8,265 research outputs found
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.Comment: 5 pages, 9 figure
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.Comment: 6 pages, 10 figure
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.Comment: 5 Pages, 6 figures. Minimal changes with respect to the previous
versio
Role of friction-induced torque in stick-slip motion
We present a minimal quasistatic 1D model describing the kinematics of the
transition from static friction to stick-slip motion of a linear elastic block
on a rigid plane. We show how the kinematics of both the precursors to
frictional sliding and the periodic stick-slip motion are controlled by the
amount of friction-induced torque at the interface. Our model provides a
general framework to understand and relate a series of recent experimental
observations, in particular the nucleation location of micro-slip instabilities
and the build up of an asymmetric field of real contact area.Comment: 6 pages, 5 figure
Nontrivial temporal scaling in a Galilean stick-slip dynamics
We examine the stick-slip fluctuating response of a rough massive
non-rotating cylinder moving on a rough inclined groove which is submitted to
weak external perturbations and which is maintained well below the angle of
repose. The experiments presented here, which are reminiscent of the Galileo's
works with rolling objects on inclines, have brought in the last years
important new insights into the friction between surfaces in relative motion
and are of relevance for earthquakes, differing from classical block-spring
models by the mechanism of energy input in the system. Robust nontrivial
temporal scaling laws appearing in the dynamics of this system are reported,
and it is shown that the time-support where dissipation occurs approaches a
statistical fractal set with a fixed value of dimension. The distribution of
periods of inactivity in the intermittent motion of the cylinder is also
studied and found to be closely related to the lacunarity of a random version
of the classic triadic Cantor set on the line.Comment: 7 pages including 6 figure
Rubber friction: role of the flash temperature
When a rubber block is sliding on a hard rough substrate, the substrate
asperities will exert time-dependent deformations of the rubber surface
resulting in viscoelastic energy dissipation in the rubber, which gives a
contribution to the sliding friction. Most surfaces of solids have roughness on
many different length scales, and when calculating the friction force it is
necessary to include the viscoelastic deformations on all length scales. The
energy dissipation will result in local heating of the rubber. Since the
viscoelastic properties of rubber-like materials are extremely strongly
temperature dependent, it is necessary to include the local temperature
increase in the analysis. At very low sliding velocity the temperature increase
is negligible because of heat diffusion, but already for velocities of order
0.01 m/s the local heating may be very important. Here I study the influence of
the local heating on the rubber friction, and I show that in a typical case the
temperature increase results in a decrease in rubber friction with increasing
sliding velocity for v > 0.01 m/s. This may result in stick-slip instabilities,
and is of crucial importance in many practical applications, e.g., for the
tire-road friction, and in particular for ABS-breaking systems.Comment: 22 pages, 27 figure
Probing Electron Correlation via Attosecond XUV Pulses in the Two-Photon Double Ionization of Helium
Recent experimental developments of high-intensity, short-pulse XUV light
sources are enhancing our ability to study electron-electron correlations. We
perform time-dependent calculations to investigate the so-called "sequential"
regime (photon energy above 54.4 eV) in the two-photon double ionization of
helium. We show that attosecond pulses allow to induce and probe angular and
energy correlations of the emitted electrons. The final momentum distribution
reveals regions dominated by the Wannier ridge break-up scenario and by
post-collision interaction.Comment: 4 pages, 5 figure
Universal features in sequential and nonsequential two-photon double ionization of helium
We analyze two-photon double ionization of helium in both the nonsequential
and sequential regime. We show that the energy spacing between the two emitted
electrons provides the key parameter that controls both the energy and the
angular distribution and reveals the universal features present in both the
nonsequential and sequential regime. This universality, i.e., independence of
photon energy, is a manifestation of the continuity across the threshold for
sequential double ionization. For all photon energies, the energy distribution
can be described by a universal shape function that contains only the spectral
and temporal information entering second-order time-dependent perturbation
theory. Angular correlations and distributions are found to be more sensitive
to the photon energy. In particular, shake-up interferences have a large effect
on the angular distribution. Energy spectra, angular distributions
parameterized by the anisotropy parameters, and total cross sections presented
in this paper are obtained by fully correlated time-dependent ab initio
calculations.Comment: 12 pages, 8 figure
The effect of Coulombic friction on spatial displacement statistics
The phenomenon of Coulombic friction enters the stochastic description of dry
friction between two solids and the statistic characterization of vibrating
granular media. Here we analyze the corresponding Fokker-Planck equation
including both velocity and spatial components, exhibiting a formal connection
to a quantum mechanical harmonic oscillator in the presence of a delta
potential. Numerical solutions for the resulting spatial displacement
statistics show a crossover from exponential to Gaussian displacement
statistics. We identify a transient intermediate regime that exhibits
multiscaling properties arising from the contribution of Coulombic friction.
The possible role of these effects during observations in diffusion experiments
is shortly discussed.Comment: 11 pages, 9 figure
Dynamics of Phononic Dissipation at the Atomic Scale: Dependence on Internal Degrees of Freedom
Dynamics of dissipation of a local phonon distribution to the substrate is a
key issue in friction between sliding surfaces as well as in boundary
lubrication. We consider a model system consisting of an excited nano-particle
which is weakly coupled with a substrate. Using three different methods we
solve the dynamics of energy dissipation for different types of coupling
between the nano-particle and the substrate, where different types of
dimensionality and phonon densities of states were also considered for the
substrate. In this paper, we present our analysis of transient properties of
energy dissipation via phonon discharge in the microscopic level towards the
substrate. Our theoretical analysis can be extended to treat realistic
lubricant molecules or asperities, and also substrates with more complex
densities of states. We found that the decay rate of the nano-particle phonons
increases as the square of the interaction constant in the harmonic
approximation.Comment: 10 pages, 6 figures, submitted to Phys. Rev.
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