Dynamically induced friction reduction in micro-structured interfaces.

We investigate the dynamic behavior of a regular array of in-plane elastic supports interposed between a sliding rigid body and a rigid substrate. Each support is modelled as a mass connected to a fixed pivot by means of radial and tangential elastic elements. Frictional interactions are considered at the interface between the supports and the sliding body. Depending on the specific elastic properties of the supports, different dynamic regimes can be achieved, which, in turn, affect the system frictional behavior. Specifically, due to transverse microscopic vibration of the supports, a lower friction force opposing the macroscopic motion of the rigid body can be achieved compared to the case where no supports are present and rubbing occurs with the substrate. Furthermore, we found that the supports static orientation plays a key role in determining the frictional interactions, thus offering the chance to specifically design the array aiming at controlling the resulting interfacial friction force.

Water entry and fall of hydrophobic and superhydrophobic Teflon spheres.

Hydrophobic and superhydrophobic solid Teflon spheres have been observed while settling in water under the action of gravity, starting from different initial conditions, and have been followed until the steady-state is reached. The superhydrophobic sphere features a nano/microtextured surface and advancing and receding water contact angles equal to, respectively, [Formula: see text] and [Formula: see text]. When impacting water from air, both spheres can entrap a conspicuous amount of air deriving from the sealing of a macro-sized air cavity formed upon impact (air cavity trapping) and standing at the rear part of the settling sphere. It is shown that this air amount, like a spindle, reduces the force coefficient exerted on the sphere, basically acting on the pressure drag. However, the air cavity trapping occurs above a critical impact velocity which for the superhydrophobic spheres is significantly lower than that pertaining to the hydrophobic one; thus a certain range of impact velocities exists at which the superhydrophobic sphere experiences a lower pressure drag and a higher mean velocity. As soon as the air cavity vanishes, the dynamics of the superhydrophobic sphere becomes indistinguishable from that of the hydrophobic one, in spite of the persistence of air within the surface micro-texture.

Fluid contact angle on solid surfaces: Role of multiscale surface roughness.

We present a simple analytical model and an exact numerical study which explain the role of roughness on different length scales for the fluid contact angle on rough solid surfaces. We show that there is no simple relation between the distribution of surface slopes and the fluid contact angle. In particular, surfaces with the same distribution of slopes may exhibit very different contact angles depending on the range of length-scales over which the surfaces have roughness.

An effective medium approach to predict the apparent contact angle of drops on super-hydrophobic randomly rough surfaces.

The apparent contact angle of large 2D drops with randomly rough self-affine profiles is numerically investigated. The numerical approach is based upon the assumption of large separation of length scales, i.e. it is assumed that the roughness length scales are much smaller than the drop size, thus making it possible to treat the problem through a mean-field like approach relying on the large-separation of scales. The apparent contact angle at equilibrium is calculated in all wetting regimes from full wetting (Wenzel state) to partial wetting (Cassie state). It was found that for very large values of the roughness Wenzel parameter (r(W) > -1/ cos θ(Y), where θ(Y) is the Young's contact angle), the interface approaches the perfect non-wetting condition and the apparent contact angle is almost equal to 180°. The results are compared with the case of roughness on one single scale (sinusoidal surface) and it is found that, given the same value of the Wenzel roughness parameter rW, the apparent contact angle is much larger for the case of a randomly rough surface, proving that the multi-scale character of randomly rough surfaces is a key factor to enhance superhydrophobicity. Moreover, it is shown that for millimetre-sized drops, the actual drop pressure at static equilibrium weakly affects the wetting regime, which instead seems to be dominated by the roughness parameter. For this reason a methodology to estimate the apparent contact angle is proposed, which relies only upon the micro-scale properties of the rough surface.

Role of statistical properties of randomly rough surfaces in controlling superhydrophobicity.

We investigate the effect of statistical properties of the surface roughness on its superhydrophobicity. In particular, we focus on the liquid-solid interfacial structure and its dependence on the coupled effect of surface statistical properties and drop pressure. We find that, for self-affine fractal surfaces with Hurst exponent H > 0.5, the transition to the Wenzel state first involves the short wavelengths of the roughness and, then, gradually moves to larger and larger scales. However, as the drop pressure is increased, at a certain point of the loading history, an abrupt transition to the Wenzel state occurs. This sudden transition identifies the critical drop pressure p(W), which destabilizes the composite interface. We find that p(W) can be strongly enhanced by increasing the mean square slope of the surface, or equivalently the Wenzel roughness parameter r(W). Our investigation shows that, even in the case of randomly rough surface, r(W) is still the most crucial parameter in determining the superhydrophobicity of the surface. An analytical approach is, then, proposed to show that, for any given value of Young's contact angle θ(Y), a threshold value (r(W))(th) = 1/(-cos θ(Y)) exists, above which the composite interface is strongly stabilized and the surface presents robust superhydrophobic properties. Interestingly, this threshold value is identical to the one that would be obtained in pure Wenzel regime to guarantee perfect superhydrophobicity.

Transvaginal estriol administration in postmenopausal women: a double blind comparative study of two different doses.

A group of 72 postmenopausal women were treated for 4 weeks with vaginal suppositories containing 0.5 or 1.0 mg of estriol. The two different doses achieved an identical significant improvement of urogenital symptoms, while a dose-related effect seen to be on climacteric complaints, according to a good absorption of estriol by the vaginal epithelium. Minimal side effects were observed and the safety of vaginal estriol treatment could advise further study about the effect of this kind of treatment on the climacteric syndrome.