Construction Of Stable Lubricant Layer On One-dimensional Cone-structured Surfaces And Droplet Directional Transport Study

The directional liquid transport behavior of one-dimensional（1D）structured surfaces has found a wide range of applications in fog collection,oil-water separation,sensing,chemical synthesis,and other fields.The directional transport velocity of liquid on solid surfaces increases with the driving force it is subjected to and decreases with the increase in hysteresis resistance at the solid-liquid interface.For conical structures of a certain size,the Laplace pressure difference induced by structural asymmetry can provide the driving force for the directional transport of droplets,and the contact angle hysteresis due to the roughness of the solid surface and the inhomogeneity of the chemical composition at the microscopic level is the source of resistance to directional transport.The construction of a lubricant film on the surface of the conical structure can effectively reduce the hysteresis resistance on the droplet.However,the Rayleigh instability effect caused by the high curvature of the 1D conical structure will cause the lubricant film to form a periodic distribution of droplets,and the Laplace pressure difference gradient induced by the conical structure will further reduce the stability of the oil film on its surface.Therefore,it is not sufficient to rely on the capillary effect brought by the mere roughness to achieve uniform and stable adhesion of oil film on the surface of 1D conical structure.How to construct a stable and uniform oil film on the 1D conical surface to reduce the hysteresis resistance to the directional motion of droplets and improve the performance of droplet directional motion has become a key problem that needs to be solved.On the other hand,in order to increase the velocity of directional transport of droplets,constructing an array of micro-grooves on the surface of the 1D conical structure helps to increase the local Laplace pressure differential gradient and improve the driving force to which the motion is subjected.Therefore,introducing a lubricating layer that reduces the hysteresis at the solid-liquid interface on the surface of the array microgroove conical structure is a reasonable way to further improve the liquid mobility.However,the pure liquid lubricant oil layer tends to cover the surface microgroove structure under surface tension,which destroys the enhancement effect of microgrooves on the driving force applied to liquid droplets.How to construct a stable lubricant oil layer on the surface of the conical structure while maintaining the micro-groove structure is the core problem facing the promotion of rapid directional mobility of droplets.The following studies were designed and conducted to address both of these questions:（1）1D copper wire conical structures were prepared by gradient electrochemical etching and the top angle of the conical structures was controlled by modulating the reaction conditions.Ti O₂films were constructed on the surface of the conical structure to provide a nanostructure to increase the capillary force between the lubricant and the substrate surface and to graft polymer PDMS molecular brushes on the surface of the conical structure using Ti O₂ photocatalytic properties,with the excess oligomer PDMS acting as a lubricant layer to reduce the hysteresis forces on the droplets.The strong van der Waals force"locks"the oligomer PDMS on the surface of the conical structure and prepared the polymer brush stabilized lubricated surfaces（PBSLS）.Wettability characterization of the grafted polymer brushes on the two-dimensional（2D）lubricant oil surface and material characterization analysis yielded a successful grafting of hydrophobic polymer molecular brushes on the surface.The chemical composition of the PDMS polymer brushes and the relationship between graft length and reaction time were then investigated,and polymer brushes in the length range of 2-13 nm were obtained by controlling the reaction time.The effects of 1D lubricant surfaces of hydrophilic and hydrophobic polymer brushes and ordinary lubricating surfaces（SLIPS）on the motion of tiny droplets condensed in fog on a conical structured surface covered with a lubricant film were investigated,focusing on the change of droplet motion velocity under prolonged fogging conditions.The adhesion of the lubricant film on the 1D copper wire structure surface of the polymer brush 1D lubricant surface and SLIPS surface after long time droplet directional motion was investigated,and the relationship and law of the lubricant film resistance to the shearing motion effect of the moving droplet was analyzed as well as the analysis of the stabilization mechanism of the lubricant film by the grafted polymer molecular brush,and finally the efficient water collection capability of the 1D conical structure of the stabilized lubricant film was characterized.The results show that the grafted polymer molecular brush on the conical structure can effectively stabilize the lubricant film on the surface,and the lubricant film can be retained for at least 6 h under continuous shear action,which can effectively reduce the adhesion of the surface to the droplets and realize the rapid movement of the droplets.（2）By grafting a layer of flexible polymer molecular brushes on the surface of the 1D grooved conical structure,the driving force of the conical surface is increased while the contact angle hysteresis is reduced.The first step was to prepare an orderly arranged 1D trench cone structure by 3D printing,and to prepare a highly adhesive PDA coating on its surface,and to use the phenolic hydroxyl group of PDA to react with PDMS monomer to form a flexible polymer PDMS molecular brush.By studying the mechanism of grafting polymer molecular brushes on the material surface and characterizing the contact angle and sliding angle of the material surface,the analysis concluded that the flexible polymer molecular brushes can effectively reduce the contact angle hysteresis of droplets and improve the droplet motion performance.The thicknesses of the PDA coating and the grafted molecular brushes on the material surface were 7 nm and 4 nm,respectively,as measured by a step meter,which did not cover the conical surface groove microstructure and thus affected the droplet motion performance.By analyzing the droplet motion behavior of the grooved conical structure（increase droplet driving force）,the smooth conical structure with flexible polymer brushes（reduce hysteresis force）,and the grooved conical structure with flexible polymer brushes（increase driving force and reduce hysteresis force）,we focus on the difference of droplet growth and transport velocity between the three structures.The experimental results show that the combination of groove and flexible polymer brush structure can effectively improve the droplet directional motion performance,and the droplet growth rate and transfer rate are effectively improved compared with the single groove conical structure surface.The anisotropic properties of the groove can increase the Laplace pressure difference,while the highly dynamic nature of the polymer brush structure can effectively reduce the droplet hysteresis force.This composite structured conical surface provides some basic research theory for the development of efficient droplet directional transport surfaces.