Preparation Of Superwetting TiO₂ Surface And Its Application In Underoil Water Droplet Manipulation

Surfaces with special wettability play important roles in biological analysis,microdroplet reactions,etc.,and smart droplet manipulation is necessary for all these applications.However,almost all related studies are happened in atmosphere,and to realize similar manipulation of water droplet in complex environment,such as oil,still remains challenge.Existing research often needs to design different types of materials to meet different requirements.It is still a challenge to achieve effective manipulations of water droplets in oil to display different functions based on the same material.Meanwhile,the effect of environmental media on surface wetting variation is still not clear.In this thesis,based on the photo-response characteristics of TiO₂,combined with experimental research and theoretical calculations,surface hydrophilic/hydrophobic transitions in both air and oil are analyzed.Meanwhile,concrete surface wettings required for droplet manipulation in different environments are specified.Based on this finding,manipulations of water droplets in oil environment are further performed.The main research results are as follows:The flat TiO₂ was prepared by the sol-gel method,and then modified with fluorosilane,based on the UV light responsiveness of TiO₂,the hydrophilic-hydrophobic conversions of the surface in both air and oil are analyzed.Results show that the surface has hydrophobicity in both air and oil,but the transition process is not synchronous.It takes 150 min and 240 min for the surface to be superhydrophilic in air,and oil,respectively.And the transition from the hydrophobicity to hydrophilicity is more difficult in polar（n-butanol）oil than in non-polar oil.Mechanism study indicates that hydroxyl groups induced by UV irradiation can increase the surface hydrophilicity.Meanwhile,out-of-step transition of surface wetting in different environments is due to the different roles played by the exposed TiO₂,which provides hydrophilicity in air,and oleophilicity/hydrophobicity in oil.Based on the clarification of the mechanism of surface wetting transition of TiO₂,a series of TiO₂ surfaces with rough structure were further designed,and corresponding study about water droplet adhesion,penetration and self-transport on these surfaces in oil environment were carried out.Fluorosilane-modified superhydrophobic TiO₂ nanotube arrays surface was prepared by anodic oxidation and gas phase modification.Based on the photo-responsive property of TiO₂,surface adhesions transitions in both air and oil environments were analyzed.In air,it needs 65 min of UV irradiation to achieve high adhesion,while in oil,the time is increased to about 95 min.The low adhesion can be recovered after heating,and needed heating time are about 130 min and 60 min in air and oil,respectively.These results indicate that it is more difficult for the surface to turn to high adhesion in oil and more difficult to return to low adhesion in air.Mechanism analysis shows that the reversible adhesion is due to the combined effect of nanostructures and surface chemistry variation.Based on the responsive adhesion controllability,applications of droplet transfer and across media droplet storage are also demonstrated.The Janus TiO₂ membrane was prepared by anodic oxidation and one-side UV irradiation.After heating treatment,underoil superhydrophilicity of the membrane will return to the superhydrophobic state,and the membrane will lose the Janus effect.Switching control of underoil water permeation can be achieved by alternating UV irradiation and heating treatment.Moreover,on the basis of local irradiation with UV light,permeation microchannels with different diameters were constructed on the membrane,and accurate control of the unidirectional permeation flux of water was further proved.Result proves that the optimal UV irradiation time is about 50 min to achieve underoil Janus effect of the membrane.The mechanism analysis shows that the underoil asymmetric wettability generated on the membrane is the necessary factor for the unidirectional water droplet penetration.Based on the Janus membrane,applications such as underoil collection and removal of water droplets and independent multi-channel reactions are demonstrated.By coating TiO₂ nanoparticles on the conical copper wire,a conical structure surface coated with TiO₂ nanoparticles was prepared.After UV irradiation,the under-oil directional self-transport of water droplets was investigated on the surface.Experimental results indicate that the high underoil hydrophilicity of the surface caused by UV irradiation is a necessary condition for the directional self-transportation,and the critical underoil contact angle is about 57°.Mechanism analysis shows that the self-transport performance of water droplets on the surface is related to the Laplace force induced by the gradient structure and the reduction of hysteresis caused by the high underoil hydrophilicity.The higher the underoil surface hydrophilicity,the faster the directional self-transportation of water droplets.Environmental resistance also has an important influence on the directional self-transport of water droplets.The greater the viscosity of the oil environment,the greater the resistance the droplets will encounter during the self-transportation process.Based on the underoil self-transport property of the TiO₂ conical structure,microdroplet reaction and demulsification of water in emulsions are also demonstrated.