915 research outputs found
A Minimal Model for Vorticity and Gradient Banding in Complex Fluids
A general phenomenological reaction-diffusion model for flow-induced phase
transitions in complex fluids is presented. The model consists of an equation
of motion for a nonconserved composition variable, coupled to a Newtonian
stress relations for the reactant and product species. Multivalued reaction
terms allow for different homogeneous phases to coexist with each other,
resulting in banded composition and shear rate profiles. The one-dimensional
equation of motion is evolved from a random initial state to its final
steady-state. We find that the system chooses banded states over homogeneous
states, depending on the shape of the stress constitutive curve and the
magnitude of the diffusion coefficient. Banding in the flow gradient direction
under shear rate control is observed for shear-thinning transitions, while
banding in the vorticity direction under stress control is observed for
shear-thickening transitions.Comment: 11 pages, submitted to Eur Phys J
Two-dimensional perturbations in a scalar model for shear banding
We present an analytical study of a toy model for shear banding, without
normal stresses, which uses a piecewise linear approximation to the flow curve
(shear stress as a function of shear rate). This model exhibits multiple
stationary states, one of which is linearly stable against general
two-dimensional perturbations. This is in contrast to analogous results for the
Johnson-Segalman model, which includes normal stresses, and which has been
reported to be linearly unstable for general two-dimensional perturbations.
This strongly suggests that the linear instabilities found in the
Johnson-Segalman can be attributed to normal stress effects.Comment: 16 pages, 10 figures, to appear in EPJE, available online first,
click DOI or http://www.springerlink.com/content/q1q0187385017628
Budding and Domain Shape Transformations in Mixed Lipid Films and Bilayer Membranes
We study the stability and shapes of domains with spontaneous curvature in
fluid films and membranes, embedded in a surrounding membrane with zero
spontaneous curvature. These domains can result from the inclusion of an
impurity in a fluid membrane, or from phase separation within the membrane. We
show that for small but finite line and surface tensions and for finite
spontaneous curvatures, an equilibrium phase of protruding circular domains is
obtained at low impurity concentrations. At higher concentrations, we predict a
transition from circular domains, or "caplets", to stripes. In both cases, we
calculate the shapes of these domains within the Monge representation for the
membrane shape. With increasing line tension, we show numerically that there is
a budding transformation from stable protruding circular domains to spherical
buds. We calculate the full phase diagram, and demonstrate a two triple points,
of respectively bud-flat-caplet and flat-stripe-caplet coexistence.Comment: 14 pages, to appear in Phys Rev
Statistical mechanics far from equilibrium: prediction and test for a sheared system
We report the complete statistical treatment of a system of particles
interacting via Newtonian forces in continuous boundary-driven flow, far from
equilibrium. By numerically time-stepping the force-balance equations of a
model fluid we measure occupancies and transition rates in simulation. The
high-shear-rate simulation data verify the invariant quantities predicted by
our statistical theory, thus demonstrating that a class of non-equilibrium
steady states of matter, namely sheared complex fluids, is amenable to
statistical treatment from first principles.Comment: 4 pages plus a 3-page pdf supplemen
Micro- vs. macro-phase separation in binary blends of poly(styrene)-poly(isoprene) and poly(isoprene)-poly(ethylene oxide) diblock copolymers
In this paper we present an experimentally determined phase diagram of binary blends of the diblock copolymers poly(styrene)-poly(isoprene) and poly(isoprene)-poly(ethylene oxide). At high temperatures, the blends form an isotropic mixture. Upon lowering the temperature, the blend macro-phase separates before micro-phase separation occurs. The observed phase diagram is compared to theoretical predictions based on experimental parameters. In the low-temperature phase the crystallisation of the poly(ethylene oxide) block influences the spacing of the ordered phase
Nano-scale mechanical probing of supported lipid bilayers with atomic force microscopy
We present theory and experiments for the force-distance curve of an
atomic force microscope (AFM) tip (radius ) indenting a supported fluid
bilayer (thickness ). For realistic conditions the force is dominated by
the area compressibility modulus of the bilayer, and, to an
excellent approximation, given by . The
experimental AFM force curves from coexisting liquid ordered and liquid
disordered domains in 3-component lipid bilayers are well-described by our
model, and provides in agreement with literature values. The liquid
ordered phase has a yield like response that we model by hydrogen bond
breaking.Comment: 6 pages, 6 figures, accepted for publication in Physical Review
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
A Survey of the Fishes of the Mulberry River, Arkansas
Announcement of plans to dam Mulberry River, Arkansas, by the United States Army Corps of Engineers has generated some dispute. Most agruments against damming the stream revolve around environmental degradation and loss of aesthetic values. This report serves as a pre-impoundment survey of the fishes of the Mulberry River so that possible effects of impoundment can be more objectively assessed. Knowledge on the fishes of the Mulberry River is severely lacking. The first study was by Jordan and Gilbert (1886) who collected in the southern U.S. in July, August, and September, 1884. They collected in many streams in Arkansas including several tributaries of the Arkansas River
Coarse-grained simulations of flow-induced nucleation in semi-crystalline polymers
We perform kinetic Monte Carlo simulations of flow-induced nucleation in
polymer melts with an algorithm that is tractable even at low undercooling. The
configuration of the non-crystallized chains under flow is computed with a
recent non-linear tube model. Our simulations predict both enhanced nucleation
and the growth of shish-like elongated nuclei for sufficiently fast flows. The
simulations predict several experimental phenomena and theoretically justify a
previously empirical result for the flow-enhanced nucleation rate. The
simulations are highly pertinent to both the fundamental understanding and
process modeling of flow-induced crystallization in polymer melts.Comment: 17 pages, 6 eps figure
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
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