197 research outputs found
Fluctuations of entropy production in the isokinetic ensemble
We discuss the microscopic definition of entropy production rate in a model
of a dissipative system: a sheared fluid in which the kinetic energy is kept
constant via a Gaussian thermostat. The total phase space contraction rate is
the sum of two statistically independent contributions: the first one is due to
the work of the conservative forces, is independent of the driving force and
does not vanish at zero drive, making the system non-conservative also in
equilibrium. The second is due to the work of the dissipative forces, and is
responsible for the average entropy production; the distribution of its
fluctuations is found to verify the Fluctuation Relation of Gallavotti and
Cohen. The distribution of the fluctuations of the total phase space
contraction rate also verify the Fluctuation Relation. It is compared with the
same quantity calculated in the isoenergetic ensemble: we find that the two
ensembles are equivalent, as conjectured by Gallavotti. Finally, we discuss the
implication of our results for experiments trying to verify the validity of the
FR.Comment: 8 pages, 4 figure
First-passage time of run-and-tumble particles
We solve the problem of first-passage time for run-and-tumble particles in
one dimension. Exact expression is derived for the mean first-passage time in
the general case, considering external force-fields and chemotactic-fields,
giving rise to space dependent swim-speed and tumble rate. Agreement between
theoretical formulae and numerical simulations is obtained in the analyzed case
studies -- constant and sinusoidal force fields, constant gradient chemotactic
field. Reported findings can be useful to get insights into very different
phenomena involving active particles, such as bacterial motion in external
fields, intracellular transport, cell migration, animal foraging
Configurational entropy of hard spheres
We numerically calculate the configurational entropy S_conf of a binary
mixture of hard spheres, by using a perturbed Hamiltonian method trapping the
system inside a given state, which requires less assumptions than the previous
methods [R.J. Speedy, Mol. Phys. 95, 169 (1998)]. We find that S_conf is a
decreasing function of packing fraction f and extrapolates to zero at the
Kauzmann packing fraction f_K = 0.62, suggesting the possibility of an ideal
glass-transition for hard spheres system. Finally, the Adam-Gibbs relation is
found to hold.Comment: 10 pages, 6 figure
Effective run-and-tumble dynamics of bacteria baths
{\it E. coli} bacteria swim in straight runs interrupted by sudden
reorientation events called tumbles. The resulting random walks give rise to
density fluctuations that can be derived analytically in the limit of non
interacting particles or equivalently of very low concentrations. However, in
situations of practical interest, the concentration of bacteria is always large
enough to make interactions an important factor. Using molecular dynamics
simulations, we study the dynamic structure factor of a model bacterial bath
for increasing values of densities. We show that it is possible to reproduce
the dynamics of density fluctuations in the system using a free run-and-tumble
model with effective fitting parameters. We discuss the dependence of these
parameters, e.g., the tumbling rate, tumbling time and self-propulsion
velocity, on the density of the bath
Run-and-tumble particles in speckle fields
The random energy landscapes developed by speckle fields can be used to
confine and manipulate a large number of micro-particles with a single laser
beam. By means of molecular dynamics simulations, we investigate the static and
dynamic properties of an active suspension of swimming bacteria embedded into
speckle patterns. Looking at the correlation of the density fluctuations and
the equilibrium density profiles, we observe a crossover phenomenon when the
forces exerted by the speckles are equal to the bacteria's propulsion
Phase diagram and complexity of mode-locked lasers: from order to disorder
We investigate mode-locking processes in lasers displaying a variable degree
of structural randomness, from standard optical cavities to multiple-scattering
media. By employing methods mutuated from spin-glass theory, we analyze the
mean-field Hamiltonian and derive a phase-diagram in terms of the pumping rate
and the degree of disorder. Three phases are found: i) paramagnetic,
corresponding to a noisy continuous wave emission, ii) ferromagnetic, that
describes the standard passive mode-locking, and iii) the spin-glass in which
the phases of the electromagnetic field are frozen in a exponentially large
number of configurations. The way the mode-locking threshold is affected by the
amount of disorder is quantified. The results are also relevant for other
physical systems displaying a random Hamiltonian, like Bose-Einstein
condensates and nonlinear optical beams.Comment: 4 pages, 2 figure
Saddles and softness in simple model liquids
We report a numerical study of saddles properties of the potential energy
landscape for soft spheres with different softness, i.e. different power n of
the interparticle repulsive potential. We find that saddle-based quantities
rescale into master curves once energies and temperatures are scaled by
mode-coupling temperature T_MCT, confirming and generalizing previous findings
obtained for Lennard-Jones like models.Comment: 2 pages, 2 figure
On G-fractional diffusion models in bounded domains
In the recent literature, the g-subdiffusion equation involving Caputo
fractional derivatives with respect to another function has been studied in
relation to anomalous diffusions with a continuous transition between different
subdiffusive regimes. In this paper we study the problem of g-fractional
diffusion in a bounded domain with absorbing boundaries. We find the explicit
solution for the initial-boundary value problem and we study the first passage
time distribution and the mean first-passage time (MFPT). An interestin outcome
is the proof that with a particular choice of the function it is possible
to obtain a finite MFPT, differently from the anomalous diffusion described by
a fractional heat equation involving the classical Caputo derivative
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