HETEROGENEOUS NANOSTRUCTURES FOR BIOMEDICAL DIAGNOSTICS - Bio-Inspired and Nanoscale Integrated Computing - page 431

Figure 16.1. Water robot strider in relation to a water strider. (http://www.math.
mit.edu/ B dhu/Striderweb/striderweb.htm)
Water striders are able to remain stationary on water with a M c ratio less than 1.
In contrast, geckos are capable of dashing across the surface of water at a M C
ratio greater than 1 (Fig. 16.2). The attractive forces that hold geckos to the
surface are van der Waals interactions between the finely divided setae and the
surface [4].
While the precise mechanical principles by which large animals such as geckos
and geese propel themselves against the surface of water remain unknown,
investigators have been inspired by nature. The endless possibilities to manipulate
the relation between body force and surface tension have generated tremendous
enthusiasm to develop novel microscale devices.
16.2.2. Surface Tension-Inspired Microfluidics
The top-to-down approach to design small devices has encountered an increasing
surface to volume ratio. While surface tension is a potential obstacle to mobilize
mechanical components, researchers have employed various driving mechanisms,
namely, thermal actuation [5], piezoelectrical property [6], electrolysis and
electrostatic forces [7], acoustic actuation [8], and alike. Recently, Eun Sok Kim
et al. have developed a focusing acoustic ejector [9] to overcome surface tension.
The droplet emerges from the liquid surface as a result of convergence of
piezoelectrically generated acoustic waves (Fig. 16.3).
Rather than avoiding surface tension, Jin (CJ) Kim's group at University of
California, Los Angeles, developed a digital droplet production utilizing the