arXiv subjects

B. N. J. Persson

Publications and source records attributed to B. N. J. Persson.

At least 91 records · Page 5Linked to original sources

arXiv2010

Fluid flow at the interface between elastic solids with randomly rough surfaces

I study fluid flow at the interface between elastic solids with randomly rough surfaces. I use the contact mechanics model of Persson to take into account the elastic interaction between the solid walls and the Bruggeman effective medium theory to account for the influence of the disorder on the fluid flow. I calculate the flow tensor which determines the pressure flow factor and, e.g., the leak-rate of static seals. I show how the perturbation treatment of Tripp can be extended to arbitrary order in the ratio between the root-mean-square roughness amplitude and the average interfacial surface separation. I introduce a matrix D(Zeta), determined by the surface roughness power spectrum, which can be used to describe the anisotropy of the surface at any magnification Zeta. I present results for the asymmetry factor Gamma(Zeta) (generalized Peklenik number) for grinded steel and sandblasted PMMA surfaces.

cond-mat.soft↗

arXiv2010

On the dependence of the leak-rate of seals on the skewness of the surface height probability distribution

Seals are extremely useful devices to prevent fluid leakage. We present experimental result which show that the leak-rate of seals depend sensitively on the skewness in the height probability distribution. The experimental data are analyzed using the critical-junction theory. We show that using the top-power spectrum result in good agreement between theory and experiment.

cond-mat.soft↗

arXiv2009

Velocity dependence of friction of confined polymers

We present molecular dynamics friction calculations for confined hydrocarbon solids with molecular lengths from 20 to 1400 carbon atoms. Two cases are considered: (a) polymer sliding against a hard substrate, and (b) polymer sliding on polymer. We discuss the velocity dependence of the frictional shear stress for both cases. In our simulations, the polymer films are very thin (approx. 3 nm), and the solid walls are connected to a thermostat at a short distance from the polymer slab. Under these circumstances we find that frictional heating effects are not important, and the effective temperature in the polymer film is always close to the thermostat temperature. In the first setup (a), for hydrocarbons with molecular lengths from 60 to 1400 carbon atoms, the shear stresses are nearly independent of molecular length, but for the shortest hydrocarbon C20H42 the frictional shear stress is lower. In all cases the frictional shear stress increases monotonically with the sliding velocity. For polymer sliding on polymer [case (b)] the friction is much larger, and the velocity dependence is more complex. For hydrocarbons with molecular lengths from 60 to 140 C-atoms, the number of monolayers of lubricant increases (abruptly) with increasing sliding velocity (from 6 to 7 layers), leading to a decrease of the friction. Before and after the layering transition, the frictional shear stresses are nearly proportional to the logarithm of sliding velocity. For the longest hydrocarbon (1400 C-atoms) the friction shows no dependence on the sliding velocity, and for the shortest hydrocarbon (20 C-atoms) the frictional shear stress increases nearly linearly with the sliding velocity.

cond-mat.soft↗

arXiv2009

Leak-rate of seals: effective medium theory and comparison with experiment

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.

cond-mat.soft↗

arXiv2009

Frictional Drag induced in Low- Dimensional Systems by Brownian Motion of Ions in Liquid Flow

We study the frictional drag force in low-dimensional systems (2D-electron and 2D-liquid systems) mediated by a fluctuating electromagnetic field which originate from Brownian motion of ions in liquid. The analysis is focused on the [2D-system-2D-system], [2D-system-semi-infinite liquid], and [2D-system-infinite liquid] configurations. We show that for 2D-electron systems the friction drag depends linearly on the relative velocity of the free carries of charge in the different media, but for 2D-liquid systems the frictional drag depends nonlinear on the relative velocity. For 2D-systems the frictional drag force induced by liquid flow may be several orders of magnitude larger than the frictional drag induced by an electronic current.

cond-mat.mes-hall↗

arXiv2009

Heat transfer between elastic solids with randomly rough surfaces

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 noncontact 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.

cond-mat.soft↗

arXiv2009

Comment on "No quantum friction between uniformly moving plates"

Quite recently Philbin et al present new theory of the van der Waals friction at zero temperature. Contrary to the previous theory they claimed that there is no "quantum friction" due to quantum fluctuation of the electromagnetic field between two parallel plates moving relative to each other. We show that this theory is incorrect.

cond-mat.other↗

arXiv2009

Leak-rate of seals: comparison of theory with experiment

Seals are extremely useful devices to prevent fluid leakage. We present experimental results for the leak-rate of rubber seals, and compare the results to a novel theory, which is based on percolation theory and a recently developed contact mechanics theory. We find good agreement between theory and experiment.

cond-mat.soft↗

arXiv2008

Experimental Interpretation of Interfacial Separation and Squeezing Pressure

The logarithmic relation between interfacial separation and squeezing pressure between randomly rough surfaces, has been predicted by both theory and experiment. However, the experimental slope between interfacial separation and logarithmic squeezing pressure, is slightly bigger than that predicted by theory. Here we present a detailed explanation on the slope difference between theory and experiment.

cond-mat.soft↗

arXiv2008

Interfacial separation between elastic solids with randomly rough surfaces: comparison of experiment with theory

We study the average separation between an elastic solid and a hard solid with a nominal flat but randomly rough surface, as a function of the squeezing pressure. We present experimental results for a silicon rubber (PDMS) block with a flat surface squeezed against an asphalt road surface. The theory shows that an effective repulse pressure act between the surfaces of the form p proportional to exp(-u/u0), where u is the average separation between the surfaces and u0 a constant of order the root-mean-square roughness, in good agreement with the experimental results.

cond-mat.soft↗

arXiv2008

Contact mechanics with adhesion: Interfacial separation and contact area

We study the adhesive contact between elastic solids with randomly rough, self affine fractal surfaces. We present molecular dynamics (MD) simulation results for the interfacial stress distribution and the wall-wall separation. We compare the MD results for the relative contact area and the average interfacial separation, with the prediction of the contact mechanics theory of Persson. We find good agreement between theory and the simulation results. We apply the theory to the system studied by Benz et al. involving polymer in contact with polymer, but in this case the adhesion gives only a small modification of the interfacial separation as a function of the squeezing pressure.

cond-mat.soft↗

arXiv2008

Van der Waals Frictional Drag induced by Liquid Flow in Low- Dimensional Systems

We study the van der Waals frictional drag force induced by liquid flow in low-dimensional systems (2D and 1D electron systems, and 2D and 1D channels with liquid). We find that for both 1D and 2D systems, the frictional drag force induced by liquid flow may be several orders of magnitude larger than the frictional drag induced by electronic current.

cond-mat.other↗

arXiv2008

Nanodroplets on rough hydrophilic and hydrophobic surfaces

We present results of Molecular Dynamics (MD) calculations on the behavior of liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On hydrophobic surfaces, the contact angle for nanodroplets depends strongly on the root mean square roughness amplitude, but it is nearly independent of the fractal dimension of the surface. Since increasing the fractal dimension increases the short-wavelength roughness, while the long-wavelength roughness is almost unchanged, we conclude that for hydrophobic interactions the short-wavelength (atomistic) roughness is not very important. We show that the nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is no contact angle hysteresis due to strong thermal fluctuations, which occur at the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however, there is strong contact angle hysteresis due to higher energy barrier. These findings may be very important for the development of artificially biomimetic superhydrophobic surfaces.

cond-mat.soft↗

arXiv2008

On the elastic energy and stress correlation in the contact between elastic solids with randomly rough surfaces

When two elastic solids with randomly rough surfaces are brought in contact, a very inhomogeneous stress distribution sigma(x) will occur at the interface. Here I study the elastic energy and the correlation function , where sigma(q) is the Fourier transform of sigma(x) and where <...> stands for ensemble average. I relate to the elastic energy stored at the interface, and I show that for self affine fractal surfaces, quite generally \sim q^{-(1+H)}, where H is the Hurst exponent of the self-affine fractal surface.

cond-mat.soft↗

arXiv2008

Capillary adhesion between elastic solids with randomly rough surfaces

I study how the contact area and the work of adhesion, between two elastic solids with randomly rough surfaces, depend on the relative humidity. The surfaces are assumed to be hydrophilic, and capillary bridges form at the interface between the solids. For elastically hard solids with relative smooth surfaces, the area of real contact and therefore also the sliding friction, are maximal when there is just enough liquid to fill out the interfacial space between the solids, which typically occurs for $d_{\rm K} \approx 3 h_{\rm rms}$, where $d_{\rm K}$ is the height of the capillary bridge and $h_{\rm rms}$ the root-mean-square roughness of the (combined) surface roughness profile. For elastically soft solids, the area of real contact is maximal for very low humidity (i.e., small $d_{\rm K}$), where the capillary bridges are able to pull the solids into nearly complete contact. In both case, the work of adhesion is maximal (and equal to $2γ{\rm cos}θ$, where $γ$ is the liquid surface tension and $θ$ the liquid-solid contact angle) when $d_{\rm K} >> h_{\rm rms}$, corresponding to high relative humidity.

cond-mat.soft↗

arXiv2008

Theory of the leak-rate of seals

Seals are extremely useful devices to prevent fluid leakage. However, the exact mechanism of roughness induced leakage is not well understood. We present a theory of the leak-rate of seals, which is based on percolation theory and a recently developed contact mechanics theory. We study both static and dynamics seals. We present molecular dynamics results which show that when two elastic solids with randomly rough surfaces are squeezed together, as a function of increasing magnification or decreasing squeezing pressure, a non-contact channel will percolate when the (relative) projected contact area, A/A_0, is of order 0.4, in accor dance with percolation theory. We suggest a simple experiment which can be used to test the theory.

cond-mat.soft↗

arXiv2008

Contact mechanics: contact area and interfacial separation from small contact to full contact

We present a molecular dynamics study of the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. The numerical calculations mainly focus on the contact area and the interfacial separation from small contact (low load) to full contact (high load). For small load the contact area varies linearly with the load and the interfacial separation depends logarithmically on the load. For high load the contact area approaches the nominal contact area (i.e., complete contact), and the interfacial separation approaches zero. The numerical results have been compared with analytical theory and experimental results. They are in good agreement with each other. The present findings may be very important for soft solids, e.g., rubber, or for very smooth surfaces, where complete contact can be reached at moderate high loads without plastic deformation of the solids.

cond-mat.soft↗

arXiv2007

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.

cond-mat.soft↗