768 research outputs found
Phase Transitions in Lyotropic Nematic Gels
In this paper, we discuss the equilibrium phases and collapse transitions of
a lyotropic nematic gel immersed in an isotropic solvent. A nematic gel
consists of a cross-linked polymer network with rod-like molecules embedded in
it. Upon decreasing the quality of the solvent, we find that a lyotropic
nematic gel undergoes a discontinuous volume change accompanied by an
isotropic-nematic transition. We also present phase diagrams that these systems
may exhibit. In particular, we show that coexistence of two isotropic phases,
of two nematic phases, or of an isotropic and a nematic phase can occur.Comment: 13 pages Revtex, 10 figures, submitted to EPJ
Fluctuations of a driven membrane in an electrolyte
We develop a model for a driven cell- or artificial membrane in an
electrolyte. The system is kept far from equilibrium by the application of a DC
electric field or by concentration gradients, which causes ions to flow through
specific ion-conducting units (representing pumps, channels or natural pores).
We consider the case of planar geometry and Debye-H\"{u}ckel regime, and obtain
the membrane equation of motion within Stokes hydrodynamics. At steady state,
the applied field causes an accumulation of charges close to the membrane,
which, similarly to the equilibrium case, can be described with renormalized
membrane tension and bending modulus. However, as opposed to the equilibrium
situation, we find new terms in the membrane equation of motion, which arise
specifically in the out-of-equilibrium case. We show that these terms lead in
certain conditions to instabilities.Comment: 7 pages, 2 figures. submitted to Europhys. Let
Energy versus information based estimations of dissipation using a pair of magnetic colloidal particles
Using the framework of stochastic thermodynamics, we present an experimental
study of a doublet of magnetic colloidal particles which is manipulated by a
time-dependent magnetic field. Due to hydrodynamic interactions, each bead
experiences a state-dependent friction, which we characterize using a
hydrodynamic model. In this work, we compare two estimates of the dissipation
in this system: the first one is energy based since it relies on the measured
interaction potential, while the second one is information based since it uses
only the information content of the trajectories. While the latter only offers
a lower bound of the former, we find it to be simple to implement and of
general applicability to more complex systems.Comment: Main text: 5 pages, 4 figures. Supplementary material: 5 pages, 5
figure
Gallavotti-Cohen-Type symmetry related to cycle decompositions for Markov chains and biochemical applications
We slightly extend the fluctuation theorem obtained in \cite{LS} for sums of
generators, considering continuous-time Markov chains on a finite state space
whose underlying graph has multiple edges and no loop. This extended frame is
suited when analyzing chemical systems. As simple corollary we derive in a
different method the fluctuation theorem of D. Andrieux and P. Gaspard for the
fluxes along the chords associated to a fundamental set of oriented cycles
\cite{AG2}.
We associate to each random trajectory an oriented cycle on the graph and we
decompose it in terms of a basis of oriented cycles. We prove a fluctuation
theorem for the coefficients in this decomposition. The resulting fluctuation
theorem involves the cycle affinities, which in many real systems correspond to
the macroscopic forces. In addition, the above decomposition is useful when
analyzing the large deviations of additive functionals of the Markov chain. As
example of application, in a very general context we derive a fluctuation
relation for the mechanical and chemical currents of a molecular motor moving
along a periodic filament.Comment: 23 pages, 5 figures. Correction
Modified Fluctuation-dissipation theorem for non-equilibrium steady-states and applications to molecular motors
We present a theoretical framework to understand a modified
fluctuation-dissipation theorem valid for systems close to non-equilibrium
steady-states and obeying markovian dynamics. We discuss the interpretation of
this result in terms of trajectory entropy excess. The framework is illustrated
on a simple pedagogical example of a molecular motor. We also derive in this
context generalized Green-Kubo relations similar to the ones derived recently
by Seifert., Phys. Rev. Lett., 104, 138101 (2010) for more general networks of
biomolecular states.Comment: 6 pages, 2 figures, submitted in EP
Coherent Backscattering of light in a magnetic field
This paper describes how coherent backscattering is altered by an external
magnetic field. In the theory presented, magneto-optical effects occur inside
Mie scatterers embedded in a non-magnetic medium. Unlike previous theories
based on point-like scatterers, the decrease of coherent backscattering is
obtained in leading order of the magnetic field using rigorous Mie theory. This
decrease is strongly enhanced in the proximity of resonances, which cause the
path length of the wave inside a scatterer to be increased. Also presented is a
novel analysis of the shape of the backscattering cone in a magnetic field.Comment: 27 pages, 5 figures, Revtex, to appear in Phys. Rev.
Stochastic model for nucleosome sliding in the presence of DNA ligands
Heat-induced mobility of nucleosomes along DNA is an experimentally
well-studied phenomenon. A recent experiment shows that the repositioning is
modified in the presence of minor-groove binding DNA ligands. We present here a
stochastic three-state model for the diffusion of a nucleosome along DNA in the
presence of such ligands. It allows us to describe the dynamics and the steady
state of such a motion analytically. The analytical results are in excellent
agreement with numerical simulations of this stochastic process.With this
model, we study the response of a nucleosome to an external force and how it is
affected by the presence of ligands.Comment: 10 pages, 8 figures, submitted to Eur. Phys. J.
Electrostatic and electrokinetic contributions to the elastic moduli of a driven membrane
We discuss the electrostatic contribution to the elastic moduli of a cell or
artificial membrane placed in an electrolyte and driven by a DC electric field.
The field drives ion currents across the membrane, through specific channels,
pumps or natural pores. In steady state, charges accumulate in the Debye layers
close to the membrane, modifying the membrane elastic moduli. We first study a
model of a membrane of zero thickness, later generalizing this treatment to
allow for a finite thickness and finite dielectric constant. Our results
clarify and extend the results presented in [D. Lacoste, M. Cosentino
Lagomarsino, and J. F. Joanny, Europhys. Lett., {\bf 77}, 18006 (2007)], by
providing a physical explanation for a destabilizing term proportional to
\kps^3 in the fluctuation spectrum, which we relate to a nonlinear ()
electro-kinetic effect called induced-charge electro-osmosis (ICEO). Recent
studies of ICEO have focused on electrodes and polarizable particles, where an
applied bulk field is perturbed by capacitive charging of the double layer and
drives flow along the field axis toward surface protrusions; in contrast, we
predict "reverse" ICEO flows around driven membranes, due to curvature-induced
tangential fields within a non-equilibrium double layer, which hydrodynamically
enhance protrusions. We also consider the effect of incorporating the dynamics
of a spatially dependent concentration field for the ion channels.Comment: 22 pages, 10 figures. Under review for EPJ
Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio
We study in-plane magnetic tunnel junctions with additional perpendicular
polarizer for subnanosecond-current-induced switching memories. The
spin-transfer-torque switching dynamics was studied as a function of the cell
aspect ratio both experimentally and by numerical simulations using the
macrospin model. We show that the anisotropy field plays a significant role in
the dynamics, along with the relative amplitude of the two spin-torque
contributions. This was confirmed by micromagnetic simulations. Real-time
measurements of the reversal were performed with samples of low and high aspect
ratio. For low aspect ratios, a precessional motion of the magnetization was
observed and the effect of temperature on the precession coherence was studied.
For high aspect ratios, we observed magnetization reversals in less than 1 ns
for high enough current densities, the final state being controlled by the
current direction in the magnetic tunnel junction cell.Comment: 6 pages, 7 figure
Fluctuation theorem and large deviation function for a solvable model of a molecular motor
We study a discrete stochastic model of a molecular motor. This discrete
model can be viewed as a \emph{minimal} ratchet model. We extend our previous
work on this model, by further investigating the constraints imposed by the
Fluctuation Theorem on the operation of a molecular motor far from equilibrium.
In this work, we show the connections between different formulations of the
Fluctuation Theorem. One formulation concerns the generating function of the
currents while another one concerns the corresponding large deviation function,
which we have calculated exactly for this model. A third formulation of FT
concerns the ratio of the probability of making one forward step to the
probability of making one backward step. The predictions of this last
formulation of the Fluctuation Theorem adapted to our model are in very good
agreement with the data of Carter and Cross [Nature, {\bf 435}, 308 (2005)] on
single molecule measurements with kinesin. Finally, we show that all the
formulations of FT can be understood from the notion of entropy production.Comment: 15 pages, 9 figure
- …