917 research outputs found
Instability and front propagation in laser-tweezed lipid bilayer tubules
We study the mechanism of the `pearling' instability seen recently in
experiments on lipid tubules under a local applied laser intensity. We argue
that the correct boundary conditions are fixed chemical potentials, or surface
tensions \Sigma, at the laser spot and the reservoir in contact with the
tubule. We support this with a microscopic picture which includes the intensity
profile of the laser beam, and show how this leads to a steady-state flow of
lipid along the surface and gradients in the local lipid concentration and
surface tension (or chemical potential). This leads to a natural explanation
for front propagation and makes several predictions based on the tubule length.
While most of the qualitative conclusions of previous studies remain the same,
the `ramped' control parameter (surface tension) implies several new
qualitative results. We also explore some of the consequences of front
propagation into a noisy (due to pre-existing thermal fluctuations) unstable
medium.Comment: 12 page latex + figures using epsf.sty to be published in Journal de
Physique II, January 199
Perspectives on the viscoelasticity and flow behavior of entangled linear and branched polymers
We briefly review the recent advances in the rheology of entangled polymers
and identify emerging research trends and outstanding challenges, especially
with respect to branched polymers. Emphasis is placed on the role of
well-characterized model systems, as well as the synergy of
synthesis-characterization, rheometry and modeling/simulations. The theoretical
framework for understanding the observed linear and nonlinear rheological
phenomena is the tube model which is critically assessed in view of its
successes and shortcomings, whereas alternative approaches are briefly
discussed. Finally, intriguing experimental findings and controversial issues
that merit consistent explanation, such as shear banding instabilities,
multiple stress overshoots in transient simple shear and enhanced steady-state
elongational viscosity in polymer solutions, are discussed, whereas future
directions such as branch point dynamics and anisotropic monomeric friction are
outlined.Comment: 25 pages, accepted for publication in Journal of Physics Condensed
Matter (August 2015
Validation of the Jarzynski relation for a system with strong thermal coupling: an isothermal ideal gas model
We revisit the paradigm of an ideal gas under isothermal conditions. A moving piston performs work on an ideal gas in a container that is strongly coupled to a heat reservoir. The thermal coupling is modeled by stochastic scattering at the boundaries. In contrast to recent studies of an adiabatic ideal gas with a piston [R.C. Lua and A.Y. Grosberg, J. Phys. Chem. B 109, 6805 (2005); I. Bena et al., Europhys. Lett. 71, 879 (2005)], the container and piston stay in contact with the heat bath during the work process. Under this condition the heat reservoir as well as the system depend on the work parameter lambda and microscopic reversibility is broken for a moving piston. Our model is thus not included in the class of systems for which the nonequilibrium work theorem has been derived rigorously either by Hamiltonian [C. Jarzynski, J. Stat. Mech. (2004) P09005] or stochastic methods [G.E. Crooks, J. Stat. Phys. 90, 1481 (1998)]. Nevertheless the validity of the nonequilibrium work theorem is confirmed both numerically for a wide range of parameter values and analytically in the limit of a very fast moving piston, i.e., in the far nonequilibrium regime
Structural disjoining potential for grain boundary premelting and grain coalescence from molecular-dynamics simulations
We describe a molecular dynamics framework for the direct calculation of the
short-ranged structural forces underlying grain-boundary premelting and
grain-coalescence in solidification. The method is applied in a comparative
study of (i) a Sigma 9 120 degress twist and (ii) a Sigma 9 {411}
symmetric tilt boundary in a classical embedded-atom model of elemental Ni.
Although both boundaries feature highly disordered structures near the melting
point, the nature of the temperature dependence of the width of the disordered
regions in these boundaries is qualitatively different. The former boundary
displays behavior consistent with a logarithmically diverging premelted layer
thickness as the melting temperature is approached from below, while the latter
displays behavior featuring a finite grain-boundary width at the melting point.
It is demonstrated that both types of behavior can be quantitatively described
within a sharp-interface thermodynamic formalism involving a width-dependent
interfacial free energy, referred to as the disjoining potential. The
disjoining potential for boundary (i) is calculated to display a monotonic
exponential dependence on width, while that of boundary (ii) features a weak
attractive minimum. The results of this work are discussed in relation to
recent simulation and theoretical studies of the thermodynamic forces
underlying grain-boundary premelting.Comment: 24 pages, 8 figures, 1 tabl
Monte Carlo Study of Short-Range Order and Displacement Effects in Disordered CuAu
The correlation between local chemical environment and atomic displacements
in disordered CuAu alloy has been studied using Monte Carlo simulations based
on the effective medium theory (EMT) of metallic cohesion. These simulations
correctly reproduce the chemically-specific nearest-neighbor distances in the
random alloy across the entire Cu\$_x\$Au\$_{1-x}\$ concentration range. In the
random equiatomic CuAu alloy, the chemically specific pair distances depend
strongly on the local atomic environment (i.e. fraction of like/unlike nearest
neighbors). In CuAu alloy with short-range order, the relationship between
local environment and displacements remains qualitatively similar. However the
increase in short-range order causes the average Cu-Au distance to decrease
below the average Cu-Cu distance, as it does in the ordered CuAuI phase. Many
of these trends can be understood qualitatively from the different neutral
sphere radii and compressibilities of the Cu and Au atoms.Comment: 9 pages, 5 figures, 2 table
The Johnson-Segalman model with a diffusion term in Couette flow
We study the Johnson-Segalman (JS) model as a paradigm for some complex
fluids which are observed to phase separate, or ``shear-band'' in flow. We
analyze the behavior of this model in cylindrical Couette flow and demonstrate
the history dependence inherent in the local JS model. We add a simple gradient
term to the stress dynamics and demonstrate how this term breaks the degeneracy
of the local model and prescribes a much smaller (discrete, rather than
continuous) set of banded steady state solutions. We investigate some of the
effects of the curvature of Couette flow on the observable steady state
behavior and kinetics, and discuss some of the implications for metastability.Comment: 14 pp, to be published in Journal of Rheolog
Instability of Myelin Tubes under Dehydration: deswelling of layered cylindrical structures
We report experimental observations of an undulational instability of myelin
figures. Motivated by this, we examine theoretically the deformation and
possible instability of concentric, cylindrical, multi-lamellar membrane
structures. Under conditions of osmotic stress (swelling or dehydration), we
find a stable, deformed state in which the layer deformation is given by \delta
R ~ r^{\sqrt{B_A/(hB)}}, where B_A is the area compression modulus, B is the
inter-layer compression modulus, and h is the repeat distance of layers. Also,
above a finite threshold of dehydration (or osmotic stress), we find that the
system becomes unstable to undulations, first with a characteristic wavelength
of order \sqrt{xi d_0}, where xi is the standard smectic penetration depth and
d_0 is the thickness of dehydrated region.Comment: 5 pages + 3 figures [revtex 4
Milky Way White Dwarfs as Sub-GeV to Multi-TeV Dark Matter Detectors
We show that Milky Way white dwarfs are excellent targets for dark matter
(DM) detection. Using Fermi and H.E.S.S. Galactic center gamma-ray data, we
investigate sensitivity to DM annihilating within white dwarfs into long-lived
or boosted mediators and producing detectable gamma rays. Depending on the
Galactic DM distribution, we set new constraints on the spin-independent
scattering cross section down to cm in the sub-GeV DM
mass range, which is multiple orders of magnitude stronger than existing
limits. For a generalized NFW DM profile, we find that our white dwarf
constraints exceed spin-independent direct detection limits across most of the
sub-GeV to multi-TeV DM mass range, achieving sensitivities as low as about
cm. In addition, we improve earlier versions of the DM capture
calculation in white dwarfs, by including the low-temperature distribution of
nuclei when the white dwarf approaches crystallization. This yields smaller
capture rates than previously calculated by a factor of a few up to two orders
of magnitude, depending on white dwarf size and the astrophysical system.Comment: 29 pages, 5 figure
Uniaxial and biaxial soft deformations of nematic elastomers
We give a geometric interpretation of the soft elastic deformation modes of
nematic elastomers, with explicit examples, for both uniaxial and biaxial
nematic order. We show the importance of body rotations in this non-classical
elasticity and how the invariance under rotations of the reference and target
states gives soft elasticity (the Golubovic and Lubensky theorem). The role of
rotations makes the Polar Decomposition Theorem vital for decomposing general
deformations into body rotations and symmetric strains. The role of the square
roots of tensors is discussed in this context and that of finding explicit
forms for soft deformations (the approach of Olmsted).Comment: 10 pages, 10 figures, RevTex, AmsTe
Loss of solutions in shear banding fluids in shear banding fluids driven by second normal stress differences
Edge fracture occurs frequently in non-Newtonian fluids. A similar
instability has often been reported at the free surface of fluids undergoing
shear banding, and leads to expulsion of the sample. In this paper the
distortion of the free surface of such a shear banding fluid is calculated by
balancing the surface tension against the second normal stresses induced in the
two shear bands, and simultaneously requiring a continuous and smooth meniscus.
We show that wormlike micelles typically retain meniscus integrity when shear
banding, but in some cases can lose integrity for a range of average applied
shear rates during which one expects shear banding. This meniscus fracture
would lead to ejection of the sample as the shear banding region is swept
through. We further show that entangled polymer solutions are expected to
display a propensity for fracture, because of their much larger second normal
stresses. These calculations are consistent with available data in the
literature. We also estimate the meniscus distortion of a three band
configuration, as has been observed in some wormlike micellar solutions in a
cone and plate geometry.Comment: 23 pages, to be published in Journal of Rheolog
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