Designing Model Systems for Enhanced Adhesion
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6/11/2007
11:59 PM
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rather a unique property of the combination of materials. The thermodynamic definition of adhesion involves the work, or energy, associated with the creation of an interface from two surfaces:
Designing Model
Systems for Enhanced Adhesion Edwin P. Chan, Christian Greiner, Eduard Arzt, and Alfred J. Crosby
Abstract Nature provides inspiration for enhanced control of adhesion through numerous examples ranging from geckos to jumping spiders. The primary strategy in these examples is the incorporation of patterns, specifically high-aspect-ratio topographic features, to ingeniously maximize adhesion forces while maintaining ease of release. Recently, considerable research efforts have been devoted toward the understanding, development, and optimization of synthetic analogues to these examples in nature. In this article, we provide insight into the mechanisms that lead to enhanced control of interfacial properties through patterning, the strategies that can be used for fabricating synthetic patterns, and an overview of experimental results that have been used to gain understanding and guidance in this emerging field.
Introduction Recently, biologists have studied the amazing ability of insects and geckos to firmly attach and rapidly detach on demand to a variety of natural terrain.1–10 Their ability to control adhesion (attachment and detachment from a surface) is in many cases connected to the long, fibrillar hyperstructures that decorate their attachment pads (see Figure 1a for micrographs comparing the attachment pads of the beetle, fly, spider, and gecko). Depending on the animal species, the fibrils can vary in complexity (sometimes hierarchically arranged), dimensions (fibrils spanning from nanometers to millimeters in diameter), and materials properties. The gecko foot in particular possesses keratinous hairs (setae), 30–130 µm long and containing hundreds of projections terminating in 200–500-nm-wide spatula-shaped structures (Figure 1a, D). These structures maximize adaptability and adhesion to a variety of rough surfaces,11–13 facilitate easy release from the surface, and enable attachment and detachment over millions of cycles.14,15 Recent experimental investigations have demonstrated that the origin of the adhesion force is a combination of van der Waals interaction and capillary effect.16,17
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For materials engineers, there is tremendous opportunity for adopting these natural inspirations in the design of patterned adhesives for a variety of applications. These “smart” adhesives can potentially demonstrate high adhesive force, ease of detachment from any surface, and self-cleaning properties to enhance repeated use. However, much like biological evolution that selects the efficient fibrillar structure for a specific species, we must explore the significant parameter space (Figure 1b) that potentially contributes to these advantageous surface properties. In this article, we provide background based on our current understanding of adhesion, especially in the context of pa
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