Preparation And Properties Of Bioinspired Structured Wet Adhesives

In natural environment,many animals,such as tree frogs,locusts and beetles,etc.,have excellent adhesion abilities on wet surfaces.It is related to the micro-nano structures covered with mucus on their toe pads,which is called wet adhesion.In spite of good progress made in the past years,many questions about the mechanism of wet adhesion remain to be addressed.The mimicking of biological adhesion systems not only deepens our understanding of the mechanism of biological wet adhesion,but also promotes the development of new materials to better meet the needs of life and industrial development.Taking tree frog as the representative of wet adhesion,wet adhesion systems strategy mimicked tree frog toe pads are proposed in this thesis.The influences of bioinspired mucus and nanostructures on the wet adhesion are systematically studied.Based on the in-depth understanding of biological wet adhesion,a novel underwater adhesive is further proposed.Inspired by the hexagonal epithelial cells on the toe pads,a structured hydrogel adhesive（SHAD）is prepared to gain insight into wet adhesion.SHAD is composed of poly（acrylamide）/poly（vinyl alcohol）（PAAm/PVA）hydrogel with elastic modulus and the surface pattern close to the tree frog toe pads.As large amount of liquid is involved in the bulk,SHAD is an ideal material to mimic the mucus secreting system.Moreover,the addition of glycerol in SHAD（named as G-SHAD）is utilized to regulate the fluid properties.The combined analysis of adhesion force and work of adhesion reveals the major contribution of direct solid-solid contact in wet adhesion.By changing the mixing ratio and the temperature of glycerol-water mixture,Stefan adhesion(F_SA)is proved to dominate capillarity forces（F_C）during separation.A fast detachment and a low temperature greatly increase the interface F_SA,which also increase the detaching distance.As a result,G-SHAD with 30%glycerol,whose viscosity is close to the mucus in tree frog,shows the best comprehensive performance,namely high adhesion,high friction force and low distance of detachment.Inspired by nanoconcave top of epidermal cells on tree frog toe pads,array of composite micropillars with nanopits on the surface（CP_p）has been designed.Polystyrene（PS）nanoparticles are mixed into polydimethylsiloxane（PDMS）and serve as the template for nanopits on the array of PS/PDMS composite micropillars.CP_pshows much larger wet adhesion as compared to the arrays of micropillars without nanopits.Under a certain loading force,most of the liquid between CP_p and the counterpart surface is squeezed out so that the liquid remained in nanopits forms multiple nanoscale liquid bridges within the contact area of single micropillar.Moreover,a large loading force could squeeze part of the liquid out of nanopits,resulting in the suction effect during the pull-off.The multiple liquid bridges,the suction effect and the solid direct contacts thus contribute to a strong wet adhesion,which could be～36.5 times of tree frog toe pads.The results suggest the function of nanoconcaves on the toe pad of tree frogs and offer a new design strategy for structured adhesive to gain strong wet adhesion.Based on the solid-solid direct contact,a novel smart adhesive（WAD）with strong,reversible underwater adhesion is prepared by copolymerizing N,N-dimethylacrylamide with crystal n-octadecyl acrylate（C18）and lauryl methacrylate segments.The flexible C18 side chain can form an intimate contact with substrates,achieving a strong adhesion.Meanwhile,the adhesion is intelligently controllable.The change of C18 chain from fixed chain at crystal state to the long and flexible chain at the amorphous state is the mainly driving force to subtly regulate the adhesion.At a high temperature,the modulus and the surface roughness on the surface of WAD are both greatly decreased,promoting the close contact with substrates.WAD is intrinsically hydrophobic,which plays a vital role in realizing strong underwater adhesion（～230 k Pa）by preventing the penetration of water to the bonding interface.Moreover,the damage of WAD can be quickly heals in 15s enabling the durability property of adhesive.In addition,WAD shows excellent underwater adhesion on surfaces with various roughnesses,under water with various Na Cl concentrations and p H conditions.The adhesive can be used as repair-patch and gripper device underwater.We believe WAD is promising for many applications in the area of transfer printing,sensors,and healthcare,and so on.