Enabled by a technology to fabricate well-defined nanogrates over a large area ͑2 ϫ 2 cm 2 ͒, we report the effect of such a surface, in both hydrophilic and hydrophobic conditions, on liquid slip and the corresponding friction reduction in microchannels. The grates are designed to be dense ͑ϳ230 nm pitch͒ but deep ͑ϳ500 nm͒ in order to sustain a large amount of air in the troughs when the grates are hydrophobic, even under pressurized liquid flow conditions ͑e.g., more than 1 bar͒. A noticeable slip ͑i.e., slip length of 100-200 nm, corresponding to 20%-30% reduction of pressure drop in a ϳ3 m high channel͒ is observed for water flowing parallel over the hydrophobic nanogrates; this is believed to be an "effective" slip generated by the nanostrips of air in the grate troughs under the liquid. The effective slip is clearer and larger in flows parallel to the nanograting patterns than in transverse, suggesting that the nanograted superhydrophobic surfaces would not only reduce friction in liquid flows under pressure but also enable directional control of the slip. This paper is the first to use nanoscale grating patterns and to measure their effect on liquid flows in microchannels.

Two-phase flows in microchannels with surface modifications are experimentally investigated. First, we investigate the shape of static and moving bubbles in microchannels with square cross-sections for different contact angles. Water and air are mixed on-chip in a cross-shaped mixing chamber. This mixing geometry allows for the production of monodisperse bubbles, the size of which can be controlled with the flow rates. We study the flow morphologies of mixtures made of pure water and air, and made of water with surfactant and air (aqueous foam), in both hydrophilic and hydrophobic microchannels. Second, we investigate the transient rheological behavior of polymer solutions when the length of the polymers is comparable to the height of the channel. The measured viscosity of the solution is several times larger than the expected value, and does not show the typical shear-thinning behavior. These experiments highlight the importance of wall properties for two-phase flows in microfluidic devices.

We experimentally investigated molecular effects of the slipno-slip boundary condition of Newtonian liquids in micro- and nanochannels as small as 350 nm. The slip was measurable for channels smaller than approximately 2 mum. The amount of slip is found to be independent of the channel size, but is a function of the shear rate, the type of liquid (polar or nonpolar molecular structure), and the morphology of the solid surface (molecular-level smoothness).

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