Applied Physics

How Wet Does It Get?

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Science  18 Feb 2005:
Vol. 307, Issue 5712, pp. 1015
DOI: 10.1126/science.307.5712.1015b

The ability to manipulate small volumes of liquids has opened up the possibility of designing a lab on a chip with micrometer-scale channels. The architectures for these microfluidic chambers can either be closed—with the typical channels, pumps, valves, and reservoirs— or open, where the flow is controlled by local changes in either the wettability of the substrate or its topography.

Seemann et al. patterned a set of rectangular grooves into chemically modified silicon to give a system that could be described by two parameters: the aspect ratio of the grooves, defined as the depth/width, and the contact angle formed between the fluid and the substrate. At contact angles greater than 45°, the fluid formed droplets that would spill over the walls of shallow channels and that transformed into filaments in deeper trenches. At lower contact angles, the wetting was complicated by pinned wedges that formed along the corners of the channels, and the filaments could take on either a positive or negative Laplace pressure. This suggests that dynamic changes in the properties of the substrate can be used to drive a fluid through a chip, keeping in mind that the chemistry performed on a chip will affect the wettability and hence the dynamics and shape of the moving fluid. — MSL

Proc. Natl. Acad. Sci. U.S.A. 102, 1848 (2005).

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