Fabrication And Fog Collecting Properties Of Biomimetic Structure Graphene Multi-Functional Surface

The depletion of water resources has become a serious environmental problem faced by mankind.To solve the problem,after billions of years of development,many organisms in nature have enjoyed the capacity of water collection and transportt.For example,the spindle knot of spider silk,the tip structure of cactus and the gradient wetting structure on the back of desert beetles,where the water droplet can be transported directionally under the wetting gradient and the Laplace pressure,thus greatly improving the fog collecting rate(FCR).Fortunately,human beings can get inspiration from nature by imitating the surface structure characteristics of these organisms to prepare artificial catchment surfaces and realize the directional transport of water droplets.This surface can also be further applied to liquid processing devices such as microfluidic chips,self-driving actuators and heat exchangers.However,in order to achieve this goal,traditional materials and processing methods usually have high cost,complex processing technology and poor surface stability,which make it difficult to produce on a large scale and restrict its wider application.Thus,we coupled a variety of biological prototypes with polymethylsiloxane(PDMS)and two-dimensional graphene lamellar structure as raw materials,combined with simple impregnation coating,laser etching,mechanical cutting and ultrasonic treatment,and fabricated one dimensional biomimetic fiber,tip gradient wetting surface,mesh gradient wetting surface,radial gradient wetting surface and Janus soft actuator.The water collecting performance,mechanism of control of wettability,directional transport behavior of droplets on different wetting surfaces and the working mechanism of Janus actuator were also studied.The main conclusions can be summarized as follows:(1)Inspired by the knots on spider silk,we fabricated a series of the bioinspired fibers with multi-level spindle knot structures by combing PDMS and graphene(G)and simple twodip coating method.The control of droplet motion was realized by adjusting surface wettability.On the special spindle knot structure,water droplets move from the junction to the spindle kont under the action of Laplace pressure and wettability gradient.When the middle region of spindle knot was changed into superhydrophobicity after laser etching,the water drop is driven by gradient wettability to move towards the joint and realized reversible transport.The results show that curvature and wettability gradient direction can effectively regulate the movement of tiny droplets,and play an important synergistic role in the process of water collection and transport.(2)Inspired by the needle-like spines of cactus and wettability gradient pattern on the back of dessert beetle,we designed a novel bioinspired surface with tip-shaped pattern by combining PDMS and G.This novel bioinspired PDMS/G surface was fabricated via the integration of laser etching and mechanical cutting,and thus the tiny droplets could be driven to the more hydrophilic region of the tip-shaped patterns.The movement velocity shows a peak of 2.42 mm/s when the wedge angle was 20°.In addition,porous graphene composite coatings from laser etch can produce obvious wettability change by tuning temperature.Thus,the tip-shaped surface after hating treatment possessed the highest collection rate about 2.75 g/cm2·h.(3)On the basis of the research contents of the first two chapters,inspired by the spider net and wettability gradient pattern on the back of dessert beetle,we designed a kind of special bioinspired hybrid wetting surface on the copper mesh.The surface containing hydrophilic and superhydrophobic areas,is successfully prepared by laser etching the PDMS/G film deposited on the pure copper mesh and then ultrasonic vibration.Due to the pyrolysis and curing of the PDMS/G film under laser etching,the laser etched copper mesh shows as superhydrophobicity,and after ultrasonic vibration,the naked area of the copper mesh shows as a hydrophilic region.Thus,based on the gradient wettability pressure the as-prepared hybrid wetting surface can quickly drive tiny water droplets moving toward more wettable regions,and the efficiency of water collection reaches 5.4 g/cm2·h.Moreover,the PDMS/G surface not only is tolerant to many stresses such as excellent anti-corrosion ability,anti-UV exposure and oil contamination,but also selfhealing simply by burning to the worn areas.Especially,our method is low-cost and easy-to-operate and time-saving.(4)Inspired from the needle-like spines of cactus and wettability gradient pattern on the back of dessert beetle,we designed the radiated pattern on different wetting surfaces by combining laser etching and mechanical cutting.Thus on the hydrophilic surface the radiated pattern drives tiny droplets moving to the more hydrophilic region of the tip shapes and realizes the unidirectional transporte from the center to the edge.We demonstrate that the surface with 1:1 hydrophilic and hydrophobic area ratio enjoys the highest collection rate,which is about 3.075 g/cm2·h.On the superhydrophobic surface water droplets move along tip shape to the center of circle and are stored.In addition,based on the above research results,we found that PDMS film etched by laser has the characteristics of Janus film to achieve oil driven response.By cutting at predigned locations,different structures of oil-sensitive actuators,including sawtooth and petallike software platforms,were successfully fabricated,and petal-like soft actuator acted as manipulators to transport the target objects.In addition,Janus actuators with different wettability surfaces exhibit different swelling behavior and different surface tension in different media.Therefore,they are expected to be candidate materials for soft actuators.At the same time,the switch between oil/water mixture and ethanol is also realized.