Sealing is at the Origin of Rubber Slipping on Wet Roads

Loss of braking power and rubber skidding on a wet road is still an open physics problem, since neither the hydrodynamical effects nor the loss of surface adhesion that are sometimes blamed really manage to explain the 20-30% observed loss of low speed tire-road friction. Here we advance a novel mechanism based on sealing of water-filled substrate pools by the rubber. The sealed-in water effectively smoothens the substrate, thus reducing the viscoelastic dissipation in bulk rubber induced by surface asperities, well established as a major friction contribution. Starting with the measured spectrum of asperities one can calculate the water-smoothened spectrum and from that the predicted friction reduction, which is of the right magnitude. The theory is directly supported by fresh tire-asphalt friction data.Comment: 5 pages, 4 figures. Published on Nature Materials (November 7th 2004

Rubber friction on wet and dry road surfaces: the sealing effect

Rubber friction on wet rough substrates at low velocities is typically 20-30% smaller than for the corresponding dry surfaces. We show that this cannot be due to hydrodynamics and propose a novel explanation based on a sealing effect exerted by rubber on substrate "pools" filled with water. Water effectively smoothens the substrate, reducing the major friction contribution due to induced viscoelastic deformations of the rubber by surface asperities. The theory is illustrated with applications related to tire-road friction.Comment: Format Revtex 4; 8 pages, 11 figures (no color); Published on Phys. Rev. B (http://link.aps.org/abstract/PRB/v71/e035428); previous work on the same topic: cond-mat/041204

Rubber friction on (apparently) smooth lubricated surfaces

We study rubber sliding friction on hard lubricated surfaces. We show that even if the hard surface appears smooth to the naked eye, it may exhibit short wavelength roughness, which may give the dominant contribution to rubber friction. That is, the observed sliding friction is mainly due to the viscoelastic deformations of the rubber by the substrate surface asperities. The presented results are of great importance for rubber sealing and other rubber applications involving (apparently) smooth surfaces.Comment: 7 pages, 15 figure

On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion

Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the Atomic Force Microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.Comment: Topical review; 82 pages, 61 figures; Format: Latex (iopart). Some figures are in Postscript Level

Contact area between a viscoelastic solid and a hard, randomly rough, substrate

We study the time-dependent contact area as a viscoelastic solid is squeezed against a randomly rough substrate. Using a recently developed contact mechanics theory we study how the contact area depends on time and on the magnification zeta. Numerical results are presented for self-affine fractal surfaces, and applications to tack, rubber friction, and sealing are given

Rubber Friction on Wet Rough Substrates at Low Sliding Velocity: The Sealing Effect

Rubber friction on wet rough substrates at low velocities is typically 20-30% smaller than for the corresponding dry surfaces. We show that this cannot be due to hydrodynamics and propose a novel explanation based on a sealing effect exerted by rubber on substrate "pools" filled with water. Water effectively smoothens the substrate, reducing the major friction contribution due to induced viscoelastic deformations of the rubber by surface asperities. The theory is illustrated with applications related to tire-road friction

Crack propagation in rubber-like materials

Crack propagation in rubber-like materials is of great practical importance but still not well understood. We study the contribution to the crack propagation energy (per unit area) G from the viscoelastic deformations of the rubber in front of the propagating crack tip. We show that G takes the standard form G(v, T) = G(0)[1 + f (v, T)] where G(0) is associated with the (complex) bond-breaking processes at the crack tip while f (v, T) is determined by the viscoelastic energy dissipation in front of the crack tip. As applications, we discuss the role of crack propagation for adhesion, rolling resistance and sliding friction for smooth surfaces, and for rubber wear

On the nature of the static friction, kinetic friction and creep

In this paper, we discuss the nature of the static and kinetic friction, and of (thermally activated) creep. We focus on boundary lubrication at high confining pressure (similar to1 GPa), as is typical for hard solids, where one or at most two layers of confined molecules separates the sliding surfaces. We find in most of our Molecular Dynamics (MD) simulations (at low sliding velocity), that the lubricant molecules are permanently attached or pinned to one of the solid walls. We describe the (flexible) lubricant-wall bonds as springs with bending elasticity. If the springs are elastically stiff, the system exhibits a very small static friction, and a (low velocity) kinetic friction which increases with increasing sliding velocity. On the other hand, if the springs are soft enough, strong elastic instabilities occur during sliding, resulting in a large static friction force F,, and a kinetic friction force F-k equal to the static friction force at low sliding velocities. In this case rapid slip events occur at the interface, characterized by velocities much higher and independent of the drive velocity v. In the MD simulations we observe that, for incommensurate systems (at low temperature), only when the lubrication film undergoes a phase transformation at the onset of slip do we observe a static friction coefficient which is appreciately larger than the kinetic friction coefficient. We give arguments for why, at very low sliding velocity (where thermally activated creep occurs), the kinetic friction force may depend linearly on In (v/v(0)), as usually observed experimentally, rather than non-linearly [- In (v/v(0))](2/3) as predicted by a simple theory of activated processes. We also discuss the role of elasticity at stop and start. We show that for "simple" rubber (at low start velocity), the static friction coefficient (mu(s)) is equal to the kinetic friction coefficient (mu(k)). In general, at non-zero temperature, the static friction coefficient is higher than the kinetic friction coefficient because of various thermally activated relaxation processes, e.g. chain interdiffusion or (thermally activated) formation of capillary bridges. However, there is no single value of the static friction coefficient, since it depends upon the initial dwell time and on rate of starting. We argue that the correct basis for the Coulomb friction law, which states that the friction force is proportional to the normal load, is not the approximate independence of the friction coefficient on the normal pressure (which often does not hold accurately anyhow), but rather it follows from the fact that for rough surfaces the area of real contact is proportional to the load, and the pressure distribution in the contact areas is independent of the load. (C) 2003 Elsevier Science B.V. All rights reserved