1.1 External stimuli-responsive smart surfaces
Smart surfaces with reversibly switchable wettability have aroused great interest because of their myriad applications as biosensors, microfluidic devices, intelligent membranes, and so on. This reversible switching can be achieved through an externally applied stimulus such as light irradiation, electrical potential, temperature, solvent, and pH. We have presented diverse methods for the fabrication of a wetting surface that is switchable from superhydrophobicity to superhydrophilicity, which combines the surface nanostructure and the smart materials in response to various external stimuli.
1.2 Superhydrophobic surfaces
In nature, the leaves of many plants exhibiting superhydrophobicity which have water contact angle greater than 150°and low contact angle hysteresis are cleaned completely by a simple rain shower: rolling water droplets remove dirt and debris from their surfaces (self-cleaning). The unusual wetting characteristics of superhydrophobic surfaces are governed by both their surface chemistry and roughness on multiple scales. To fabricate such surfaces, we have proposed various methods, such as supramolecualr organosilane, layer-by-layer deposition method, texturing of metal surface, and micellization of block copolymer.
1.3 Anisotropic wettability
Surfaces with controlled anisotropic wettability have the advantage of limiting liquid flow to a desired direction, which has advantage potential applications in microfluidic devices, evaporation-driven formation of patterns and easy-clean coatings, etc. Anisotropic wetting behavior has been observed on one-dimensional patterned surfaces achieved either through chemical pattering or surface roughness. In nature, anisotropic wettability has been found on the rice leaf, and it has been mimicked by combining layer-by-layer assembly and the anisotropic wavy surface.
2.1 Biomimetic adhesives
The climbing ability of gecko lizards has attracted human attention for more than two millennia. The gecko footpad areas are covered with hundreds of thousands of setae with a density of 5300/mm2. Each seta is branched into hundreds of spatulas with dimensions of ~100nm. Multiple attempts to mimic gecko’s foot-hair have been reported during the last few years. We have focused on fabricating synthetic gecko’s foot-hair using polymers and hierarchically branched AAO nanotemplates. Our approach shows a way to manufacture self-cleaning, re-attachable dry adhesives.
Self-assembled monolayers (SAMs) are molecularly thin films of supramolecules, which are excellent for the model study of investigating the adhesion behavior in nanometer-scale, i.e., nano-adhesion. Systematic research on the nano-adhesion behavior of SAMs of various structures is being performed using atomic force microscope (AFM) and Johnson-Kendall-Roberts (JKR) apparatus.
2.3 Johnson-Kendall-Roberts (JKR) apparatus
JKR apparatus is the instrument for evaluating the surface energy (γ) or intrinsic work of adhesion (W) between solid materials, which is based on contact mechanics. The excess deformation induced by interfacial attraction is mainly considered when measuring the change in contact area by applied load. We can obtain methodological advantages over conventional instruments such as contact angle meter, peel tester, etc. Intrinsic properties, such as W, are obtained by maintaining the contact in the equilibrium state during loading process. Non-equilibrium properties, such as effective adhesion energy (G), are also obtained by varying the unloading condition after contact, which gives us additional important information at the interface between the two materials. We expect this instrument can be applied in various fields such as nano-adhesion in self-assembled monolayers, adhesion of micro-particles, bio-interfacial adhesion, etc.
The study of biomineralization draw great interest in light of understanding the complex process in the formation of biomineralized structures as well as application of biominetic approach to design materials with controlled morphology and shape, desirable size in nanometer and unique properties. A key feature of biomineralization is involvement of biological macrmolecules such as proteins, nomally glycoproteins or phosphproteins. Advantages in biomimetic approach lie that mineralization is achieved under ambient pH, temperatures and use of water in constrast to harsh conditions and solvents for synthesis many advanced materials
3.2 Polymer induced crystallization of amorphous CaCO3
Inspired by biomineralization process observed in seashell nacre, the biomimetic mineralization of calcium carbonate has received much attention in recent times. This, combined with a broader understanding of how organic molecules in biological systems affect the nucleation and growth of biominerals, has led to the design of new hybrid materials. Calcium carbonate is an attractive model mineral because its various crystal forms are easily recognizable, and its morphology has been the subject in biomineralization processes. We therefore have dealt with polymer-induced crystallization of ACC films and the sequential formation of CaCO3 structures assisted by the acidic polymer additives.
4.1 Highly transparent anti-reflection film
Anti-reflection coatings are widely used to reduce the surface reflection of optical devices. In nature, the moths' eyes have an anti-reflection property: their surfaces are covered with a natural nanostructured film which eliminates reflections, allowing the moth to see well in the dark, without reflections to give its location away to predators. We have proposed a versatile and cost effective method for preparing anti-reflection surfaces with moth eye-like surface topographies.