414 research outputs found
Active nematics on a substrate: giant number fluctuations and long-time tails
We construct the equations of motion for the coupled dynamics of order
parameter and concentration for the nematic phase of driven particles on a
solid surface, and show that they imply (i) giant number fluctuations, with a
standard deviation proportional to the mean and (ii) long-time tails in the autocorrelation of the particle velocities in dimensions
despite the absence of a hydrodynamic velocity field. Our predictions can be
tested in experiments on aggregates of amoeboid cells as well as on layers of
agitated granular matter.Comment: Submitted to Europhys Lett 26 Aug 200
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
The pressure-volume-temperature relationship of cellulose
Pressure–volume–temperature (PVT) mea- surements of a-cellulose with different water contents, were performed at temperatures from 25 to 180 °C and pressures from 19.6 to 196 MPa. PVT measurements allowed observation of the combined effects of pressure and temperature on the specific volume during cellulose thermo-compression. All isobars showed a decrease in cellulose specific volume with temperature. This densification is associated with a transition process of the cellulose, occurring at a temperature defined by the inflection point Tt of the isobar curve. Tt decreases from 110 to 40 °C with pressure and is lower as moisture content increases. For isobars obtained at high pressures and high moisture contents, after attaining a minimum, an increase in volume is observed with temperature that may be related to free water evaporation. PVT a-cellulose experimental data was compared with predicted values from a regression analysis of the Tait equations of state, usually applied to synthetic polymers. Good correla- tions were observed at low temperatures and low pressures. The densification observed from the PVT experimental data, at a temperature that decreases with pressure, could result from a sintering phenomenon, but more research is needed to actually understand the cohesion mechanism under these conditions
Strong Phase Separation in a Model of Sedimenting Lattices
We study the steady state resulting from instabilities in crystals driven
through a dissipative medium, for instance, a colloidal crystal which is
steadily sedimenting through a viscous fluid. The problem involves two coupled
fields, the density and the tilt; the latter describes the orientation of the
mass tensor with respect to the driving field. We map the problem to a 1-d
lattice model with two coupled species of spins evolving through conserved
dynamics. In the steady state of this model each of the two species shows
macroscopic phase separation. This phase separation is robust and survives at
all temperatures or noise levels--- hence the term Strong Phase Separation.
This sort of phase separation can be understood in terms of barriers to
remixing which grow with system size and result in a logarithmically slow
approach to the steady state. In a particular symmetric limit, it is shown that
the condition of detailed balance holds with a Hamiltonian which has
infinite-ranged interactions, even though the initial model has only local
dynamics. The long-ranged character of the interactions is responsible for
phase separation, and for the fact that it persists at all temperatures.
Possible experimental tests of the phenomenon are discussed.Comment: To appear in Phys Rev E (1 January 2000), 16 pages, RevTex, uses
epsf, three ps figure
Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: Molecular Dynamics Simulation
The scission kinetics of bottle-brush molecules in solution and on an
adhesive substrate is modeled by means of Molecular Dynamics simulation with
Langevin thermostat. Our macromolecules comprise a long flexible polymer
backbone with segments, consisting of breakable bonds, along with two side
chains of length , tethered to each segment of the backbone. In agreement
with recent experiments and theoretical predictions, we find that bond cleavage
is significantly enhanced on a strongly attractive substrate even though the
chemical nature of the bonds remains thereby unchanged.
We find that the mean bond life time decreases upon adsorption by
more than an order of magnitude even for brush molecules with comparatively
short side chains $N=1 \div 4$. The distribution of scission probability along
the bonds of the backbone is found to be rather sensitive regarding the
interplay between length and grafting density of side chains. The life time
declines with growing contour length as ,
and with side chain length as . The probability
distribution of fragment lengths at different times agrees well with
experimental observations. The variation of the mean length of the
fragments with elapsed time confirms the notion of the thermal degradation
process as a first order reaction.Comment: 15 pages, 7 figure
Constraining The Universal Lepton Asymmetry
The relic cosmic background neutrinos accompanying the cosmic microwave
background (CMB) photons may hide a universal lepton asymmetry orders of
magnitude larger than the universal baryon asymmetry. At present, the only
direct way to probe such an asymmetry is through its effect on the abundances
of the light elements produced during primordial nucleosynthesis. The relic
light element abundances also depend on the baryon asymmetry, parameterized by
the baryon density parameter (eta_B = n_B/n_gamma = 10^(-10)*eta_10), and on
the early-universe expansion rate, parameterized by the expansion rate factor
(S = H'/H) or, equivalently by the effective number of neutrinos (N_nu = 3 +
43(S^2 - 1)/7). We use data from the CMB (and Large Scale Structure: LSS) along
with the observationally-inferred relic abundances of deuterium and helium-4 to
provide new bounds on the universal lepton asymmetry, finding for eta_L, the
analog of eta_B, 0.072 +/- 0.053 if it is assumed that N_nu = 3 and, 0.115 +/-
0.095 along with N_nu = 3.3^{+0.7}_{-0.6}, if N_nu is free to vary
Isocurvature perturbations in extra radiation
Recent cosmological observations, including measurements of the CMB
anisotropy and the primordial helium abundance, indicate the existence of an
extra radiation component in the Universe beyond the standard three neutrino
species. In this paper we explore the possibility that the extra radiation has
isocurvatrue fluctuations. A general formalism to evaluate isocurvature
perturbations in the extra radiation is provided in the mixed inflaton-curvaton
system, where the extra radiation is produced by the decay of both scalar
fields. We also derive constraints on the abundance of the extra radiation and
the amount of its isocurvature perturbation. Current observational data favors
the existence of an extra radiation component, but does not indicate its having
isocurvature perturbation. These constraints are applied to some particle
physics motivated models. If future observations detect isocurvature
perturbations in the extra radiation, it will give us a hint to the origin of
the extra radiation.Comment: 41 pages, 8 figures; version accepted for publication in JCA
Periodic and Quasiperiodic Motion of an Elongated Microswimmer in Poiseuille Flow
We study the dynamics of a prolate spheroidal microswimmer in Poiseuille flow
for different flow geometries. When moving between two parallel plates or in a
cylindrical microchannel, the swimmer performs either periodic swinging or
periodic tumbling motion. Although the trajectories of spherical and elongated
swimmers are qualitatively similar, the swinging and tumbling frequency
strongly depends on the aspect ratio of the swimmer. In channels with reduced
symmetry the swimmers perform quasiperiodic motion which we demonstrate
explicitely for swimming in a channel with elliptical cross section
WMAP 5-year constraints on lepton asymmetry and radiation energy density: Implications for Planck
In this paper we set bounds on the radiation content of the Universe and
neutrino properties by using the WMAP-5 year CMB measurements complemented with
most of the existing CMB and LSS data (WMAP5+All),imposing also self-consistent
BBN constraints on the primordial helium abundance. We consider lepton
asymmetric cosmological models parametrized by the neutrino degeneracy
parameter and the variation of the relativistic degrees of freedom, due to
possible other physical processes occurred between BBN and structure formation
epochs. We find that WMAP5+All data provides strong bounds on helium mass
fraction and neutrino degeneracy parameter that rivals the similar bounds
obtained from the conservative analysis of the present data on helium
abundance. We also find a strong correlation between the matter energy density
and the redshift of matter-radiation equality, z_re, showing that we observe
non-zero equivalent number of relativistic neutrinos mainly via the change of
the of z_re, rather than via neutrino anisotropic stress claimed by the WMAP
team. We forecast that the CMB temperature and polarization measurements
observed with high angular resolutions and sensitivities by the future Planck
satellite will reduce the errors on these parameters down to values fully
consistent with the BBN bounds
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