10,390 research outputs found
The nature of the observed free-electron-like state in a PTCDA monolayer on Ag(111)
A free-electron like band has recently been observed in a monolayer of PTCDA
(3,4,9,10-perylene tetracarboxylic dianhydride) molecules on Ag(111) by
two-photon photoemission [Schwalb et al., Phys. Rev. Lett. 101, 146801 (2008)]
and scanning tunneling spectroscopy [Temirov et al., Nature 444, 350 (2006)].
Using density functional theory calculations, we find that the observed
free-electron like band originates from the Shockley surface state band being
dramatically shifted up in energy by the interaction with the adsorbed
molecules while it acquires also a substantial admixture with a molecular band
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
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.Comment: 5 pages, 9 color figures. Some figures are in Postscript Level 3
format. Minimal changes with respect to the previous version. Added doi and
reference to the published article also inside the pape
Elastohydrodynamics for soft solids with surface roughness: transient effects
A huge number of technological and biological systems involves 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
Theory of friction: contribution from fluctuating electromagnetic field
We calculate the friction force between two semi-infinite solids in relative
parallel motion (velocity ), and separated by a vacuum gap of width . 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 , and
present a detailed discussion of the limiting cases of small and large and
.Comment: 15 pages, No figure
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.Comment: 16 pages, 14 figure
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.Comment: 6 pages, 10 figure
Numerical Investigation of Second Mode Attenuation over Carbon/Carbon Surfaces on a Sharp Slender Cone
We have carried out axisymmetric numerical simulations of a spatially
developing hypersonic boundary layer over a sharp 7-half-angle cone
at inspired by the experimental investigations by Wagner (2015).
Simulations are first performed with impermeable (or solid) walls with a
one-time broadband pulse excitation applied upstream to determine the most
convectively-amplified frequencies resulting in the range 260kHz -- 400kHz,
consistent with experimental observations of second-mode instability waves.
Subsequently, we introduce harmonic disturbances via continuous periodic
suction and blowing at 270kHz and 350kHz. For each of these forcing frequencies
complex impedance boundary conditions (IBC), modeling the acoustic response of
two different carbon/carbon (C/C) ultrasonically absorptive porous surfaces,
are applied at the wall. The IBCs are derived as an output of a pore-scale
aeroacoustic analysis -- the inverse Helmholtz Solver (iHS) -- which is able to
return the broadband real and imaginary components of the surface-averaged
impedance. The introduction of the IBCs in all cases leads to a significant
attenuation of the harmonically-forced second-mode wave. In particular, we
observe a higher attenuation rate of the introduced waves with frequency of
350kHz in comparison with 270kHz, and, along with the iHS impedance results, we
establish that the C/C surfaces absorb acoustic energy more effectively at
higher frequencies.Comment: AIAA-SciTech 201
Theory of adhesion: role of surface roughness
We discuss how surface roughness influence the adhesion between elastic
solids. We introduce a Tabor number which depends on the length scale or
magnification, and which gives information about the nature of the adhesion at
different length scales. We consider two limiting cases relevant for (a)
elastically hard solids with weak adhesive interaction (DMT-limit) and (b)
elastically soft solids or strong adhesive interaction (JKR-limit). For the
former cases we study the nature of the adhesion using different adhesive force
laws (, , where is the wall-wall separation). In
general, adhesion may switch from DMT-like at short length scales to JKR-like
at large (macroscopic) length scale. We compare the theory predictions to the
results of exact numerical simulations and find good agreement between theory
and the simulation results
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