7,257 research outputs found
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
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
Hydrodynamic Trapping of Swimming Bacteria by Convex Walls
Swimming bacteria display a remarkable tendency to move along flat surfaces for prolonged times. This behavior may have a biological importance but can also be exploited by using microfabricated structures to manipulate bacteria. The main physical mechanism behind the surface entrapment of swimming bacteria is, however, still an open question. By studying the swimming motion of Escherichia coli cells near microfabricated pillars of variable size, we show that cell entrapment is also present for convex walls of sufficiently low curvature. Entrapment is, however, markedly reduced below a characteristic radius. Using a simple hydrodynamic model, we predict that trapped cells swim at a finite angle with the wall and a precise relation exists between the swimming angle at a flat wall and the critical radius of curvature for entrapment. Both predictions are quantitatively verified by experimental data. Our results demonstrate that the main mechanism for wall entrapment is hydrodynamic in nature and show the possibility of inhibiting cell adhesion, and thus biofilm formation, using convex features of appropriate curvature
Focusing and imaging with increased numerical apertures through multimode fibers with micro-fabricated optics
The use of individual multimode optical fibers in endoscopy applications has
the potential to provide highly miniaturized and noninvasive probes for
microscopy and optical micromanipulation. A few different strategies have been
proposed recently, but they all suffer from intrinsically low resolution
related to the low numerical aperture of multimode fibers. Here, we show that
two-photon polymerization allows for direct fabrication of micro-optics
components on the fiber end, resulting in an increase of the numerical aperture
to a value that is close to 1. Coupling light into the fiber through a spatial
light modulator, we were able to optically scan a submicrometer spot (300 nm
FWHM) over an extended region, facing the opposite fiber end. Fluorescence
imaging with improved resolution is also demonstrated.Comment: 5 pages, 3 figure
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