The wetting/non-wetting properties of a liquid drop in contact with a chemically hydrophobic rough surface (thermodynamic contact angle theta(e)>pi/2) are studied for the case of an extremely idealized rough profile: the liquid drop is considered to lie on a simple sinusoidal profile. Depending on surface geometry and pressure values, it is found that the Cassie and Wenzel states can coexist. But if the amplitude h of the substrate is sufficiently large the only possible stable state is the Cassie one, whereas if h is below a certain critical value hcr a transition to the Wenzel state occurs. Since in many potential applications of such super-hydrophobic surfaces, liquid drops often collide with the substrate (e.g. vehicle windscreens), in the paper the critical drop pressure pW is calculated at which the Cassie state is no longer stable and the liquid jumps into full contact with the substrate (Wenzel state). By analyzing the asymptotic behavior of the systems in the limiting case of a large substrate corrugation, a simple criterion is also proposed to calculate the minimum height asperity h necessary to prevent the Wenzel state from being formed, to preserve the super-hydrophobic properties of the substrate, and, hence, to design a robust super-hydrophobic surface.

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In this paper we discuss the adhesion of a thin elastic plate to a randomly rough hard substrate. It is shown that at small magnification (long length scales) the plate, because of its higher compliance, is able to adhere in apparent full contact to the long wavelength corrugation of the underlying surface. That is, at length scales longer than the plate thickness, the gain in the adhesion energy upon the contact with the substrate overcomes the repulsive elastic energy produced by the elastic deformations, and the plate is able to fill out the large cavities of the rigid substrate. This produces a larger area of contact and an enhanced capability to adhere to a rough surface in comparison to the semi-infinite elastic solid case. However, at large enough magnification (small length scales) the plate behaves as a semi-infinite solid, and, depending on the roughness statistical properties, the area of true atomic contact may be much smaller than the nominal contact area.

We present a theoretical approach to estimate the fluid leakage in flat seals. The approach is based on the analogy between the seal-substrate interface and a porous medium. We assume that the interface is constituted of a random distribution of noncontact patches (the pores) and small but numerous contact spots (islands). Leakage may occur only through the pores, of which the lateral size and height are distributed according to a probability density function that we calculate on the basis of a recent theory of contact mechanics. Our theoretical approach is based on a percolation scheme that has never been proposed before and we believe it could be useful to stimulate further theoretical or experimental investigations. Within this percolation scheme we apply critical path analysis to calculate the hydraulic conductivity of the medium and compare our predictions with other calculations very recently presented to the scientific community.

This paper is concerned with the shifting behavior of a metal belt CVT. The calculations are performed for the chain belt case by using a one-dimensional model of the belt: the radial thickness of the belt is neglected. The friction forces are modeled on the basis of the Coulomb friction hypothesis. The deformation of the belt, i.e., the variation of its transversal width, is shown to be negligible with respect to the variation of the local groove width caused by the elastic deformation of the pulleys and by the clearance in the bearings. The particular shape of the deformed pulley is described on the basis of Sattler model (1999) who showed that the variation of the groove angle and that one of the local groove width of the pulley can be easily described by simple trigonometric formulas. The paper shows that the characteristic behavior of the transmission during slow shifting maneuvers, referred to as “creep mode,” is caused by the bending of the pulleys, that is to say for rigid pulleys no “creep mode” can be observed. Moreover, the model shows that increasing the rate of change of speed ratio a transition from the “creep-mode” to the so called “slip-mode” behavior of the variator takes place, as experimentally observed.

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