12,413 research outputs found
Stochastic Hydrodynamic Synchronization in Rotating Energy Landscapes
Hydrodynamic synchronization provides a general mechanism for the spontaneous
emergence of coherent beating states in independently driven mesoscopic
oscillators. A complete physical picture of those phenomena is of definite
importance to the understanding of biological cooperative motions of cilia and
flagella. Moreover, it can potentially suggest novel routes to exploit
synchronization in technological applications of soft matter. We demonstrate
that driving colloidal particles in rotating energy landscapes results in a
strong tendency towards synchronization, favouring states where all beads
rotate in phase. The resulting dynamics can be described in terms of activated
jumps with transition rates that are strongly affected by hydrodynamics leading
to an increased probability and lifetime of the synchronous states. Using
holographic optical tweezers we quantitatively verify our predictions in a
variety of spatial configurations of rotors.Comment: Copyright (2013) by the American Physical Societ
Directed transport of active particles over asymmetric energy barriers
We theoretically and numerically investigate the transport of active colloids
to target regions, delimited by asymmetric energy barriers. We show that it is
possible to introduce a generalized effective temperature that is related to
the local variance of particle velocities. The stationary probability
distributions can be derived from a simple diffusion equation in the presence
of an inhomogeneous effective temperature resulting from the action of external
force fields. In particular, transitions rates over asymmetric energy barriers
can be unbalanced by having different effective temperatures over the two
slopes of the barrier. By varying the type of active noise, we find that equal
values of diffusivity and persistence time may produce strongly varied
effective temperatures and thus stationary distributions
Aging under Shear: Structural Relaxation of a Non-Newtonian Fluid
The influence of an applied shear field on the dynamics of an aging colloidal
suspension has been investigated by the dynamic light scattering determination
of the density autocorrelation function. Though a stationary state is never
observed, the slow dynamics crosses between two different non-equilibrium
regimes as soon as the structural relaxation time approaches the inverse shear
rate. In the shear dominated regime (at high shear rate values) the structural
relaxation time is found to be strongly sensitive to shear rate while aging
proceeds at a very slow rate. The effect of shear on the detailed shape of the
density autocorrelation function is quantitatively described assuming that the
structural relaxation process arises from the heterogeneous superposition of
many relaxing units each one independently coupled to shear with a parallel
composition rule for timescales.Comment: 5 pages, 5 figure
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
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
Colloidal attraction induced by a temperature gradient
Colloidal crystals are of extreme importance for applied research, such as
photonic crystals technology, and for fundamental studies in statistical
mechanics. Long range attractive interactions, such as capillary forces, can
drive the spontaneous assembly of such mesoscopic ordered structures. However
long range attractive forces are very rare in the colloidal realm. Here we
report a novel strong and long ranged attraction induced by a thermal gradient
in the presence of a wall. Switching on and off the thermal gradient we can
rapidly and reversibly form stable hexagonal 2D crystals. We show that the
observed attraction is hydrodynamic in nature and arises from thermal induced
slip flow on particle surfaces. We used optical tweezers to directly measure
the force law and compare it to an analytic prediction based on Stokes flow
driven by Marangoni forces.Comment: 4 pages, 4 figure
Testing circumstellar disk lifetimes in young embedded clusters associated with the Vela Molecular Ridge
Context. The Vela Molecular Ridge hosts a number of young embedded star
clusters in the same evolutionary stage. Aims. The main aim of the present work
is testing whether the fraction of members with a circumstellar disk in a
sample of clusters in the cloud D of the Vela Molecular Ridge, is consistent
with relations derived for larger samples of star clusters with an age spread.
Besides, we want to constrain the age of the young embedded star clusters
associated with cloud D. Methods. We carried out L (3.78 microns) photometry on
images of six young embedded star clusters associated with cloud D of the Vela
Molecular Ridge, taken with ISAAC at the VLT. These data are complemented with
the available HKs photometry. The 6 clusters are roughly of the same size and
appear to be in the same evolutionary stage. The fraction of stars with a
circumstellar disk was measured in each cluster by counting the fraction of
sources displaying a NIR excess in colour-colour (HKsL) diagrams. Results. The
L photometry allowed us to identify the NIR counterparts of the IRAS sources
associated with the clusters. The fraction of stars with a circumstellar disk
appears to be constant within errors for the 6 clusters. There is a hint that
this is lower for the most massive stars. The age of the clusters is
constrained to ~1-2 Myr. Conclusions. The fraction of stars with a
circumstellar disk in the observed sample is consistent with the relations
derived from larger samples of star clusters and with other age estimates for
cloud D. The fraction may be lower for the most massive stars. Our results
agree with a scenario where all intermediate and low-mass stars form with a
disk, whose lifetime is shorter for higher mass stars.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
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
Polar features in the flagellar propulsion of E. coli bacteria
E. coli bacteria swim following a run and tumble pattern. In the run state
all flagella join in a single helical bundle that propels the cell body along
approximately straight paths. When one or more flagellar motors reverse
direction the bundle unwinds and the cell randomizes its orientation. This
basic picture represents an idealization of a much more complex dynamical
problem. Although it has been shown that bundle formation can occur at either
pole of the cell, it is still unclear whether this two run states correspond to
asymmetric propulsion features. Using holographic microscopy we record the 3D
motions of individual bacteria swimming in optical traps. We find that most
cells possess two run states characterised by different propulsion forces,
total torque and bundle conformations. We analyse the statistical properties of
bundle reversal and compare the hydrodynamic features of forward and backward
running states. Our method is naturally multi-particle and opens up the way
towards controlled hydrodynamic studies of interacting swimming cells
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