Preparation Of Anisotropic Wetting Surfaces Based On Patterned Micro-/Nano-Structures And Their Applications

In recent years,Micro/nano-scale(ordered)patterned surface and interfacial materials,owing to their special physical and chemical properties(unique magnetic,electrical,optical and chemical properties,etc.),have attracted a lot of attention by the scientific researchers and applied in many important areas.Among the properties,the special wetting properties of patterned materials are paid more attention because of its great application prospect.By learning about nature,scientists fabricated nano/micro collaborative patterned materials utilizing bionic method,and realized the special wetting properties of animals and plants,such as butterfly wings-,spider silk-,and cactus-like wetting behavior.That is,directional water transportation on the surface automatically,or anisotropic wetting surface.Anisotropic wetting surfaces with automatically directional water transportation properties have very important application prospects in intelligent devices,self-cleaning,microfluidic,printing,and theoretical research.In the initial stage of this paper,the reported anisotropic wetting surfaces mainly focus on the unidirectional wetting of water in air,as for anisotropic wetting of liquids with low surface energy in air or under water,or anisotropic wetting of water under oil media still remains unexplored.Controlling the wetting behavior of liquids with wide surface tension range is difficult.At the time,there are only Lotus leaf-like structure to transport water,for directionally transporting of liquids with low surface tension and high viscous is more to be investigated.For liquids with other special properties,such as acid and base,it is even more difficult to realize responsive anisotropic wetting simultaneously.The functional surfaces mentioned above have important application value in both basic research and practical applications,including oil and water separation and freshwater collection,etc.Therefore,this paper aims to prepare anisotropic wetting surfaces with different functions under different conditions,and to improve the application value of the anisotropic wetting surface.Utilizing colloidal lithography,photolithography,and chemical modification as basic processing means,patterned micro/nano-structure with special properties is constructed,and anisotropic wetting of different functional liquids is realized,and then the application of anisotropic wetting surface in liquid transportation,microfluidic,and sensor is investigated.In chapter 2,we prepared silicon cylinder arrays(Si-CAs)based on colloidal lithography,and asymmetrically modified two types of molecules with surface tension differences onto two sides of the Si-CAs.We realized anisotropic wetting of liquids under various media utilizing the chemistry-asymmetric Janus structure,and proposed the preparation strategy.Essentially,the molecules with different surface energies modified on the Si-CAs asymmetrically cause the anisotropic wetting behavior of liquids.The as-prepared Janus Si-CAs could control water unidirectional wetting in air or oil as well as oil unidirectional wetting in air or water media.We propose a strategy for preparing anisotropic wetting surfaces under arbitrary media(air,water,oil),which is to find two types of molecules;one of the molecules is easy to be wetted by the liquid under the corresponding media,while the other one is not.By introducing PNIPAM molecule onto one part of Janus Si-CAs,thermal-responsive Janus PNIPAM-MHA Si-CAs is prepared.Such surface switches between isotropic and unidirectional wetting property under various media when the temperature is below or above the LCST of PNIPAM molecule.Because of the excellent unidirectional property and intelligent switch of liquids wettability of the Janus structure under various media,the as-prepared anisotropic wetting surfaces show a broad range of applications in future smart device systems and theoretical research.In chapter 3,utilizing "Janus" systems,we prepared an anisotropic wetting surface that could manipulate the wetting behavior of liquids in a wide surface tension range.Through selecting and asymmetrically modifying two types of molecules with considerable surface tension difference onto two sides of the Si-CAs,the as-prepared asymmetric substrate could direct the unidirectional wetting of liquids in a wide surface tension range(high surface tension water ? low surface tension oil),which shows an extraordinary ability of fluid control.Reason of the excellent phenomena is investigated and the preparation strategy is proposed.Preparing the anisotropic wetting surface requires two factors.One is finding two molecules with considerable large surface energy difference to modify two sides of the cylinders asymmetrically and the other is the upper surface of the Janus Si-CA needs to possess a slow wetting speed for oil.Then,we prepared different functional MF chips for flowing control of liquids in a wide surface tension range in MFs utilizing the "Janus" Si-CAs coupled with a T-shaped PDMS microchannel.When a typical PDMS channel was chosen,a surface tension admeasuring apparatus for liquids was prepared,and the chip could be used to estimate the surface tension of unknown liquids.If the microchannel was fabricated from PFS-blended PDMS,the as-prepared chip showed great abilities to manipulate the liquids in a wide surface tension range(even oil)in the unidirectional flow in MFs.Therefore,because of the large range of fluid control in MFs(open or conventional system),the Janus surface is a great tool to be applied in MFs and would show a broad range of applications in the future MF systems.In chapter 4,utilizing "Janus" systems,we realized smart pH-responsive surfaces that could autonomously induce unidirectional wetting of acid and base with reversed directions,and applied the surface in liquid transportation and acid/base detector.Through selecting typical pH-responsive groups,amino and carboxyl groups,and asymmetrically modifying them onto two sides of the Si-CAs.The as-prepared Janus Si-CAs could direct unidirectional wetting of strong acid and base toward corresponding directions that are modified by amino or carboxyl groups.Through investigating the wetting behavior of liquids with other pH values,it was found that the molecules we selected show high sensitivity to strong acid and base.As decreasing in the acidity or basicity of the liquids,the wetting behavior was changed from unidirectional wetting to anisotropic wetting.With further decreasing(pH = 3.99,7.04,9.99),the liquids nearly show isotropic wetting behavior.The smart phenomena are caused by asynchronous protonation/deprotonation behavior on two sides of the asymmetric surfaces.Acid/base detector could be accomplished based on the phenomena.We proposed the precondition of the smart asymmetric surfaces for simultaneously regulating the wetting behavior of acid and base,which are relatively large differences of CA value between two sides of the Si-CAs when acidic and basic conditions are given.Following the principle,through investigating the wetting behavior of liquids with various pH values on amino or carboxyl groups modified Si-CAs,we identified the operating condition of the Janus Si-CAs.In addition to acid and base,the Janus surfaces could direct unidirectional wetting of water toward amino or mixed carboxyl groups modified directions when the surfaces are treated by strong acid or base.Furthermore,owing to the remarkable responsive behavior,such smart surfaces are great tools to be applied in microfluidics for fluid control,sensors,and oil/water separation.The surfaces would show a broad range of applications in the future stimuli-responsive materials and interfacial chemistry fields.In chapter 5,utilizing "Janus" systems,we realized functional surface that could coordinately induce wetting of water,and understand the mechanism of anisotropic wetting surface for water wetting.Through photolithography,two "Janus" structures with different directions were integrated in a single substrate,resulting in a patterned complicated asymmetric anisotropic wetting surface.Water shows unidirectional wetting on the patterned asymmetric surface toward resultant direction of the two wetting directions.Morphology of the structure,the distance between nanopillars,and size of the photolithography pattern do not affect the wetting performance.After investigating different molecules modified patterned chemistry-asymmetric nanostructure arrays,we found that the larger the surface energy difference between the modified materials is,the better the wetting anisotropy of water on it.Based on our patterning method,a lot of functional surfaces may be integrated in a single surface,and these multifunctional surfaces will cater the demand of future scientific development.