1,516 research outputs found
Quantum hydrodynamics for supersolid crystals and quasicrystals
Supersolids are theoretically predicted quantum states that break the
continuous rotational and translational symmetries of liquids while preserving
superfluid transport properties. Over the last decade, much progress has been
made in understanding and characterizing supersolid phases through numerical
simulations for specific interaction potentials. The formulation of an
analytically tractable framework for generic interactions still poses
theoretical challenges. By going beyond the usually considered quadratic
truncations, we derive a systematic higher-order generalization of the
Gross-Pitaevskii mean field model in conceptual similarity with the
Swift-Hohenberg theory of pattern formation. We demonstrate the tractability of
this broadly applicable approach by determining the ground state phase diagram
and the dispersion relations for the supersolid lattice vibrations in terms of
the potential parameters. Our analytical predictions agree well with numerical
results from direct hydrodynamic simulations and earlier quantum Monte-Carlo
studies. The underlying framework is universal and can be extended to
anisotropic pair potentials with complex Fourier-space structure.Comment: 18 pages, 10 figures; supplementary information available on reques
Low Reynolds number hydrodynamics of asymmetric, oscillating dumbbell pairs
Active dumbbell suspensions constitute one of the simplest model system for
collective swimming at low Reynolds number. Generalizing recent work, we derive
and analyze stroke-averaged equations of motion that capture the effective
hydrodynamic far-field interaction between two oscillating, asymmetric
dumbbells in three space dimensions. Time-averaged equations of motion, as
those presented in this paper, not only yield a considerable speed-up in
numerical simulations, they may also serve as a starting point when deriving
continuum equations for the macroscopic dynamics of multi-swimmer suspensions.
The specific model discussed here appears to be particularly useful in this
context, since it allows one to investigate how the collective macroscopic
behavior is affected by changes in the microscopic symmetry of individual
swimmers.Comment: 10 pages, to appear in EPJ Special Topic
Relativistic Equilibrium Distribution by Relative Entropy Maximization
The equilibrium state of a relativistic gas has been calculated based on the
maximum entropy principle. Though the relativistic equilibrium state was long
believed to be the Juttner distribution, a number of papers have been published
in recent years proposing alternative equilibrium states. However, some of
these papers do not pay enough attention to the covariance of distribution
functions, resulting confusion in equilibrium states. Starting from a fully
covariant expression to avoid this confusion, it has been shown in the present
paper that the Juttner distribution is the maximum entropy state if we assume
the Lorentz symmetry.Comment: Six pages, no figure
Improved bounds on entropy production in living systems
Living systems maintain or increase local order by working against the Second
Law of Thermodynamics. Thermodynamic consistency is restored as they dissipate
heat, thereby increasing the net entropy of their environment. Recently
introduced estimators for the entropy production rate have provided major
insights into the thermal efficiency of important cellular processes. In
experiments, however, many degrees of freedom typically remain hidden to the
observer, and in these cases, existing methods are not optimal. Here, by
reformulating the problem within an optimization framework, we are able to
infer improved bounds on the rate of entropy production from partial
measurements of biological systems. Our approach yields provably optimal
estimates given certain measurable transition statistics. In particular, it can
reveal non-zero heat production rates even when non-equilibrium processes
appear time symmetric and so may pretend to obey detailed balance. We
demonstrate the broad applicability of this framework by providing improved
bounds on the entropy production rate in a diverse range of biological systems
including bacterial flagella motors, growing microtubules, and calcium
oscillations within human embryonic kidney cells.Comment: Expanded discussion section on entropy production bounds for
reversible observables. 19 pages, 10 figure
Information transmission and signal permutation in active flow networks
© 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft. Recent experiments show that both natural and artificial microswimmers in narrow channel-like geometries will self-organise to form steady, directed flows. This suggests that networks of flowing active matter could function as novel autonomous microfluidic devices. However, little is known about how information propagates through these far-from-equilibrium systems. Through a mathematical analogy with spin-ice vertex models, we investigate here the input-output characteristics of generic incompressible active flow networks (AFNs). Our analysis shows that information transport through an AFN is inherently different from conventional pressure or voltage driven networks. Active flows on hexagonal arrays preserve input information over longer distances than their passive counterparts and are highly sensitive to bulk topological defects, whose presence can be inferred from marginal input-output distributions alone. This sensitivity further allows controlled permutations on parallel inputs, revealing an unexpected link between active matter and group theory that can guide new microfluidic mixing strategies facilitated by active matter and aid the design of generic autonomous information transport networks
Stationarity, soft ergodicity, and entropy in relativistic systems
Recent molecular dynamics simulations show that a dilute relativistic gas
equilibrates to a Juettner velocity distribution if ensemble velocities are
measured simultaneously in the observer frame. The analysis of relativistic
Brownian motion processes, on the other hand, implies that stationary
one-particle distributions can differ depending on the underlying
time-parameterizations. Using molecular dynamics simulations, we demonstrate
how this relativistic phenomenon can be understood within a deterministic model
system. We show that, depending on the time-parameterization, one can
distinguish different types of soft ergodicity on the level of the one-particle
distributions. Our analysis further reveals a close connection between time
parameters and entropy in special relativity. A combination of different
time-parameterizations can potentially be useful in simulations that combine
molecular dynamics algorithms with randomized particle creation, annihilation,
or decay processes.Comment: 4 page
Psychosocial Predictors of Metabolic Syndrome among Latino Groups in the Multi-Ethnic Study of Atherosclerosis (MESA).
ObjectiveWe sought to determine the contribution of psychological variables to risk for metabolic syndrome (MetS) among Latinos enrolled in the Multi-Ethnic Study of Atherosclerosis (MESA), and to investigate whether social support moderates these associations, and whether inflammatory markers mediate the association between psychological variables and MetS.Research design and methodsCross-sectional analyses at study baseline were conducted with a national Latino cohort (n = 1,388) that included Mexican Americans, Dominican Americans, Puerto Rican Americans and Central/South Americans. Hierarchical logistic regression analyses were conducted to test the effects of psychosocial variables (chronic stress, depressive symptoms, and social support) on MetS. In addition, separate subgroup-specific models, controlling for nationality, age, gender, socioeconomic position, language spoken at home, exercise, smoking and drinking status, and testing for the effects of chronic stress, depressive symptoms and inflammation (IL-6, CRP, fibrinogen) in predicting risk for MetS were conducted.ResultsIn the overall sample, high chronic stress independently predicted risk for MetS, however this association was found to be significant only in Mexican Americans and Puerto Rican Americans. Social support did not moderate the associations between chronic stress and MetS for any group. Chronic stress was not associated with inflammatory markers in either the overall sample or in each group.ConclusionsOur results suggest a differential contribution of chronic stress to the prevalence of MetS by national groups
Retrospective evaluation of the association between admission blood glucose and l-lactate concentrations in ponies and horses with gastrointestinal disease (2008-2016): 545 cases
A recent study described increased l‐lactate concentrations in ponies with gastrointestinal disease compared to horses, but blood glucose (BG) concentrations were not considered. The study tested the hypothesis that BG and l‐lactate concentrations are correlated in horses and ponies with gastrointestinal disease and that BG concentrations, not equid type (pony vs horse), are an independent predictor of L‐lactate concentrations. It was further hypothesized that equid type was an independent predictor of BG concentrations
Vortex line entanglement in active Beltrami flows
Over the last decade, substantial progress has been made in understanding the
topology of quasi-2D non-equilibrium fluid flows driven by ATP-powered
microtubules and microorganisms. By contrast, the topology of 3D active fluid
flows still poses interesting open questions. Here, we study the topology of a
spherically confined active flow using 3D direct numerical simulations of
generalized Navier-Stokes (GNS) equations at the scale of typical microfluidic
experiments. Consistent with earlier results for unbounded periodic domains,
our simulations confirm the formation of Beltrami-like bulk flows with
spontaneously broken chiral symmetry in this model. Furthermore, by leveraging
fast methods to compute linking numbers, we explicitly connect this chiral
symmetry breaking to the entanglement statistics of vortex lines. We observe
that the mean of linking number distribution converges to the global helicity,
consistent with the asymptotic result by Arnold. Additionally, we characterize
the rate of convergence of this measure with respect to the number and length
of observed vortex lines, and examine higher moments of the distribution. We
find that the full distribution is well described by a k-Gamma distribution, in
agreement with an entropic argument.Comment: 18 pages, 8 figure
SAR-Based Vibration Estimation Using the Discrete Fractional Fourier Transform
A vibration estimation method for synthetic aperture radar (SAR) is presented based on a novel application of the discrete fractional Fourier transform (DFRFT). Small vibrations of ground targets introduce phase modulation in the SAR returned signals. With standard preprocessing of the returned signals, followed by the application of the DFRFT, the time-varying accelerations, frequencies, and displacements associated with vibrating objects can be extracted by successively estimating the quasi-instantaneous chirp rate in the phase-modulated signal in each subaperture. The performance of the proposed method is investigated quantitatively, and the measurable vibration frequencies and displacements are determined. Simulation results show that the proposed method can successfully estimate a two-component vibration at practical signal-to-noise levels. Two airborne experiments were also conducted using the Lynx SAR system in conjunction with vibrating ground test targets. The experiments demonstrated the correct estimation of a 1-Hz vibration with an amplitude of 1.5 cm and a 5-Hz vibration with an amplitude of 1.5 mm
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