Low Adhesive Surfaces that Adapt to Changing Environments

2009 WILEY-VCH Verlag Gmb Over millions of years of evolution, soil animals have developed surface cuticle structures that help avoid soil adhesion at varying moisture levels. In contrast, in materials science, the technical methods that are well known to reduce adhesion with low surface energy textured surfaces in air and hydrophilic-gel coatings in water perform less efficiently in a changing environment. Inspired by biology and natural materials, the low-adhesion principles occurring in nature have led us to the fabrication of a biomimetic nanostructured composite coating consisting of surface-grafted hydrophobic nanoparticles embedded into a hydrophilic polyethylene oxide molecular brush. This responsive coating undergoes reconstruction from the morphology of rigid hydrophobic asperities, which hide the collapsed polymer brush in air, to the morphology of a hydrophilic brush-like layer engulfing the nanoparticles underwater. Due to this reconstruction, the coating demonstrates low adhesion to hydrophilic, hydrophobic, and amphiphilic materials, in both dry and wet environments. This finding is useful for the development of materials with low-adhesive surfaces, to be used in various applications where adsorption of contaminants or binding to other materials must be reduced. Adhesion arises from the balance of attractive and repulsive forces acting between contacting surfaces. In most cases, the adhesion phenomenon takes place in two major surrounding environments: air with variable humidity (vapors) and liquid water with various solutes (solutions). Both media may carry polar, nonpolar, and amphiphilic molecules and particles at variable concentrations. For the simplified case of a vacuum, adhesion depends on two major contributions: van der Waals and Coulomb interactions. For water vapor, capillary condensation on hydrophilic surfaces usually contributes to enhanced adhesion. In solutions, the situation is muchmore complex, andmany mechanisms strongly affect the variation in adhesion forces from attractive to repulsive. A regime that allows near exclusion of adhesion between surfaces and potential contaminants plays a key role in microelectromechanical systems (MEMS), self-cleaning coatings in air, and antifouling coatings in solutions. Similar to many living organisms, engineered devices often have to survive and operate in everchanging environments, where cyclical changes from vapors to solutions repeatedly endanger their function and intactness. Two distinctive methods are currently used in science and technology to solve problems of low-level adhesion in humid air and aqueous solutions. For decreasing adhesion in humid air, self-assembled hydrophobic coatings are applied to decrease capillary condensation and van der Waals interactions, which are lowest for fluorinated and siloxane-based materials. Surface modification with hydrophobic materials involves the assembly of silane and thiol monolayers and fluorocarbon plasma deposition. Another important method for decreasing adhesion attempts to mimic mechanisms found in nature. In lotus leaves, for example, adhesion of water droplets and dust particles is decreased by the presence of a hierarchically structured rough surface, which decreases the contact area, one of the main factors determining adhesion forces. Adhesion also depends on the elastic modulus of the surface material. For soft materials, large contact areas result in strong adhesive forces, even though interfacial forces may be weak. PDMS networks are a good example of low-surface-energy materials that may be adhesive due to the low-elastic modulus. It has been shown that increase in PDMS stiffness (by increasing the degree of crosslinking) results in lower adhesiveness. If two rigid materials are placed in contact with each other, the effective contact area is actually much lower than themacroscopic geometrical one. This difference is due to the asperities, which increase the interplanar distance between the contacting surfaces. Surface energy and contact area considerations suggest that a nonadhesive surface should possess a low-surface energy (hydrophobicity), multiscale surface roughness, and high elastic modulus. Patterning of lowand high-aspect-ratio asperities on the surface of poly(methyl methacrylate) and polyurethane acrylate has been shown to cause a gradual decrease in both adhesive and frictional forces. Fluorination of the patterned surfaces provided further decreases of surface adhesiveness, and environmental humidity has only a minor effect on the adhesive and tribological properties of these fluorinated surfaces. However, these methods fail in aqueous solutions. Hydrophobic interactions and adsorption of amphiphilic species result in the recovery of adhesiveness betweenmaterials in solutions. Protein molecules show a high affinity for low-surface-energy materials due to the attractive interactions between hydrophobic moieties of protein and substrates. The same methods that are used to change surface morphology of hydrophobic surfaces to achieve low adhesiveness in air, in fact, result in an increase in the amount of adsorbed solute species. This causes many