Other Applications - Hierarchical Nanostructures for Energy Devices - page 284

Besides the theoretical approach, many fabrication methods of gecko-
mimetic structures have been developed to obtain hierarchical structures
through various methods 129-132 to replicate as close as possible the topo-
graphic features found in geckos and possibly satisfying all seven requisites
for dry adhesion proposed by Autumn et al. 133 Greiner et al. 130 fabricated
hierarchical pillars in PDMS with the base pillars which were 50 mmin
diameter and 200 mm in length. The top round-ended pillars were 10 mmin
diameter with aspect ratios ranging from 0.5 to 2. Contrary to what was
expected, the pull-off test showed a pull-off force an order of magnitude
lower for these hierarchical structures. This was attributed to the lower
packing density of the hierarchical structure when compared to a linear
structure. Jeong et al. 132 had similar observations to Greiner et al., but
Murphy et al. 131 reported a two-level hierarchical structure, which has higher
adhesion when compared to the linear structures as shown in Figure 12.11 and
12.12. The hierarchical structure might have enabled the reduction of the
effective modulus of the material, thus leading to an increase in the contact area
experienced by the hemispherical indenter. The materials reported in these
investigations were typically soft polymers (elastic modulus of 3-19.8 MPa).
Kustandi et al. 129 were the only ones who reported the use of a stiff polymer

d n 3 r 4 n g | 8

.

Figure 12.11 Scanning electron micrographs of three-level hierarchical polyurethane

fibers: (a) 400-mm-diameter curved base fibers; (b) base fiber tip with
midlevel 50-mm-diameter fibers; (c) midlevel fibers in detail; (d) terminal
third level fibers at the tip of the midlevel fibers are 3 mmindiameter
and 20 mm in height and have 5-mm-diameter flat mushroom tips.
Reproduced with permission from ref. 131. Copyright 2013, American
Chemical Society.