4,798 research outputs found
Semiclassical quantization of multidimensional systems
Low order classical perturbation theory is used to obtain semiclassical eigenvalues for a system of three anharmonically coupled oscillators. The results in the low energy region studied here agree well with the "exact" quantum values. The latter had been calculated by matrix diagonalization using a large basis set
Flow-induced currents in nanotubes: a Brownian dynamics approach
Motivated by recent experiments [Science {\bf 299}, 1042 (2003)] reporting
that carbon nanotubes immersed in a flowing fluid displayed an electric current
and voltage, we numerically study the behaviour of a collection of Brownian
particles in a channel, in the presence of a flow field applied on similar but
slower particles in a wide chamber in contact with the channel. For a suitable
range of shear rates, we find that the flow field induces a unidirectional
drift in the confined particles, and is stronger for narrower channels. The
average drift velocity initially rises with increasing shear rate, then shows
saturation for a while, thereafter starts decreasing, in qualitative agreement
with recent theoretical studies [Phys. Rev. B {\bf 70}, 205423 (2004)] based on
Brownian drag and ``loss of grip''. Interestingly, if the sign of the
interspecies interaction is reversed, the direction of the induced drift
remains the same, but the flow-rate at which loss of grip occurs is lower, and
the level of fluctuations is higher.Comment: 7 pages, 9 figure
Flow-induced voltage and current generation in carbon nanotubes
New experimental results, and a plausible theoretical understanding thereof,
are presented for the flow-induced currents and voltages observed in
single-walled carbon nanotube samples. In our experiments, the electrical
response was found to be strongly sublinear -- nearly logarithmic -- in the
flow speed over a wide range, and its direction could be controlled by an
electrochemical biasing of the nanotubes. These experimental findings are
inconsistent with the conventional idea of a streaming potential as the
efficient cause. Here we present a new, physically appealing, Langevin-equation
based treatment of the nanotube charge carriers, assumed to be moving under
coulombic forcing by the correlated ionic fluctuations, advected by the liquid
in flow. The resulting 'Doppler-shifted' force-force correlation, as seen by
the charge carriers drifting in the nanotube, is shown to give a strongly
sublinear response, broadly in agreement with experiments.Comment: 11 pages including 3 figures. To appear in Phys. Rev B (2004
Evolution, present status and issues concerning small tank systems in Sri Lanka [Small tanks in Sri Lanka: evolution, present status and issues]
Tank irrigationSmall scale systemsHistoryIrrigation systemsDesignMaintenanceHydrologyPollutionIrrigation managementCultivationFarming systemsWells
Trapping and sorting active particles: motility-induced condensation & smectic defects
We present an experimental realization of the collective trapping phase
transition [Kaiser et al., PRL 108, 268307 (2012)], using motile polar granular
rods in the presence of a V-shaped obstacle. We offer a theory of this
transition based on the interplay of motility-induced condensation and
liquid-crystalline ordering and show that trapping occurs when persistent
influx overcomes the collective expulsion of smectic defect structures. In
agreement with the theory, our experiments find that a trap fills to the brim
when the trap angle is below a threshold , while all
particles escape for . Our simulations support a further
prediction, that goes down with increasing rotational noise. We
exploit the sensitivity of trapping to the persistence of directed motion to
sort particles based on the statistical properties of their activityComment: 6 pages, 5 figures, for supplementary mpg files, see
"https://www.dropbox.com/sh/3cmswfoysdn0sb6/AACpEp-G3768B6Y62nDFj_Hea?dl=0".
This paper supersedes our earlier version arXiv:1603.08535 and contains
substantial new results including revised theoretical treatmen
A Dynamic Renormalization Group Study of Active Nematics
We carry out a systematic construction of the coarse-grained dynamical
equation of motion for the orientational order parameter for a two-dimensional
active nematic, that is a nonequilibrium steady state with uniaxial, apolar
orientational order. Using the dynamical renormalization group, we show that
the leading nonlinearities in this equation are marginally \textit{irrelevant}.
We discover a special limit of parameters in which the equation of motion for
the angle field of bears a close relation to the 2d stochastic Burgers
equation. We find nevertheless that, unlike for the Burgers problem, the
nonlinearity is marginally irrelevant even in this special limit, as a result
of of a hidden fluctuation-dissipation relation. 2d active nematics therefore
have quasi-long-range order, just like their equilibrium counterpartsComment: 31 pages 6 figure
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