Quantifying frictional drag reduction properties of superhydrophobic metal oxide nanostructures.

We measure a frictional drag reducing property of various superhydrophobic metal oxide nanostructures by quantifying their effective slip length. Scalable chemical methods tailored to each metal substrate are applied to grow oxide nanostructures on copper (Cu), aluminum (Al), and titanium (Ti), respectively. In particular, three different types of oxide nanostructures are grown on titanium substrate by changing the chemical composition to investigate the morphological influence on the slip length. Microchannels containing metal oxide nanostructures are fabricated based on microfluidic sticker method, while the slip length is unambiguously determined by measuring the ratio of the volume flow rate over superhydrophobic surface to that over the flat surface simultaneously. The slip length is measured to be ~8 μm on Cu nanostructures, while it is measured to be ~3 μm on Al nanostructures. For Ti nanostructures, the measured slip lengths range in 1-2.5 μm, where they increase proportionally with the structural pitch of nanostructures, agreeing with the theoretical prediction. We believe that our results will be useful in applying scalable low-cost metal oxide nanostructures to underwater applications by providing their frictional characteristics.