Other Applications - Hierarchical Nanostructures for Energy Devices - page 282

d n 3 r 4 n g | 8

Figure 12.9 The effect of branches on the boiling heat transfer: as the number of
branch increases in the hierarchical nanostructures, the CHF increases.
Reproduced with permission from ref. 108. Copyright 2013, The Japan
Society of Applied Physics.

saturation temperature of coolant from chip temperature) and heat flux
(multiplication of voltage supplied to and current through the heater) were
evaluated in the custom-built pool boiling heat transfer set-up while the
voltage in the heater was increased gradually. The convective heat transfer
behavior changed to pool boiling heat transfer, resulting in increased HTC and
CHF. Furthermore, we observed a decrease in boiling incipient temperature
compared to bare chips. The fabrication method for nanostructures on the IC
chips is straightforward and provides an effective way of cooling electronics by
boiling heat transfer, which has the potential for integration into advanced
electronic package configurations such as 3-D stacked structures.

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12.2.5 Gecko-inspired Hierarchical Adhesive
The evolution of biological systems has resulted in hierarchical nano- and
microfibrillar structures with diverse mechanical, optical, and sensing
functionalities. 109-111 These excellent and unique structure-related prop-
erties found in biological systems have inspired researchers to generate ar-
tificial materials, mimicking, for example, the amazing adhesion abilities of
gecko feet, 112-117 the self-cleaning superhydrophobic surface of the lotus
leaf, 118-119 and hairy fluid sensor arrays on the body of fish 120 and crick-
ets. 121 In particular, the hierarchical nanofibrillar structures of synthetic
gecko adhesives enable binding to almost any surface by van der Waals
interactions. 122 A gecko's superb ability to adhere to surfaces is widely
credited to the large attachment area of approximately 220 mm 2 of hier-
archical and fibrillar structures on its feet. 123 This combination provides the
gecko toe-pad with the means to effectively engage a high percentage of the
spatulae, at each step, to any kind of surface topography. 123 The gecko foot
has micro/nano-sized hierarchical structures. The fibrils begin from rows of