Figure 1

Concept of large effective slip by a nanoengineered superhydrophobic surface in Couette flow. Liquid sits on hydrophobic structures by surface tension. The majority of the liquid boundary is with air, where shear stress is much smaller. The hydrophobic structures need to be relatively tall and populated in submicron scale to sustain the liquid levitation and to produce the effective slip over air under realistic conditions (i.e., pressure ranging up to 1 atm).Reuse & Permissions

Figure 2

SEM image of a nanoturf surface made by the black silicon method [24]. The inset shows the apparent contact angle of water droplet (i.e., $\sim 180°$) on the nanoturf surface after the hydrophobic coating with Teflon.Reuse & Permissions

Figure 3

Schematic drawing of a cone-and-plate rheometer, showing the geometric parameters and the reference frames for the analysis of the Couette flow within it.Reuse & Permissions

Figure 4

Experimental results for slip length. Hydrophobic nanoturf (△) shows dramatic slip for both (a) water ($\sim 20\mu \mathrm{m}$) and (b) glycerin ($\sim 50\mu \mathrm{m}$), while all other surfaces show slip comparable to measurement uncertainties. The superhydrophobicity of the nanoturf surface was preserved throughout the experiment with the slip length undiminished over time, suggesting that the large effective slip by the air trapped on the hydrophobic nanoturf surface is not temporary. The secondary flow was verified experimentally by observing the torque versus shear rate data [27]. As the secondary flow became present, the torque increased not linearly but exponentially with shear rate, which was reflected in the results as the onset of the slip length reduction. Although the data were not shown here, the torque over the hydrophobic nanoturf was much smaller than the other surfaces even in the secondary flow regime, suggesting that the drag reduction by the large effective slip is still valid in the unstable flow regime.Reuse & Permissions