reported for the first time.In addition,a new mode of droplet splitting,i.e.,hole splitting,which will produce some subfragments during splitting,was also found.By analyzing the splitting time,it is found that it depends on We at low We while the splitting time shows a We independent property at high We number,distinguishing two different splitting mechanisms.For engineering applications,the stable splitting region is welcomed.A stable splitting region criterion based on the dimensionless stripe width is obtained using a simple energy model,which paves the way for tuning the splitting of nanoscale droplets.Reducing the contact time of droplets with solid surfaces is important in the design of ice-anti surfaces,designing and preparing the microstructure surface is an effective means to reduce the contact time.Nanoscale droplets impacting on point texture decorated surfaces promote a new bouncing behavior to significantly reduce the contact time of nanodroplets on solid surfaces,i.e.,ring bouncing.The contact time of impacting nanodroplets with ring bouncing behavior is significantly reduced due to the retraction time saved by creating internal and external contact line retractions.However,ring bouncing is not always produced,and in addition to ring bouncing there are outcomes such as regular bouncing,adhesion,and splash.By constructing an outcome phase diagram and combining them with energy analysis,the criterion for the ring bouncing region is effectively established to enable the regulation of nanodrop contact time on the solid surface.Finally,by testing the contact time of droplets with diameters of 8,10 and 14 nm for ring bouncing on the solid surfaces decorated by point texture,it was found that the smaller the ratio of droplet size to point texture diameter,the more significant the contact time reduction effect was.This can be explained by the fact that a lower ratio of droplet diameter to point texture diameter leads to a shorter retraction distance after internal fragmentation and thus a better contact time reduction effect.In this study,the surface decorated by point texture can reduce the contact time by a maximum of 52%,which is better than the current studies on reducing the contact time of droplets at the nanoscale.This paper focus the dynamic characteristics behind the oblique impact of nanoscale droplets on solid surfaces and the vertical impact on decorated surfaces,to reveals the physical mechanism of nanoscale droplet inclined impact on different wetting surfaces of superhydrophobic and non-superhydrophobic,and explores the methods of droplet splitting and reducing contact time.The findings of this paper will lays a solid theoretical foundation for many high-tech technologies based on nanoscale droplet.