136 research outputs found
Casimir interaction between two concentric cylinders: exact versus semiclassical results
The Casimir interaction between two perfectly conducting, infinite,
concentric cylinders is computed using a semiclassical approximation that takes
into account families of classical periodic orbits that reflect off both
cylinders. It is then compared with the exact result obtained by the
mode-by-mode summation technique. We analyze the validity of the semiclassical
approximation and show that it improves the results obtained through the
proximity theorem.Comment: 28 pages, 5 figures include
Capillary pressure of van der Waals liquid nanodrops
The dependence of the surface tension on a nanodrop radius is important for
the new-phase formation process. It is demonstrated that the famous Tolman
formula is not unique and the size-dependence of the surface tension can
distinct for different systems. The analysis is based on a relationship between
the surface tension and disjoining pressure in nanodrops. It is shown that the
van der Waals interactions do not affect the new-phase formation thermodynamics
since the effect of the disjoining pressure and size-dependent component of the
surface tension cancel each other.Comment: The paper is dedicated to the 80th anniversary of A.I. Rusano
Influence of Humidity on Microtribology of Vertically Aligned Carbon Nanotube Film
The aim of this study is to probe the influence of water vapor environment on
the microtribological properties of a forestlike vertically aligned carbon
nanotube (VACNT) film, deposited on a silicon (001) substrate by chemical vapor
deposition. Tribological experiments were performed using a gold tip under
relative humidity varying from 0 to 100%. Very low adhesion forces and high
friction coefficients of 0.6 to 1.3 resulted. The adhesion and friction forces
were independent of humidity, due probably to the high hydrophobicity of VACNT.
These tribological characteristics were compared to those of a diamond like
carbon (DLC) sample
Lattice Boltzmann for Binary Fluids with Suspended Colloids
A new description of the binary fluid problem via the lattice Boltzmann
method is presented which highlights the use of the moments in constructing two
equilibrium distribution functions. This offers a number of benefits, including
better isotropy, and a more natural route to the inclusion of multiple
relaxation times for the binary fluid problem. In addition, the implementation
of solid colloidal particles suspended in the binary mixture is addressed,
which extends the solid-fluid boundary conditions for mass and momentum to
include a single conserved compositional order parameter. A number of simple
benchmark problems involving a single particle at or near a fluid-fluid
interface are undertaken and show good agreement with available theoretical or
numerical results.Comment: 10 pages, 4 figures, ICMMES 200
Stages of steady diffusion growth of a gas bubble in strongly supersaturated gas-liquid solution
Gas bubble growth as a result of diffusion flux of dissolved gas molecules
from the surrounding supersaturated solution to the bubble surface is studied.
The condition of the flux steadiness is revealed. A limitation from below on
the bubble radius is considered. Its fulfillment guarantees the smallness of
fluctuation influence on bubble growth and irreversibility of this process.
Under the conditions of steadiness of diffusion flux three stages of bubble
growth are marked out. With account for Laplace forces in the bubble intervals
of bubble size change and time intervals of these stages are found. The trend
of the third stage towards the self-similar regime of the bubble growth, when
Laplace forces in the bubble are completely neglected, is described
analytically.Comment: 22 page
Effective interactions between inclusions in complex fluids driven out of equilibrium
The concept of fluctuation-induced effective interactions is extended to
systems driven out of equilibrium. We compute the forces experienced by
macroscopic objects immersed in a soft material driven by external shaking
sources. We show that, in contrast with equilibrium Casimir forces induced by
thermal fluctuations, their sign, range and amplitude depends on specifics of
the shaking and can thus be tuned. We also comment upon the dispersion of these
shaking-induced forces, and discuss their potential application to phase
ordering in soft-materials.Comment: 10 pages, 8 figures, to appear in PR
Nanofriction mechanisms derived from the dependence of friction on load and sliding velocity from air to UHV on hydrophilic silicon
This paper examines friction as a function of the sliding velocity and
applied normal load from air to UHV in a scanning force microscope (SFM)
experiment in which a sharp silicon tip slides against a flat Si(100) sample.
Under ambient conditions, both surfaces are covered by a native oxide, which is
hydrophilic. During pump-down in the vacuum chamber housing the SFM, the
behavior of friction as a function of the applied normal load and the sliding
velocity undergoes a change. By analyzing these changes it is possible to
identify three distinct friction regimes with corresponding contact properties:
(a) friction dominated by the additional normal forces induced by capillarity
due to the presence of thick water films, (b) higher drag force from ordering
effects present in thin water layers and (c) low friction due to direct
solid-solid contact for the sample with the counterbody. Depending on
environmental conditions and the applied normal load, all three mechanisms may
be present at one time. Their individual contributions can be identified by
investigating the dependence of friction on the applied normal load as well as
on the sliding velocity in different pressure regimes, thus providing
information about nanoscale friction mechanisms
Modeling the break-up of nano-particle clusters in aluminum- and magnesium-based metal matrix nano-composites
Aluminum- and magnesium-based metal matrix nano-composites with ceramic nano-reinforcements promise low weight with high durability and superior strength, desirable properties in aerospace, automobile, and other applications. However, nano-particle agglomerations lead to adverse effects on final properties: large-size clusters no longer act as dislocation anchors, but instead become defects; the resulting particle distribution will be uneven, leading to inconsistent properties. To prevent agglomeration and to break-up clusters, ultrasonic processing is used via an immersed sonotrode, or alternatively via electromagnetic vibration. A study of the interaction forces holding the nano-particles together shows that the choice of adhesion model significantly affects estimates of break-up force and that simple Stokes drag due to stirring is insufficient to break-up the clusters. The complex interaction of flow and co-joint particles under a high frequency external field (ultrasonic, electromagnetic) is addressed in detail using a discrete-element method code to demonstrate the effect of these fields on de-agglomeration
Three-body interactions in colloidal systems
We present the first direct measurement of three-body interactions in a
colloidal system comprised of three charged colloidal particles. Two of the
particles have been confined by means of a scanned laser tweezers to a
line-shaped optical trap where they diffused due to thermal fluctuations. Upon
the approach of a third particle, attractive three-body interactions have been
observed. The results are in qualitative agreement with additionally performed
nonlinear Poissson-Boltzmann calculations, which also allow us to investigate
the microionic density distributions in the neighborhood of the interacting
colloidal particles
Interaction and flocculation of spherical colloids wetted by a surface-induced corona of paranematic order
Particles dispersed in a liquid crystal above the nematic-isotropic phase
transition are wetted by a surface-induced corona of paranematic order. Such
coronas give rise to pronounced two-particle interactions. In this article, we
report details on the analytical and numerical study of these interactions
published recently [Phys. Rev. Lett. 86, 3915 (2001)]. We especially
demonstrate how for large particle separations the asymptotic form of a Yukawa
potential arises. We show that the Yukawa potential is a surprisingly good
description for the two-particle interactions down to distances of the order of
the nematic coherence length. Based on this fact, we extend earlier studies on
a temperature induced flocculation transition in electrostatically stabilized
colloidal dispersions [Phys. Rev. E 61, 2831 (2000)]. We employ the Yukawa
potential to establish a flocculation diagram for a much larger range of the
electrostatic parameters, namely the surface charge density and the Debye
screening length. As a new feature, a kinetically stabilized dispersion close
to the nematic-isotropic phase transition is found.Comment: Revtex v4.0, 16 pages, 12 Postscript figures. Accepted for
publication in Phys. Rev.
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