arXiv subjects

B. N. J. Persson

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

At least 109 records · Page 6Linked to original sources

arXiv2007

Molecular dynamics study of contact mechanics: contact area and interfacial separation from small to full contact

We report a molecular dynamics study of the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. We study 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 to the nominal contact area (i.e., complete contact), and the interfacial separation approaches to zero. The present results 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

Contact mechanics: relation between interfacial separation and load

I study the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. I derive a relation between the (average) interfacial separation $u$ and the applied normal squeezing pressure $p$. I show that for non-adhesive inte raction and small applied pressure, p is proportional to exp (-u/u_0), in good agreement with recent experimental observation.

cond-mat.soft↗

arXiv2007

Effect of surface roughness and adsorbates on superlubricity

We study the sliding of elastic solids in adhesive contact with flat and rough interfaces. We consider the dependence of the sliding friction on the elastic modulus of the solids. For elastically hard solids with planar surfaces with incommensurate surface structures we observe extremely low friction (superlubricity), which very abruptly increases as the elastic modulus decreases. We show that even a relatively small surface roughness or a low concentration of adsorbates may completely kill the superlubricity state.

cond-mat.soft↗

arXiv2006

How do liquids confined at the nanoscale influence adhesion?

Liquids play an important role in adhesion and sliding friction. They behave as lubricants in human bodies especially in the joints. However, in many biological attachment systems they acts like adhesives, e.g. facilitating insects to move on ceilings or vertical walls. Here we use molecular dynamics to study how liquids confined at the nanoscale influence the adhesion between solid bodies with smooth and rough surfaces. We show that a monolayer of liquid may strongly affect the adhesion.

cond-mat.soft↗

arXiv2006

Influence of surface roughness on superhydrophobicity

Superhydrophobic surfaces, with liquid contact angle theta greater than 150 degree, have important practical applications ranging from self-cleaning window glasses, paints, and fabrics to low-friction surfaces. Many biological surfaces, such as the lotus leaf, have hierarchically structured surface roughness which is optimized for superhydrophobicity through natural selection. Here we present a molecular dynamics study of liquid droplets in contact with self-affine fractal surfaces. Our results indicate that the contact angle for nanodroplets depends strongly on the root-mean-square surface roughness amplitude but is nearly independent of the fractal dimension D_f of the surface.

cond-mat.soft↗

arXiv2006

Rubber friction on smooth surfaces

We study the sliding friction for viscoelastic solids, e.g., rubber, on hard flat substrate surfaces. We consider first the fluctuating shear stress inside a viscoelastic solid which results from the thermal motion of the atoms or molecules in the solid. At the nanoscale the thermal fluctuations are very strong and give rise to stress fluctuations in the MPa-range, which is similar to the depinning stresses which typically occur at solid-rubber interfaces, indicating the crucial importance of thermal fluctuations for rubber friction on smooth surfaces. We develop a detailed model which takes into account the influence of thermal fluctuations on the depinning of small contact patches (stress domains) at the rubber-substrate interface. The theory predicts that the velocity dependence of the macroscopic shear stress has a bell-shaped f orm, and that the low-velocity side exhibits the same temperature dependence as the bulk viscoelastic modulus, in qualitative agreement with experimental data. Finally, we discuss the influence of small-amplitude substrate roughness on rubber sliding friction.

cond-mat.soft↗

arXiv2006

Resonant photon tunneling enhancement of the radiative heat transfer

We study the dependence of the heat transfer between two semi-infinite solids on the dielectric properties of the bodies. We show that the heat transfer at short separation between the solids may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the heat transfer is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes. We study the dependence of the heat flux between two metal surfaces on the electron concentration using the non-local optic dielectric approach, and co mpare with the results obtained within local optic approximation.

cond-mat.mtrl-sci↗

arXiv2006

Adsorbate vibrational mode enhancement of radiative heat transfer

We show that the radiative heat transfer between two solid surfaces at short separation may increase by many order of magnitude when the surfaces are covered by adsorbates. In this case the heat transfer is determined by resonant photon tunneling between adsorbate vibrational modes. We propose an experiment to check the theory.

cond-mat.mtrl-sci↗

arXiv2006

Theory of friction: contribution from fluctuating electromagnetic field

We calculate the friction force between two semi-infinite solids in relative parallel motion (velocity $V$), and separated by a vacuum gap of width $d$. The friction force result from coupling via a fluctuating electromagnetic field, and can be considered as the dissipative part of the van der Waals interaction. We consider the dependence of the friction force on the temperature $T$, and present a detailed discussion of the limiting cases of small and large $V$ and $d$.

cond-mat.mtrl-sci↗

arXiv2006

Frictional drag between quantum wells mediated by fluctuating electromagnetic field

We use the theory of the fluctuating electromagnetic field to calculate the frictional drag between nearby two-and three dimensional electron systems. The frictional drag results from coupling via a fluctuating electromagnetic field, and can be considered as the dissipative part of the van der Waals interaction. In comparison with other similar calculations for semiconductor two-dimensional system we include retardation effects. We consider the dependence of the frictional drag force on the temperature $T$, electron density and separation $d$. We find, that retardation effects become dominating factor for high electron densities, corresponding thing metallic film, and suggest a new experiment to test the theory. The relation between friction and heat transfer is also briefly commented on.

cond-mat.mtrl-sci↗

arXiv2006

Radiative heat transfer between nanostructures

We simplify the formalism of Polder and Van Hove [Phys.Rev.B {\bf 4}, 3303(1971)], which was developed to calculate the heat transfer between macroscopic and nanoscale bodies of arbitrary shape, dispersive and adsorptive dielectric properties. In the non-retarded limit, at small distances between the bodies, the problem is reduced to the solution of an electrostatic problem. We apply the formalism to the study of the heat transfer between: (a) two parallel semi-infinite bodies, (b) a semi-infinite body and a spherical body, and (c) that two spherical bodies. We consider the dependence of the heat transfer on the temperature $T$, the shape and the separation $d$. We determine when retardation effects become important.

cond-mat.mtrl-sci↗

arXiv2006

Dissipative Van der Waals interaction between a small particle and a metal surface

We use a general theory of the fluctuating electromagnetic field to calculate the friction force acting on a small neutral particle, e.g., a physisorbed molecule, or a nanoscale object with arbitrary dispersive and absorptive dielectric properties, moving near a metal surface. We consider the dependence of the electromagnetic friction on the temperature $T$, the separation $d$, and discuss the role of screening, non-local and retardation effects. We find that for high resistivity materials, the dissipative van der Waals interaction can be an important mechanism of vibrational energy relaxation of physisorbed molecules, and friction for microscopic solids. Several controversial topics related to electromagnetic dissipative shear stress is considered. The problem of local heating of the surface by an STM tip is also briefly commented on.

cond-mat.mtrl-sci↗

arXiv2006

Non-contact friction between nanostructures

We calculate the van der Waals friction between two semi-infinite solids in normal relative motion and find a drastic difference in comparison with the parallel relative motion. The case of the good conductors is investigated in details both within the local optic approximation, and using a non-local optic dielectric approach. We show that the friction may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the friction is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes. The theory is compared to atomic force microscope experimental data.

cond-mat.mtrl-sci↗

arXiv2006

Resonant photon tunneling enhancement of the van der Waals friction

We study the van der Waals friction between two flat metal surfaces in relative motion. For good conductors we find that normal relative motion gives a much larger friction than for parallel relative motion. The friction may increase by many order of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the friction is determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes.

cond-mat.mtrl-sci↗

arXiv2006

Adsorbate induced enhancement of electrostatic non-contact friction

We study the non-contact friction between an atomic force microscope tip and a metal substrate in the presence of bias voltage. The friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the tip surface by the bias voltage. We show that the friction can be enhanced by many orders of magnitude if the ads orbate layer can support acoustic vibrations. The theory predicts the magnitude and the distance dependence of friction in a good agreement with recent puzzling non-contact friction experiment \cite{Stipe}. We demonstrate that even an isolated adsorbate can produce high enough friction to be measured experimentally.

cond-mat.mtrl-sci↗

arXiv2006

Adsorbate vibrational modes enhancement of radiative heat transfer and van der Waals friction

We study the dependence of the heat transfer and the van der Waals friction between two semi-infinite solids on the dielectric properties of the bodies. We show that the heat transfer and van der Waals friction at short separation between the solids may increase by many orders of magnitude when the surfaces are covered by adsorbates, or can support low-frequency surface plasmons. In this case the heat transfer and van der Waals friction are determined by resonant photon tunneling between adsorbate vibrational modes, or surface plasmon modes. The enhancement of the van der Waals friction is especially large when in the adsorbed layer there is an acoustic branch for the vibrations parallel to the surface like in the case of Cs adsorption on Cu(100) surface. In this case we show that even for separation $d=10$nm, the van der Waals friction induced by adsorbates can be so large that it can be measured with the present state-of-art equipment. The van an der Waals friction is characterized by a strong distance dependence ($\sim 1/d^6$), and at the small distances it can be much larger than \textit{the electrostatic} friction observed in \cite{Stipe}. \vskip 0.3cm \textit{Keywords}: non-contact friction, van der Waals friction, radiative heat transfer, atomic force microscope, adsorbate vibrational mode

cond-mat.mtrl-sci↗

arXiv2006

Enhancement of noncontact friction between closely spaced bodies by two-dimensional systems

. We consider the effect of an external bias voltage and the spatial variation of the surface potential, on the damping of cantilever vibrations. The electrostatic friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the surface of the tip by the bias voltage and spatial variation of the surface potential. A similar effect arises when the tip is oscillating in the electrostatic field created by charged defects in a dielectric substrate. The electrostatic friction is compared with the van der Waals friction originating from the fluctuating electromagnetic field due to quantum and thermal fluctuation of the current density inside the bodies. We show that the electrostatic and van der Waals friction can be greatly enhanced if on the surfaces of the sample and the tip there are two-dimension (2D) systems, e.g. a 2D-electron system or incommensurate layers of adsorbed ions exhibiting acoustic vibrations. We show that the damping of the cantilever vibrations due to the electrostatic friction may be of similar magnitude as the damping observed in recent experiments of Stipe \textit{et al} [B.C.Stipe, H.J.Mamin, T.D.Stowe, T.W.Kenny, and D.Rugar, Phys.Rev. Lett.% \textbf{87}, 0982001]. We also show that at short separation the van der Waals friction may be large enough to be measured experimentally.

cond-mat.mtrl-sci↗

arXiv2006

Quantum field theory of the van der Waals friction

The van der Waals friction between two semi-infinite solids, and between a small neutral particle and semi-infinite solid is reconsidered on the basis of thermal quantum field theory in the Matsubara formulation. The calculation of the friction to linear order in the sliding velocity is reduced to the finding of the equilibrium Green functions. Thus this approach cab be extended for bodsies with complex geometry. The friction calculated in this approach agrees with the friction calculated using a dynamical modification of the Lifshitz theory, which is based on the fluctuation-dissipation therem. We show that the van der Waals fricxtion can be measured in non-contact friction experiment using state-of-the art equipment.

cond-mat.mtrl-sci↗