Effective slip and friction reduction in nanograted superhydrophobic microchannels

Abstract

Enabled by a technology to fabricate well-defined nanogrates over a large area $(2\times 2{\mathrm{cm}}^2)$ , 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 ($\sim 230\mathrm{nm}$ pitch) but deep $(\sim 500\mathrm{nm})$ 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 $1\mathrm{bar}$ ). A noticeable slip (i.e., slip length of $100–200\mathrm{nm}$ , corresponding to 20%–30% reduction of pressure drop in a $\sim 3\mathrm{\mu }\mathrm{m}$ high channel) 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.