We investigate the hydrodynamic friction properties of superhydrophobic surfaces and quantify their superlubricating potential. On such surfaces, the contact of the liquid with the solid roughness is minimal, while most of the interface is a liquid-gas one, resulting in strongly reduced friction. We obtain scaling laws for the effective slip length at the surface in terms of the generic surface characteristics ͑roughness length scale, depth, solid fraction of the interface, etc.͒. These predictions are successfully compared to numerical results in various geometries ͑grooves, posts or holes͒. This approach provides a versatile framework for the description of slip on these composite surfaces. Slip lengths up to 100 m are predicted for an optimized patterned surface.

We present in this letter an experimental characterization of liquid flow slippage over superhydrophobic surfaces made of carbon nanotube forests, incorporated in microchannels. We make use of a µ-PIV (Particule Image Velocimetry) technique to achieve the submicrometric resolution on the flow profile necessary for accurate measurement of the surface hydrodynamic properties. We demonstrate boundary slippage on the Cassie superhydrophobic state, associated with slip lengths of a few microns, while a vanishing slip length is found in the Wenzel state, when the liquid impregnates the surface. Varying the lateral roughness scale L of our carbon nanotube forest-based superhydrophobic surfaces, we demonstrate that the slip length varies linearly with L in line with theoretical predictions for slippage on patterned surfaces.

We perform gas flow experiments in a shallow microchannel, 1.14±0.02 μm deep, 200 μm wide, etched in glass and covered by an atomically flat silicon wafer. The dimensions of the channel are accurately measured by using profilometry, optical microscopy and interferometric optical microscopy. Flow-rate and pressure drop measurements are performed for helium and nitrogen, in a range of averaged Knudsen numbers extending up to 0.8 for helium and 0.6 for nitrogen. This represents an extension, by a factor of 3 or so, of previous studies. We emphasize the importance of the averaged Knudsen number which is identified as the basic control parameter of the problem. From the measurements, we estimate the accommodation factor for helium to be equal to 0.91±0.03 and that for nitrogen equal to 0.87±0.06. We provide estimates for second-order effects, and compare them with theoretical expectations. We estimate the upper limit of the slip flow regime, in terms of the averaged Knudsen number, to be 0.3±0.1, for the two gases.

We measure velocity profiles in water flowing through thin microchannels, using particle image velocimetry combined with a nanopositioning system. From the velocity profiles, we determine the slip lengths in two cases: Smooth hydrophilic glass surfaces, and smooth hydrophobic glass surfaces, grafted with a monolayer of silane. The slip length is determined within (+/-100 nm) , i.e., five times more accurately than previous work. In all cases, we find that the slip length is below 100 nm.

This paper provides an experimental investigation into the use of leading edge serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil's leading edge (LE) and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz to 8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (∆PWL) is sensitive to the amplitude, 2h of the LE serrations but much less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude act as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils.

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We image the flow of complex fluids in microchannels of controlled geometry using tracers. The spatial resolution allows us to access quantitatively the bulk nonlinear rheology and wall slip, as we show on model polymer solutions. In perspective this strategy should prove useful for the study of heterogeneous flows of more complex fluids.

The ability to self-assemble was evaluated for a large variety of amphiphilic block copolymers, including poly(ethyleneoxide-b-ε-caprolactone), poly(ethyleneoxide-b-d,l-lactide), poly(ethyleneoxide-b-styrene), poly(ethyleneoxide-b-butadiene) and poly(ethyleneoxide-b-methylmethacrylate). Different methods of formation are discussed, such as cosolvent addition, film hydration or electroformation. The influence of experimental parameters and macromolecular structures on the size and morphology of the final self-assembled structures is investigated and critically compared with the literature. The same process is carried out regarding the characterization of these structures. This analysis demonstrates the great care that should be taken when dealing with such polymeric assemblies. If the morphology of such assemblies can be predicted to some extent by macromolecular parameters like the hydrophilic/hydrophobic balance, those parameters cannot be considered as universal. In addition, external experimental parameters (methods of preparation, use of co-solvent, …) appeared as critical key parameters to obtain a good control over the final structure of such objects, which are very often not at thermodynamic equilibrium but kinetically frozen. A principal component analysis is also proposed, in order to examine the important parameters for forming the self-assemblies. Here again, the hydrophilic/hydrophobic fraction is identified as an important parameter.