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O-rings made from foam rubber are often used in sealing applications. Foam rubber have low (macroscopic) elastic modulus E 0 resulting in a low nominal contact pressure when squeezed against a countersurface. In most cases the foam rubber is covered by a thin surface film with the effective elastic modulus E 1 > E 0 . We show that the nominal contact pressure may not be high enough for the contact area to percolate and the O-ring seal will leak. For the leakage calculations we use the Persson multiscale contact mechanics theory, and the (modified) Bruggeman effective medium theory for the fluid flow conductivity. The experimental input for the theory are surface roughness power spectrum, which was obtained from stylus topography measurements, and the elastic properties ( E 0 and E 1 ) of the rubber O-ring. As an application of this calculation method, we have used the preliminary as well as the final results of the laboratory gas tightness tests of the 136 New Small Wheel Micromegas Quadruplets performed at CERN, from February 2019 to May 2021, in the framework of the ATLAS Experiment upgrade. In the integration quality control, a novel method for gas tightness measurement, that we have called "Flow Rate Loss", has been used as a baseline method. [ABSTRACT FROM AUTHOR]

Bodies made from elastically stiff material usually bind very weakly unless the surfaces are flat and extremely smooth. In direct wafer bonding flat surfaces bind by capillary bridges and by the van der Waals interaction, which act between all solid objects. Here we study the dependency of the work of adhesion on the humidity and surface roughness in hydrophilic direct wafer bonding. We show that the long-wavelength roughness (usually denoted waviness) has a negligible influence on the strength of wafer bonding (the work of adhesion) from the menisci that form from capillary condensation of water vapor. AFM surface topography for Si(100) wafer [ABSTRACT FROM AUTHOR]

When a body is exposed to external forces large local stresses may occur at the surface because of surface roughness. Surface stress concentration is important for many application and in particular for fatigue due to pulsating external forces. For randomly rough surfaces, I calculate the probability distribution of surface stress in response to a uniform external tensile stress with the displacement vector field parallel to the rough surface. I present numerical simulation results for the stress distribution σ (x , y) and show that in a typical case the maximum local tensile stress may be ∼ 10 times bigger than the applied stress. I discuss the role of the stress concentration on plastic deformation and surface crack generation and propagation. [ABSTRACT FROM AUTHOR]

In most practical applications, surface roughness is characterized by just one or two parameters (numbers). I show that the standard maximum surface height parameters fluctuate strongly between different surface realizations (or measurements), and should not be used in the design of engineering components. I show how some roughness parameters depend on the size of the roll-off region in the surface roughness power spectra, and introduce a new height parameter which is very reproducible. The numerical results presented agree well with experimental observations. [ABSTRACT FROM AUTHOR]

I study the heat transfer in systems of weakly interacting particles at low confining pressure in the normal atmosphere. The particles have surface roughness with self-affine fractal properties, as expected for mineral particles produced by fracture, e.g., by crunching brittle materials in a mortar. I show that for small particles (say < 10 μ m ) with hydrophilic surfaces, at large humidity water capillary bridges dominate the heat transfer, while for big particles heat transfer via the air dominates. This differs from vacuum conditions where the propagating electromagnetic (EM) waves give the dominant heat transfer for large particles, while for small particles both the evanescent EM-waves and the phononic contribution from the area of real contact are most important. [ABSTRACT FROM AUTHOR]

A press fit, also known as interference fit or friction fit, is a form of fastening between two tight fitting mating parts (usually two bodies with cylinder or conical surfaces) that produces a joint which is held together by friction after the parts are pushed together. I discuss the influence of surface roughness on the design of press fits. This topic has been addressed in the engineering community but only on an empirical level without a scientific backup. Here, I will apply the Persson contact mechanics theory to show how to include the surface roughness in the design criteria. I argue that one should use what I denote as the cylinder "stylus width" rather than the "caliper width" when determining the influence of the surface roughness of the compression (also denoted as the interference). In the classical approach using the caliper width, the compression is assumed to be independent of the elastic properties of the solids, but in the more accurate approach presented here using the stylus width the compression depends on the elastic properties and on the surface roughness power spectra of the involved solids. A detailed discussion of the relation between the root-mean-square roughness amplitude h rms and the maximum asperity height h max , of interest in its own right, is also presented as it is needed for determining the relation between the stylus and caliper derived compression's. [ABSTRACT FROM AUTHOR]

When an electric current flow across an interface between two contacting solids a change in the electric potential occur over the interface. One part of the electric contact resistance is due to the constrictions at asperity contact regions. Barber has shown that the constriction resistance can be related to the interfacial contact stiffness. I discuss the relation between the classical (approximate) Holm theory for the constriction resistance and the (exact) theory involving the contact stiffness. [ABSTRACT FROM AUTHOR]

Interfacial surface roughness can result in fluid leakage of seals, and in the design of seals it is standard to give an upper limit for the surface root-mean-square (rms) roughness amplitude h rms . However, h rms is determined mainly by the long-wavelength roughness, which is (nearly) irrelevant for the sealing. I discuss the parameters which determine the leakage of seals, and present results for static rubber seals with circular cross-section (like rubber O-rings). I also study the influence of the fluid pressure on the interfacial surface separation and the leakrate. [ABSTRACT FROM AUTHOR]

Most asteroids with a diameter larger than ∼ 300 m are rubble piles, i.e., consisting of more than one solid object. All asteroids are rotating but almost all asteroids larger than ∼ 300 m rotate with a period longer than 2.3 hours , which is the critical period where the centrifugal force equals the gravitational force. This indicates that there are nearly no adhesive interaction forces between the asteroid fragments. We show that this is due to the surface roughness of the asteroid particles which reduces the van der Waals interaction between the particles by a factor of 100 for micrometer sized particles and even more for larger particles. We show that surface roughness results in an interaction force which is independent of the size of the particles, in contrast to the linear size dependency expected for particles with smooth surfaces. Thus, two stone fragments of size 100 nm attract each other with the same non-gravitational force as two fragments of size 10 m . [ABSTRACT FROM AUTHOR]

We study the leakage of fluids (liquids or gases) in syringes with glass barrel, steel plunger and rubber O-ring stopper. The leakrate depends on the interfacial surface roughness and on the viscoelastic properties of the rubber. Random surface roughness is produced by sandblasting the rubber O-rings. We present a very simple theory for gas flow which takes into account both the diffusive and ballistic flow. The theory shows that the interfacial fluid flow (leakage) channels are so narrow that the gas flow is mainly ballistic (the so called Knudsen limit). We compare the leakrate obtained using air and helium. For barrels filled with water we observe no leakage even if leakage occurs for gases. We interpret this as resulting from capillary (Laplace pressure or surface energy) effects. [ABSTRACT FROM AUTHOR]

We show that the Persson-Brener theory of crack propagation in viscoelastic solids gives a viscoelastic fracture energy factor G / G 0 = 1 + f which is nearly the same as the viscoelastic factor obtained using the cohesive-zone model. We also discuss finite size effects and comment on the use of crack propagation theories for "solids" with a viscoelastic modulus that vanishes at zero frequency. [ABSTRACT FROM AUTHOR]

Face masks are used to trap particles (or fluid drops) in a porous material (filter) in order to avoid or reduce the transfer of particles between the human lungs (or mouth and nose) and the external environment. The air exchange between the lungs and the environment is assumed to occur through the face mask filter. However, if the resistance to air flow through the filter is high some air (and accompanied particles) will leak through the filter-skin interface. In this paper I will present a model study of the side-leakage problem. [ABSTRACT FROM AUTHOR]

We study the influence of rubber transfer films on the sliding friction between rectangular rubber blocks and a concrete surface. We present experimental results for the friction coefficient for a rubber compound sliding on a concrete surface contaminated by another rubber compound, for two different pairs (A, B) and (C, D) of rubber compounds. For the same rubber compounds, we present theory results which illustrate the relative importance of the viscoelastic and adhesive contribution to the sliding friction. We correlate the calculated rubber friction with the nature of the observed transfer films (or wear processes). [ABSTRACT FROM AUTHOR]

We study the nominal (ensemble averaged) contact pressure p(x) acting on a cylinder squeezed in contact with an elastic half space with random surface roughness. The contact pressure is Hertzian-like for α < 0.01 and Gaussian-like for α > 10 , where the dimensionless parameter α = h rms / δ is the ratio between the root-mean-square roughness amplitude and the penetration for the smooth surfaces case (Hertz contact). [ABSTRACT FROM AUTHOR]

I consider fluid flow at the interface between solids with random roughness. For anisotropic roughness, obtained by stretching isotropic roughness in the x-direction by a factor of γ 1 / 2 and in the y-direction with a factor of γ - 1 / 2 , I give an argument for why the flow conductivity in the critical junction theory should be proportional to γ in the x-direction and proportional to 1 / γ in the y-direction. [ABSTRACT FROM AUTHOR]

When a crack propagates in a viscoelastic solid, energy dissipation can occur very far from the crack tip where the stress field may be very different from the r - 1 / 2 singular form expected close to the crack tip. Most theories of crack propagation focus on the near crack tip region. Remarkable, here I show that a simple theory which does not account for the nature of the stress field in the near crack tip region results in a crack propagation energy in semiquantitative agreement with a theory based on the stress field in the near crack tip region. I consider both opening and closing crack propagation and show that for closing crack propagation in viscoelastic solids, some energy dissipation processes must occur in the crack tip process zone. The theory is illustrated by new experimental results for the adhesive interaction between a silica glass ball and a silicone rubber surface. [ABSTRACT FROM AUTHOR]

We review a theory of crack propagation in viscoelastic solids. We consider both cracks in infinite systems and in finite-sized systems. As applications of the theory we consider two adhesion problems, namely pressure sensitive adhesives and the ball-flat adhesion problem. We also study crack propagation in the pig skin dermis, which is of medical relevance, and rubber wear in the context of tires. [ABSTRACT FROM AUTHOR]

The contact between rough metallic bodies almost always involves plastic flow in the area of real contact. We performed indentation experiments on sandblasted aluminum surfaces to explore the plastic deformation of asperities and modeled the contact mechanics using the boundary element method, combined with a simple numerical procedure to take into account the plastic flow. The theory can quantitatively describe the modification of the roughness by the plastic flow. Since the long-wavelength roughness determines the fluid leakage of metallic seals in most cases, we predict that the leakage can be estimated based on the elastoplastic contact mechanics model employed here. [ABSTRACT FROM AUTHOR]

Metallic seals are crucial machine elements in many important applications, e.g., in ultrahigh vacuum systems. Due to the high elastic modulus of metals, and the surface roughness which exists on all solid surfaces, if no plastic deformation would occur one expects in most cases large fluid flow channels between the contacting metallic bodies, and large fluid leakage. However, in most applications plastic deformation occurs, at least at the asperity level, which allows the surfaces to approach each other to such an extent that fluid leakage often can be neglected. In this study, we present an experimental set-up for studying the fluid leakage in metallic seals. We study the water leakage between a steel sphere and a steel body (seat) with a conical surface. The experimental results are found to be in good quantitative agreement with a (fitting-parameter-free) theoretical model. The theory predicts that the plastic deformations reduce the leak-rate by a factor ≈ 8 . [ABSTRACT FROM AUTHOR]

I discuss fluid flow at the interface between solids with anisotropic roughness. I show that the Bruggeman effective medium theory and the critical junction theory give nearly the same results for the fluid flow conductivity. This shows that, in most cases, the surface roughness observed at high magnification is irrelevant for fluid flow problems such as the leakage of static seals, and fluid squeeze-out. The effective medium theory predicts that the fluid flow conductivity vanishes at the relative contact area A/A0 = 0.5 independent of the anisotropy. However, the effective medium theory does not solve the elastic contact mechanics problem but is based on a purely geometric argument. Thus, for anisotropic roughness the contact area may percolate at different values of A/A0 depending on the direction. We discuss how this may be taken into account in the effective medium and critical junction theories. [ABSTRACT FROM AUTHOR]

We study a simple model of a conveyor belt. We calculate the relation between the torque M and the belt–pulley slip s. We present numerical results for the M(s) relation, and for the frictional energy dissipation as a function of the slip, which is important for the rubber wear. The theory includes both the belt compliance and the pulley tread block stiffness. The theory shows that to minimize the friction and the related rubber wear, the pulley–belt system should be operated not at the slip where the torque is maximal, but at a lower slip. Thus, the belt–pulley system should be slightly "oversized" for the best performance. [ABSTRACT FROM AUTHOR]

We derive approximate mean field equations for the fluid flow between elastic solids with randomly rough surfaces including interfacial fluid slip and shear thinning. We present numerical results for the fluid flow and friction factors for realistic systems, in particular, we consider the case of an elastic cylinder with random surface roughness in relative sliding contact with a flat rigid (low-energy) counter-surface. We present experimental data for the sliding friction between rubber stoppers and glass barrels lubricated with baked-on silicone oil. We find that the frictional shear stress acting in the rubber asperity contact regions is nearly velocity independent for velocities in the 10-1000μm/s range, and very small τ f ≈ 0. 04 MPa, while for bare glass in silicone oil τ f is much larger and velocity dependent. [ABSTRACT FROM AUTHOR]

In their interesting study, Hu et al. (Tribol Lett 68:8, 2019, 10.1007/s11249-019-1247-7) have shown that for a simple "harmonium" solid model, the slip-induced motion of surface atoms is close to critically damped. This result is in fact well known from studies of vibrational damping of atoms and molecules at surfaces. However, for real practical cases, the situation may be much more complex and the conclusions of Hu et al are invalid. [ABSTRACT FROM AUTHOR]

We discuss the origin of the low friction observed for some elastically soft materials, such as hydrogels and the human cartilage. When a soft elastic solid is squeezed against another solid in a fluid, if the nominal contact pressure is high enough, after long enough contact time the contact area percolates, resulting in islands of confined fluid. For charged surfaces, with charges of equal sign, the osmotic pressure in the area of 'real contact' may be large enough to keep the surfaces separated at nanometer separation. In this case the solid contact pressure (here the osmotic pressure) in the area of 'real contact' will be nearly independent of the external load. The finite surface separation in the area of 'real contact' results in a very small (breakloose) friction force even after long time of stationary contact. [ABSTRACT FROM AUTHOR]

A huge number of technological and biological systems involve the lubricated contact between rough surfaces of soft solids in relative accelerated motion. Examples include dynamical rubber seals and the human joints. In this study, we consider an elastic cylinder with random surface roughness in accelerated sliding motion on a rigid, perfectly flat (no roughness) substrate in a fluid. We calculate the surface deformations, interface separation and the contributions to the friction force and the normal force from the area of real contact and from the fluid. The driving velocity profile as a function of time is assumed to be either a sine function, or a linear multi-ramp function. We show how the squeeze-in and squeeze-out processes, occurring in accelerated sliding, quantitatively affect the Stribeck curve with respect to the steady sliding. Finally, the theory results are compared to experimental data. [ABSTRACT FROM AUTHOR]

This article has been retracted. Please see the Retraction Notice for more detail: https://doi.org/10.1134/S0021364023360018 [ABSTRACT FROM AUTHOR]

Fluid leakage out of mechanical equipment such as gearboxes, hydraulic systems, or fuel tanks could cause serious problems and thus should be avoided. Seals are extremely useful devices for preventing such fluid leakages. We have developed a theoretical approach for calculation of the leak rate of stationary rubber seals and the friction force for dynamic seals. The theory is based on a recently developed theory of contact mechanics, which we briefly review. To test the theory, we have performed both simple model experiments and experiments on engineering seal systems. We have found good agreement between the calculated and measured results, and hence our theory has the potential to improve the future design of efficient seals. We briefly review the processes that determine rubber friction on lubricated smooth and rough substrate surfaces. We present experimental friction results for lubricated surfaces, obtained using a simple Leonardo da Vinci setup. The data is analyzed using the Persson rubber friction and contact mechanics theory. [ABSTRACT FROM AUTHOR]

Quantum Cherenkov radiation and quantum friction at the relative sliding of two transparent plates with the refractive index n have been studied in a fully relativistic theory. Radiation appears at velocities above the threshold value, v > v = 2 nc/( n + 1). The contribution from s-polarized electromagnetic waves dominates near the threshold velocity. However, in the ultrarelativistic case ( v → c), contributions from both polarizations are much larger than those in a nonrelativistic theory and a new contribution from the mixing of waves with different polarizations appears. The numerical results are supplemented by analytical calculations near the threshold velocity and the speed of light. [ABSTRACT FROM AUTHOR]

Quantum Cherenkov radiation and quantum friction at the motion of a small neutral particle parallel to the surface of a transparent dielectric with the refractive index n have been studied in a fully relativistic theory. Radiation appears at velocities above the threshold value, v > v = c/ n. The friction force in the particle-plate configuration has been derived from the friction force in the plate-plate configuration under the assumption that one of the plates is significantly decharged. A decrease in the kinetic energy of the particle near the threshold velocity is due to its radiation and near the speed of light is determined by the heat power absorbed by the particle in the rest frame. The powers of quantum and classical Cherenkov radiation can be comparable in the relativistic case. [ABSTRACT FROM AUTHOR]

We measured the Cl concentration of the lateral intercellular spaces (LIS) of MDCK cell monolayers, grown on glass coverslips, by video fluorescence microscopy. Monolayers were perfused at 37 degrees C either with HEPES-buffered solutions containing 137 mM Cl or bicarbonate/CO2-buffered solutions containing 127 mM Cl. A mixture of two fluorescent dyes conjugated to dextrans (MW 10,000) was microinjected into domes and allowed to diffuse into the nearby LIS. The Cl-sensitive dye, ABQ-dextran, was selected because of its responsiveness at high Cl concentrations; a Cl-insensitive dye, Cl-NERF-dextran, was used as a reference. Both dyes were excited at 325 nm, and ratios of the fluorescence intensity at spectrally distinct emission wavelengths were obtained from two intensified CCD cameras, one for ABQ-dextran the other for Cl-NERF-dextran. LIS Cl concentration was calibrated in situ by treating the monolayer with digitonin or ouabain and varying the perfusate Cl between 0 and 137 mM (HEPES buffer) or between 0 and 127 mM (bicarbonate/CO2 buffer). LIS Cl in HEPES-buffered solutions averaged 176 +/- 19 mM (n = 12), calibrated with digitonin, and 170 +/- 9 mM (n = 12), calibrated with ouabain. LIS Cl in bicarbonate/CO2-buffered solutions averaged 174 +/- 10 mM (n = 7) using the ouabain calibration. The Cl concentration of MDCK cell domes, measured with Cl-sensitive microelectrodes and by microspectrofluorimetry, did not differ significantly. Images of the LIS at 3 focal planes, near the tight junction, midway and basal, failed to reveal any gradients in Cl concentration along the LIS. LIS Cl changed rapidly in response to perfusate Cl with characteristic times of 0.8 +/- 0.1 min (n = 21) for Cl decrease and 0.3 +/- 0.04 min (n = 21) for Cl increase. In conclusion, (i) Cl concentration is higher in the LIS than in the bathing medium, (ii) no gradients of Cl along the depth of LIS are detectable, (iii) junctional Cl permeability is high. [ABSTRACT FROM AUTHOR]

When a rubber block is squeezed against a nominal flat but rough surface, the rubber bottom surface will penetrate into the substrate roughness profile. The relation between penetration depth $$w$$ (or the average interfacial separation $$\bar{u}$$ ) and the applied squeezing pressure $$p$$ determines the (perpendicular) contact stiffness $$K=\hbox {d}p/\hbox {d}w=-\hbox {d}p/\hbox {d}\bar{u}$$ , which is important for many applications. We have measured the relation between $$p$$ and $$\bar{u}$$ for a rubber block squeezed against 28 different concrete and asphalt road surfaces. We find a linear relation between $${\mathrm{log}}p$$ and $$\bar{u}$$ , in agreement with theory predictions. The measured stiffness values correlate rather well with the theory prediction. [ABSTRACT FROM AUTHOR]

The energy dissipation in the contact regions between solids in sliding contact can result in high local temperatures which may strongly affect the friction. This is the case for rubber sliding on road surfaces at speeds above 1 mm/s. I derive equations which describe the frictional heating for arbitrary (non-uniform) motion, taking into account that some of the frictional energy is produced inside the rubber due to the internal friction in rubber. Numerical results are presented for one limiting case for steady sliding. [ABSTRACT FROM AUTHOR]

The fluid flow through a seal interface depends on the percolating non-contact channels morphology, size and length, and on the interfacial surface energies. In particular, hydrophobic interfaces may expel fluids and decrease the fluid flow of seals, while increasing the sliding friction. We present results of interfacial fluid flow experiments on a hydrostatic column device which demonstrate how interfacial hydrophobicity can block fluid flow at interfaces and reduce the leak rate of seals. The presented results may help to understand the role of interfacial hydrophobicity in many practical applications, some of which we discuss briefly in this paper, e.g., rubber wiper blades on hydrophobic (usually wax-coated) glass, the locomotion of insects on surfaces in water, and syringes. Graphical abstract: [ABSTRACT FROM AUTHOR]

Most natural surfaces and surfaces of engineering interest, e.g., polished or sandblasted surfaces, are self-affine fractal over a wide range of length scales, with the fractal dimension $$D_{\mathrm{f}} = 2.15\pm 0.15$$ . We give several examples illustrating this and a simple argument, based on surface fragility, for why the fractal dimension usually is <2.3. A kinetic model of sandblasting is presented, which gives surface topographies and surface roughness power spectra in good agreement with experiments. [ABSTRACT FROM AUTHOR]

The dependence of the thermal component of the Casimir force and Casimir friction between graphene sheets on the drift velocity of charge carriers in one of the sheets has been analyzed. It has been shown that the drift motion results in the measurable change in the thermal Casimir force owing to the Doppler effect. The thermal Casimir force, as well as Casimir friction, increases strongly in the case of resonant photon tunneling, when the energy of an emitted photon coincides with the excitation energy of an electron-hole pair. In the case of resonant photon tunneling, the dominant contribution to the Casimir friction even at temperatures above room temperature comes from quantum friction caused by quantum fluctuations. Quantum friction can be detected in an experiment on the friction drag between graphene sheets in a high electric field. [ABSTRACT FROM AUTHOR]

The surface topography of the human wrist skin is studied using an optical method and the surface roughness power spectrum is obtained. The Persson contact mechanics theory is used to calculate the contact area for different magnifications, for both dry and wet condition of the skin. For dry skin, plastic yielding becomes important and will determine the area of contact observed at the highest magnification. The measured friction coefficient [M.J. Adams et al., Tribol Lett 26:239, 2007] on both dry and wet skin can be explained assuming that a frictional shear stress σ ≈ 15 MPa acts in the area of real contact during sliding. This frictional shear stress is typical for sliding on polymer surfaces, and for thin (nanometer) confined fluid films. The big increase in the friction, which has been observed for glass sliding on wet skin as the skin dries up, can be explained as resulting from the increase in the contact area arising from the attraction of capillary bridges. This effect is predicted to operate as long as the water layer is thinner than ∼14 μm, which is in good agreement with the time period (of order 100 s) over which the enhanced friction is observed (it takes about 100 s for ∼14 μm water to evaporate at 50% relative humidity and at room temperature). We calculate the dependency of the sliding friction coefficient on the sliding speed on lubricated surfaces (Stribeck curve). We show that sliding of a sphere and of a cylinder gives very similar results if the radius and load on the sphere and cylinder are appropriately related. When applied to skin the calculated Stribeck curve is in good agreement with experiment, except that the curve is shifted by one velocity-decade to higher velocities than observed experimentally. We explain this by the role of the skin and underlying tissues viscoelasticity on the contact mechanics. [ABSTRACT FROM AUTHOR]

We study the time dependency of the interfacial separation and of the area of real contact between a soft elastic cylinder and a rigid solid with a randomly rough surface, squeezed with normal approach in a fluid. This problem is relevant for biological, as well as for bio-medical, seals and tires applications. An ad-hoc numerical scheme is developed to solve the transient mixed-EHD homogenized lubrication problem. We show that, for the soft contact case, the transition from the EHD to the boundary regime can be much more efficiently studied within a simplified (Grubin-like) problem formulation then with the full numerical scheme. This is not only of great conceptional importance, but also of practical importance as the latter calculation is much simpler and faster than the full scheme calculation. [ABSTRACT FROM AUTHOR]

The dynamics of fluid flow at the interface between elastic solids with rough surfaces depends sensitively on the area of real contact, in particular close to the percolation threshold, where an irregular network of narrow flow channels prevails. In this paper, numerical simulation and experimental results for the contact between elastic solids with isotropic and anisotropic surface roughness are compared with the predictions of a theory based on the Persson contact mechanics theory and the Bruggeman effective medium theory. The theory predictions are in good agreement with the experimental and numerical simulation results and the (small) deviation can be understood as a finite-size effect. The fluid squeeze-out at the interface between elastic solids with randomly rough surfaces is studied. We present results for such high contact pressures that the area of real contact percolates, giving rise to sealed-off domains with pressurized fluid at the interface. The theoretical predictions are compared to experimental data for a simple model system (a rubber block squeezed against a flat glass plate), and for prefilled syringes, where the rubber plunger stopper is lubricated by a high-viscosity silicon oil to ensure functionality of the delivery device. For the latter system we compare the breakloose (or static) friction, as a function of the time of stationary contact, to the theory prediction. [ABSTRACT FROM AUTHOR]

We have measured the friction force acting on a rubber block slid on a concrete surface. We used both unfilled and filled (with carbon black) styrene butadiene (SB) rubber and have varied the temperature from −10 °C to 100 °C and the sliding velocity from 1 μm/s to 1000 μm/s. We find that the experimental data at different temperatures can be shifted into a smooth master-curve, using the temperature-frequency shifting factors obtained from measurements of the bulk viscoelastic modulus. The experimental data has been analyzed using a theory which takes into account the contributions to the friction from both the substrate asperity-induced viscoelastic deformations of the rubber, and from shearing the area of real contact. For filled SB rubber the frictional shear stress σ in the area of real contact results mainly from the energy dissipation at the opening crack on the exit side of the rubber-asperity contact regions. For unfilled rubber we instead attribute σ to shearing of a thin rubber smear film, which is deposited on the concrete surface during run in. We observe very different rubber wear processes for filled and unfilled SB rubber, which is consistent with the different frictional processes. Thus, the wear of filled SB rubber results in micrometer-sized rubber particles which accumulate as dry dust, which is easily removed by blowing air on the concrete surface. This wear process seams to occur at a steady rate. For unfilled rubber a smear film forms on the concrete surface, which cannot be removed even using a high-pressure air stream. In this case the wear rate appears to slow down after some run in time period. [ABSTRACT FROM AUTHOR]

We study the fluid flow at the interface between elastic solids with randomly rough surfaces. We derive (approximate) analytical expressions for the fluid flow factors which enter in the equation describing the fluid flow, and for the frictional shear stress factors which enter in the equation for the frictional shear stress. Numerical results for a rubber cylinder with surface roughness sliding on a flat lubricated substrate, under 'low' and 'high' pressure conditions, are presented and discussed. Finally we discuss the role of the fluid-induced elastic deformations of the surface roughness profile. [ABSTRACT FROM AUTHOR]

We calculate the friction force acting on a hard cylinder or spherical ball rolling on a flat surface of a viscoelastic solid. The rolling-friction coefficient depends non-linearly on the normal load and the rolling velocity. For a cylinder rolling on a viscoelastic solid characterized by a single relaxation time Hunter has obtained an exact result for the rolling friction, and our result is in very good agreement with his result for this limiting case. The theoretical results are also in good agreement with experiments of Greenwood and Tabor. We suggest that measurements of rolling friction over a wide range of rolling velocities and temperatures may constitute a useful way to determine the viscoelastic modulus of rubber-like materials. [ABSTRACT FROM AUTHOR]

We study the time dependency of the (average) interfacial separation between an elastic solid with a flat surface and a rigid solid with a randomly rough surface, squeezed together in a fluid. As an application we discuss fluid squeeze-out between a tire tread block and a road surface. Some implications for the leakage of seals are discussed, and experimental results are presented to test the theory. The theoretical prediction for the leak rate as a function of the fluid pressure difference is in good agreement with the experimental data for all fluid pressures up to “lift-off”, where the fluid pressure equals the nominal rubber-substrate squeezing pressure. [ABSTRACT FROM AUTHOR]

Seals are extremely useful devices to prevent fluid leakage. We present an effective-medium theory of the leak rate of rubber seals, which is based on a recently developed contact mechanics theory. We compare the theory with experimental results for seals consisting of silicon rubber in contact with sandpaper and sand-blasted PMMA surfaces. [ABSTRACT FROM AUTHOR]

We study the heat transfer between elastic solids with randomly rough surfaces. We include both the heat transfer from the area of real contact, and the heat transfer between the surfaces in the non-contact regions. We apply a recently developed contact mechanics theory, which accounts for the hierarchical nature of the contact between solids with roughness on many different length scales. For elastic contact, at the highest (atomic) resolution the area of real contact typically consists of atomic (nanometer) sized regions, and we discuss the implications of this for the heat transfer. For solids with very smooth surfaces, as is typical in many modern engineering applications, the interfacial separation in the non-contact regions will be very small, and for this case we show the importance of the radiative heat transfer associated with the evanescent electromagnetic waves which exist outside of all bodies. [ABSTRACT FROM AUTHOR]