206 research outputs found
Magnetic Dipole Absorption of Radiation in Small Conducting Particles
We give a theoretical treatment of magnetic dipole absorption of
electromagnetic radiation in small conducting particles, at photon energies
which are large compared to the single particle level spacing, and small
compared to the plasma frequency. We discuss both diffusive and ballistic
electron dynamics for particles of arbitrary shape.
The conductivity becomes non-local when the frequency is smaller than the
frequency \omega_c characterising the transit of electrons from one side of the
particle to the other, but in the diffusive case \omega_c plays no role in
determining the absorption coefficient. In the ballistic case, the absorption
coefficient is proportional to \omega^2 for \omega << \omega_c, but is a
decreasing function of \omega for \omega >> \omega_c.Comment: 25 pages of plain TeX, 2 postscipt figure
Extension of nano-confined DNA: quantitative comparison between experiment and theory
The extension of DNA confined to nanochannels has been studied intensively
and in detail. Yet quantitative comparisons between experiments and model
calculations are difficult because most theoretical predictions involve
undetermined prefactors, and because the model parameters (contour length, Kuhn
length, effective width) are difficult to compute reliably, leading to
substantial uncertainties. Here we use a recent asymptotically exact theory for
the DNA extension in the "extended de Gennes regime" that allows us to compare
experimental results with theory. For this purpose we performed new
experiments, measuring the mean DNA extension and its standard deviation while
varying the channel geometry, dye intercalation ratio, and ionic buffer
strength. The experimental results agree very well with theory at high ionic
strengths, indicating that the model parameters are reliable. At low ionic
strengths the agreement is less good. We discuss possible reasons. Our approach
allows, in principle, to measure the Kuhn length and effective width of a
single DNA molecule and more generally of semiflexible polymers in solution.Comment: Revised version, 6 pages, 2 figures, 1 table, supplementary materia
Effect of turbulence on collisional growth of cloud droplets
We investigate the effect of turbulence on the collisional growth of um-sized
droplets through high- resolution numerical simulations with well resolved
Kolmogorov scales, assuming a collision and coalescence efficiency of unity.
The droplet dynamics and collisions are approximated using a superparticle
approach. In the absence of gravity, we show that the time evolution of the
shape of the droplet-size distribution due to turbulence-induced collisions
depends strongly on the turbulent energy-dissipation rate, but only weakly on
the Reynolds number. This can be explained through the energy dissipation rate
dependence of the mean collision rate described by the Saffman-Turner collision
model. Consistent with the Saffman-Turner collision model and its extensions,
the collision rate increases as the square root of the energy dissipation rate
even when coalescence is invoked. The size distribution exhibits power law
behavior with a slope of -3.7 between a maximum at approximately 10 um up to
about 40 um. When gravity is invoked, turbulence is found to dominate the time
evolution of an initially monodisperse droplet distribution at early times. At
later times, however, gravity takes over and dominates the collisional growth.
We find that the formation of large droplets is very sensitive to the turbulent
energy dissipation rate. This is due to the fact that turbulence enhances the
collisional growth between similar sized droplets at the early stage of
raindrop formation. The mean collision rate grows exponentially, which is
consistent with the theoretical prediction of the continuous collisional growth
even when turbulence-generated collisions are invoked. This consistency only
reflects the mean effect of turbulence on collisional growth
Collective versus single-particle effects in the optical spectra of finite electronic quantum systems
We study optical spectra of finite electronic quantum systems at frequencies
smaller than the plasma frequency using a quasi-classical approach. This
approach includes collective effects and enables us to analyze how the nature
of the (single-particle) electron dynamics influences the optical spectra in
finite electronic quantum systems. We derive an analytical expression for the
low-frequency absorption coefficient of electro-magnetic radiation in a finite
quantum system with ballistic electron dynamics and specular reflection at the
boundaries: a two-dimensional electron gas confined to a strip of width a (the
approach can be applied to systems of any shape and electron dynamics --
diffusive or ballistic, regular or irregular motion). By comparing with results
of numerical computations using the random-phase approximation we show that our
analytical approach provides a qualitative and quantitative understanding of
the optical spectrum.Comment: 4 pages, 3 figure
Strength distribution of repeatedly broken chains
We determine the probability distribution of the breaking strength for chains
of N links, which have been produced by repeatedly breaking a very long chain.Comment: 4 pages, 1 figur
Coagulation by Random Velocity Fields as a Kramers Problem
We analyse the motion of a system of particles suspended in a fluid which has
a random velocity field. There are coagulating and non-coagulating phases. We
show that the phase transition is related to a Kramers problem, and use this to
determine the phase diagram, as a function of the dimensionless inertia of the
particles, epsilon, and a measure of the relative intensities of potential and
solenoidal components of the velocity field, Gamma. We find that the phase line
is described by a function which is non-analytic at epsilon=0, and which is
related to escape over a barrier in the Kramers problem. We discuss the
physical realisations of this phase transition.Comment: 4 pages, 3 figure
Caustic formation in a non-Gaussian model for turbulent aerosols
Caustics in the dynamics of heavy particles in turbulence accelerate particle
collisions. The rate at which these singularities form depends
sensitively on the Stokes number St, the non-dimensional inertia parameter.
Exact results for this sensitive dependence have been obtained using Gaussian
statistical models for turbulent aerosols. However, direct numerical
simulations of heavy particles in turbulence yield much larger
caustic-formation rates than predicted by the Gaussian theory. In order to
understand possible mechanisms explaining this difference, we analyse a
non-Gaussian statistical model for caustic formation in the limit of small St.
We show that at small St, depends sensitively on the tails of the
distribution of Lagrangian fluid-velocity gradients. This explains why
different authors obtained different St-dependencies of in
numerical-simulation studies. The most-likely gradient fluctuation that induces
caustics at small St, by contrast, is the same in the non-Gaussian and Gaussian
models. Direct-numerical simulation results for particles in turbulence show
that the optimal fluctuation is similar, but not identical, to that obtained by
the model calculations.Comment: 12 pages, 3 figures, 1 tabl
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