Fabrication Of Patterned Non-Uniform Wetting Surfaces And Their Applications In Droplet Manipulations

Abilities of droplet storage,transport,etc.endow the patterned non-unifom wetting surfaces with great potential in the development of next-generation miniaturized platforms for applications ranging from new drug candidates to water collection.However,the current methods to fabricate such non-uniform wetting surfaces cannot meet the demand for diverse droplet manipulations.In this dissertation,patterned non-uniform wetting surfaces were fabricated by site-selectively adjusting micro-structures or chemical composition of specific materials using micro-and nano-manufacturing techniques such as chemical/electrochemical machining and micro-milling;the phenomenon and mechanism of droplet adhesion,transport,mixing and seperation on the patterned surfaces were discussed,which create both theoretical and technical foundations for the design and manufacture of new intelligent micro-droplet manipulation platforms.The main research contents and results are as follows:(1)Hydrophobic/superhydrophobic patterned surfaces were fabricated on metal substrates via electrolyte jet machining/through-mask chemical oxidation and surface energy reduction techniques;isotropic/anisotropic sliding on the fabricated patterened surfaces was analyzed based on the droplet sliding tests.Results showed that the isotropic/anisotropic sliding resistance of droplets is a function of the microstructures,macroscopic shape and size of the hydrophobic region and can be predicted by classical model for sliding resistance(Furmidge equation);lossless droplet manipulations such as droplet storage and transfer can be achieved by controlling the isotropic/anisotropic sliding resistance via pattern design.(2)Hydrophilic micro-grooves were fabricated on the superhydrophobic metal background using micro-milling technique,mechanism of anisotropic droplet sliding on the hydrophilic micro-grooves and its application in gravity-driven droplet manipulations were discussed.Results indicated that both contact angles and sliding angles of droplets on the fabricated micro-grooves showed significant anisotropy in directions parallel and perpendicular to the groove;the sliding anisotropy can be quantitatively controlled by grooves width,number,spacing and branch angles;driven by gravity,the significant sliding anisotropy can guide droplets to slide rapidly along the groove and enables droplet mixing on open-air surfaces with a mixing efficiency of 167 μL/s;this open-air droplet mixing method has advantages of low cost,free of bubble trapping and the possibility to access the surface directly when compared with 3D microfluidic devices with close channels.(3)Superhydrophilic patterns were fabricated on superhydrophobic metal background by through-mask electrochemical ething and the Laplace-pressure-gradient driven droplet manipulations on the fabracated patterns were discussed.Results showed that the fluorosilane layer on the superhydrophobic background can be site-selectively removed and superhydrophilic patterns can therefore be created via pitting and pitting expansion;evenly removing most of the fluorosilane layer at high-potential and then wiping off the remaining part at low-potential is the key to fabricate homogeneous superhydrophilic patterns using through-mask electrochemical etching method;Laplace-pressure-gradient driven droplet manipulations including droplet transport and mixing were performed on the fabricated wedge-shaped superhydrophilic patterns;the droplet transport speed and mixing volumes can be controlled by the branch angle and relative position of the patterns.(4)Underwater oleophilic/superoleophobic patterned surface was fabricated on copper foils using a through-mask chemical oxidation method,and a new category of underwater oil droplet transport process that is driven by Laplace pressure differential was proposed based on the surface;the main factors that influence flow speed and transported volumes,as well as their influencing rules were analyzed.Results indicated that through-mask chemical oxidation can create oleophilic/superoleophobic patterned surface on copper foil by site-selectively construct nanorod cluster structures;oil droplets dispensed on oleophilic round patterns(reservoirs)which are connected by oleophilic channel will spontaneously be transported because of Laplace pressure differential;flow speed and transported volumes can be quantitatively controlled by the size of reservoirs,channels,and the properties of oils;the patterning techniques and underwater oil transport process were extended to more complex patterns with multiple reservoirs for smart oil separation and mixing.