Other Applications - Hierarchical Nanostructures for Energy Devices - page 285

(PMMA) in their effort to fabricate hierarchical structures by sequentially
using two porous alumina templates, but clumping due to the densely
packed pillars was observed on the fabricated films and the adhesion force
of these structures could not be determined accurately. Ho et al. 123 studied
several structural variations, however, their results were inconclusive in
proving the advantages of a hierarchical structure. They used a multitiered
porous anodic alumina template and capillary force assisted nanoimprint-
ing and successfully fabricated a gecko-inspired hierarchical topography of
branched nanopillars on a stiff polymer. The hierarchical topography im-
proved the shear adhesion force over a topography of linear structures by
150%. The effective stiffness of the hierarchical branched structure was
lower than that of the linear structure and the reduction in effective stiffness
favored a more ecient bending of the branched topography and a better
compliance to a test surface, hence resulting in a higher area of residual
deformation. As the area of residual deformation increased, so did the shear
adhesion force. The branched pillar topography also showed a marked
increase in hydrophobicity, which is an essential property in the practical
applications of these structures for self-cleaning in dry adhesion con-
ditions. 123 As well as the dry adhesion of the gecko-inspired adhesive,
Ho et al. 123

d n 3 r 4 n g | 8

reported an adhesive that can work in both dry and wet

.

Figure 12.12 Adhesion vs. preload data for unstructured, single-level micro, single-
level macro, and double-level samples against a 12-mm-diameter glass
hemisphere. Error bars represent standard deviations. The double-
level fibers generally exhibit the highest adhesion.
Reproduced with permission from ref. 131. Copyright 2013, American
Chemical Society.