Mechanics of Self-Similar Hierarchical Adhesive Structures Inspired by Gecko Feet - Structural Interfaces and Attachments in Biology - page 195

optimal adhesion can be realized below a critical contact size, which can be related
to the intrinsic capability of a small scale material to tolerate crack-like flaws
[ 25 - 28 ] and Glassmaker et al. [ 24 ] demonstrated that fibrillar structures with
slender elastic fibrils can significantly enhance the adhesion strength. Northen
and Turner [ 29 ] made use of massively parallel MEMS processing technology to
produce hierarchical hairy adhesive materials containing single slender pillars
coated with polymer nanorods and reported significantly improved adhesion in
such multiscale systems.
In contrast to the increasing volume of research on robust adhesion, the question
of how adhesion is released upon animal movement has so far received relatively
little attention. Autumn et al. [ 2 ] reported experimental data that the pull-off force
of an individual seta of gecko depends strongly on the pulling angle. Gao and Chen
[ 27 ] numerically simulated the pull-off force of a single seta and found that the
asymmetrical alignment of seta allows the pull-off force to vary strongly (more than
an order of magnitude) with the direction of pulling.
Previous studies have provided significant insights into various aspects of
adhesion mechanisms in biology. However, a general understanding is still lacking
with respect to a number of critical issues. First, robust adhesion at the level of a
single hair or fiber does not automatically address the problem of robust adhesion
on rough surfaces at macroscopic scales. It has been shown that size reduction can
result in optimal adhesion strength at the level of a single fiber [ 7 , 22 - 24 ]. However,
it is not clear how this size-induced optimization might work at the system level
of hierarchical structures. Similarly, releasable adhesion at the level of a single seta
[ 2 , 7 ] does not provide full explanations on how releasable adhesion is achieved in
macroscopic contact. The present chapter is aimed to address the basic mechanics
principles which underline these issues.
10.3 Bottom-Up Designed Hierarchical Structures
for Robust Adhesion
10.3.1 Flaw Tolerant Adhesion of a Single Fiber
Adhesive contact between elastic objects usually fails by propagation of crack-like
flaws initiated at poor contact regions around surface asperities, impurities, trapped
contaminants, etc. As an external load is applied to pull the contacting objects apart,
stress concentration is induced near the edges of contact regions around surface
asperities. With increasing load, the intensity of stress concentration at the largest
interfacial flaw will first reach a critical level and the contact starts to fail by crack
growth and coalescence. Under this circumstance, the adhesion strength is not
optimal because only a small fraction of material is highly stressed at any instant
of loading. From the robustness point of view, it would be best to seek a design of