Study On The Directional Movement Of Nano Substances On The Wedge Surface

With the development of nanotechnology,it is possible to manipulate and detect substances at the microscale.Manipulating the directional movement of a substance on a solid interface is of great importance in chemistry,biology,medicine and other fields,such as microfluidics and micromotors.Recently,researchers have proposed various effective methods to induce the movement of substances,including the active methods using external energy(electricity,heat,acoustics,light and magnetism)to drive substances movement and passive methods that take advantage of the gradients in the interface properties or structures.However,most of the researches can only achieve oneway and short-distance drive,and realizing the control of direction and continuous movement is still a challenge.With the development of computer simulation technology,molecular dynamics simulation method has become a powerful tool to study the properties of substances at the nanoscale.In this research,based on the all-atom molecular dynamics simulations,the directional movement of nano-droplets and continuous movement of nano-particles on the wedge-shaped gradient surface are studied.Water droplets are widely used as molecule carriers and play an important role in drug delivery and energy collection.First,the movement of nanodroplets on a wedge-shaped gradient surface is studied and its mechanism is discussed.It is found that the initial position and initial wetting state of nanodroplets can tune their movement direction.Results show that the droplets at different positions on the groove surface move in different directions,namely,the droplets move towards the converging or diverging end of the groove when they are placed near the converging or diverging end,respectively.In addition,the droplets at the same position on the groove but in different wetting states move in different directions,that is,the Cassie and Wenzel droplets move towards the converging and diverging end of the groove,respectively,and they always keep their initial wetting state during the movement.By analyzing the interaction energy between the droplet and the groove,it is concluded that the opposite movements of the droplet are attributed to the opposite roles played by the groove substrate and upper layers,namely,the substrate drives the droplet towards the diverging side,while the upper layers induce the droplet to the converging side.The results suggest that the energy barrier between the two wetting states(Cassie and Wenzel state)increases with the groove height,which means that the transition between the two states is difficult when the groove height exceeds a certain height.Thus,the droplets keep their initial wetting state and move in the opposite directions under the different influence of the substrate and upper layers.Furthermore,it is found that the graphene and hexagonal boron nitride(h-BN)in-plane heterostructure can drive the water droplet to move from the converging end of the wedgeshaped h-BN track to the diverging end.In order for the nano particles to move over a long distance,two identical grooves are connected together and the substrate is optimized,and hence realize the continuous directional movement of nano particles on the wedged interface.The analysis illustrates that the angular substrate provides an auxiliary driving force at the junction to induce the nanoflake to overcome the energy barrier and moves continuously.Moreover,the sustained transport is closely related to the surface energy and groove structure: the nanoflake is more likely to move continuously on the groove with lower surface energy and smaller substrate and groove opening angle.In the present paper,the spontaneous movement of substances(nano droplet and nano particles)on the wedge-shaped gradient surface is studied,and the physical mechanism is analyzed.This study realizes the manipulation of the droplet movement direction on the wedge-shaped groove by adjusting the initial position and wetting state of the water droplets,expands the application of in-plane heterostructure and provides an idea for the continuous movement of nanoparticles.This research provides new ideas for the study of substance movement control on the interface,and is expected to be benefit for the design of interfaces for driving the nano-objects.