Sustainable drag reduction in turbulent Taylor-Couette flows by depositing sprayable superhydrophobic surfaces.

We demonstrate a reduction in the measured inner wall shear stress in moderately turbulent Taylor-Couette flows by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of reduction becomes progressively larger as the Reynolds number increases up to a value of 22% at Re=8.0×10(4). We show that the mean skin friction coefficient C(f) in the presence of the superhydrophobic coating can be fitted to a modified Prandtl-von Kármán-type relationship of the form (C(f)/2)(-1/2)=Mln (Re(C(f)/2)(1/2))+N+(b/Δr)Re(C(f)/2)(1/2) from which we extract an effective slip length of b≈19 µm. The dimensionless effective slip length b(+)=b/δ(ν), where δ(ν) is the viscous length scale, is the key parameter that governs the drag reduction and is shown to scale as b(+)∼Re(1/2) in the limit of high Re.

Autophilic effect: wetting of hydrophobic surfaces by surfactant solutions.

This paper resolves questions in the literature regarding the autophilic effect (i.e., movement of surfactant past the advancing contact line-leading to an increase in drop radius beyond that due to the advance) and its importance to quasi-static sessile drop wetting. Various systems (SDS, HTAB, and MEGA 10 surfactant solutions at three concentrations each and pure water and ethylene glycol on hydrophobic Teflon and OTS-coated silicon) are probed to determine the existence, time constant, and magnitude of the autophilic effect, using quasi-static advancing and receding sessile drops. From spreading results and advancing contact angle measurements, it is inferred that the autophilic effect does not occur for our systems (in contradiction of some literature) for the following reasons. First, no relation exists between the time constant for spreading and surfactant concentration, meaning the spreading seen is likely inertial in cause and not due to surfactants. Second, advancing contact angle decreases between tests on clean surfaces and those pre-exposed to surfactant, ruling out the possibility that the autophilic effect is faster than the advance. Third, spreading is seen after the end of the advance over both clean and pre-exposed surfaces, ruling out the possibility that the autophilic effect is slower than the advance. Finally, the pure liquids spread in a similar fashion to surfactant solutions on Teflon and similar contact angle measurements are seen for surfactant solutions and pure liquids of similar surface tension.