797 research outputs found
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
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
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
Lattice Boltzmann Simulations of Liquid Crystal Hydrodynamics
We describe a lattice Boltzmann algorithm to simulate liquid crystal
hydrodynamics. The equations of motion are written in terms of a tensor order
parameter. This allows both the isotropic and the nematic phases to be
considered. Backflow effects and the hydrodynamics of topological defects are
naturally included in the simulations, as are viscoelastic properties such as
shear-thinning and shear-banding.Comment: 14 pages, 5 figures, Revte
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
Equilibrium onions?
We demonstrate the possibility of a stable equilibrium multi-lamellar ("onion") phase in pure lamellar systems (no excess solvent) due to a sufficiently negative Gaussian curvature modulus. The onion phase is stabilized by non-linear elastic moduli coupled to a polydisperse size distribution (Apollonian packing) to allow space-filling without appreciable elastic distortion. This model is compared to experiments on copolymer-decorated lamellar surfactant systems, with reasonable qualitative agreement
Concentrating Membrane Proteins Using Asymmetric Traps and AC Electric Fields
Membrane proteins are key components of the plasma membrane and are responsible for control of chemical ionic gradients, metabolite and nutrient transfer, and signal transduction between the interior of cells and the external environment. Of the genes in the human genome, 30% code for membrane proteins (Krogh et al. J. Mol. Biol.2001, 305, 567). Furthermore, many FDA-approved drugs target such proteins (Overington et al. Nat. Rev. Drug Discovery2006, 5, 993). However, the structure-function relationships of these are notably sparse because of difficulties in their purification and handling outside of their membranous environment. Methods that permit the manipulation of membrane components while they are still in the membrane would find widespread application in separation, purification, and eventual structure-function determination of these species (Poo et al. Nature1977, 265, 602). Here we show that asymmetrically patterned supported lipid bilayers in combination with AC electric fields can lead to efficient manipulation of charged components. We demonstrate the concentration and trapping of such components through the use of a “nested trap” and show that this method is capable of yielding an approximately 30-fold increase in the average protein concentration. Upon removal of the field, the material remains trapped for several hours as a result of topographically restricted diffusion. Our results indicate that this method can be used for concentrating and trapping charged membrane components while they are still within their membranous environment. We anticipate that our approach could find widespread application in the manipulation and study of membrane proteins
Nonequilibrium statistical mechanics of shear flow: invariant quantities and current relations
In modeling nonequilibrium systems one usually starts with a definition of
the microscopic dynamics, e.g., in terms of transition rates, and then derives
the resulting macroscopic behavior. We address the inverse question for a class
of steady state systems, namely complex fluids under continuous shear flow: how
does an externally imposed shear current affect the microscopic dynamics of the
fluid? The answer can be formulated in the form of invariant quantities, exact
relations for the transition rates in the nonequilibrium steady state, as
discussed in a recent letter [A. Baule and R. M. L. Evans, Phys. Rev. Lett.
101, 240601 (2008)]. Here, we present a more pedagogical account of the
invariant quantities and the theory underlying them, known as the
nonequilibrium counterpart to detailed balance (NCDB). Furthermore, we
investigate the relationship between the transition rates and the shear current
in the steady state. We show that a fluctuation relation of the
Gallavotti-Cohen type holds for systems satisfying NCDB.Comment: 24 pages, 11 figure
Phase Separation of Rigid-Rod Suspensions in Shear Flow
We analyze the behavior of a suspension of rigid rod-like particles in shear
flow using a modified version of the Doi model, and construct diagrams for
phase coexistence under conditions of constant imposed stress and constant
imposed strain rate, among paranematic, flow-aligning nematic, and log-rolling
nematic states. We calculate the effective constitutive relations that would be
measured through the regime of phase separation into shear bands. We calculate
phase coexistence by examining the stability of interfacial steady states and
find a wide range of possible ``phase'' behaviors.Comment: 23 pages 19 figures, revised version to be published in Physical
Review
Alphavirus Replicon Particles Expressing TRP-2 Provide Potent Therapeutic Effect on Melanoma through Activation of Humoral and Cellular Immunity
Malignant melanoma is the deadliest form of skin cancer and is refractory to conventional chemotherapy and radiotherapy. Therefore alternative approaches to treat this disease, such as immunotherapy, are needed. Melanoma vaccine design has mainly focused on targeting CD8+ T cells. Activation of effector CD8+ T cells has been achieved in patients, but provided limited clinical benefit, due to immune-escape mechanisms established by advanced tumors. We have previously shown that alphavirus-based virus-like replicon particles (VRP) simultaneously activate strong cellular and humoral immunity against the weakly immunogenic melanoma differentiation antigen (MDA) tyrosinase. Here we further investigate the antitumor effect and the immune mechanisms of VRP encoding different MDAs.VRP encoding different MDAs were screened for their ability to prevent the growth of the B16 mouse transplantable melanoma. The immunologic mechanisms of efficacy were investigated for the most effective vaccine identified, focusing on CD8+ T cells and humoral responses. To this end, ex vivo immune assays and transgenic mice lacking specific immune effector functions were used. The studies identified a potent therapeutic VRP vaccine, encoding tyrosinase related protein 2 (TRP-2), which provided a durable anti-tumor effect. The efficacy of VRP-TRP2 relies on a novel immune mechanism of action requiring the activation of both IgG and CD8+ T cell effector responses, and depends on signaling through activating Fcγ receptors.This study identifies a VRP-based vaccine able to elicit humoral immunity against TRP-2, which plays a role in melanoma immunotherapy and synergizes with tumor-specific CD8+ T cell responses. These findings will aid in the rational design of future immunotherapy clinical trials
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