148 research outputs found
Colloidal motility and pattern formation under rectified diffusiophoresis
In this letter, we characterize experimentally the diffusiophoretic motion of
colloids and lambda- DNA toward higher concentration of solutes, using
microfluidic technology to build spatially- and temporally-controlled
concentration gradients. We then demonstrate that segregation and spatial
patterning of the particles can be achieved from temporal variations of the
solute concentration profile. This segregation takes the form of a strong
trapping potential, stemming from an osmotically induced rectification
mechanism of the solute time-dependent variations. Depending on the spatial and
temporal symmetry of the solute signal, localization patterns with various
shapes can be achieved. These results highlight the role of solute contrasts in
out-of-equilibrium processes occuring in soft matter
Dynamic clustering in active colloidal suspensions with chemical signaling
In this paper, we explore experimentally the phase behavior of a dense active
suspension of self- propelled colloids. In addition to a solid-like and a
gas-like phase observed for high and low densities, a novel cluster phase is
reported at intermediate densities. This takes the form of a stationary
assembly of dense aggregates, with an average size which grows with activity as
a linear function of the self-propelling velocity. While different possible
scenarii can be considered to account for these observations - such as a
generic velocity weakening instability recently put forward -, we show that the
experimental results are reproduced by a chemotactic aggregation mechanism,
originally introduced to account for bacterial aggregation, and accounting here
for diffusiophoretic chemical interaction between colloidal swimmers.Comment: supplementary video :http://
www-lpmcn.univ-lyon1.fr/~lbocquet/Movie-Theurkauff-SI.av
Sedimentation of active colloidal suspensions
In this paper, we investigate experimentally the non-equilibrium steady state
of an active colloidal suspension under gravity field. The active particles are
made of chemically powered colloids, showing self propulsion in the presence of
an added fuel, here hydrogen peroxide. The active suspension is studied in a
dedicated microfluidic device, made of permeable gel microstructures. Both the
microdynamics of individual colloids and the global stationary state of the
suspension under gravity - density profiles, number fluctuations - are measured
with optical microscopy. This allows to connect the sedimentation length to the
individual self-propelled dynamics, suggesting that in the present dilute
regime the active colloids behave as 'hot' particles. Our work is a first step
in the experimental exploration of the out-of-equilibrium properties of
artificial active systems.Comment: 4 pages, 4 figure
Artificial Rheotaxis
Motility is a basic feature of living microorganisms, and how it works is
often determined by environmental cues. Recent efforts have focused on develop-
ing artificial systems that can mimic microorganisms, and in particular their
self-propulsion. Here, we report on the design and characterization of syn-
thetic self-propelled particles that migrate upstream, known as positive rheo-
taxis. This phenomenon results from a purely physical mechanism involving the
interplay between the polarity of the particles and their alignment by a
viscous torque. We show quantitative agreement between experimental data and a
simple model of an overdamped Brownian pendulum. The model no- tably predicts
the existence of a stagnation point in a diverging flow. We take advantage of
this property to demonstrate that our active particles can sense and
predictably organize in an imposed flow. Our colloidal system represents an
important step towards the realization of biomimetic micro-systems withthe
ability to sense and respond to environmental changesComment: Published in Science Advances [Open access journal of Science
Magazine
Self Assembled Particles
A self-assembling structure using non-equilibrium driving forces leading to 'living crystals' and other maniputable particles with a complex dynamics. The dynamic self-assembly assembly results from a competition between self-propulsion of particles and an attractive interaction between the particles. As a result of non-equilibrium driving forces, the crystals form, grow, collide, anneal, repair themselves and spontaneously self-destruct, thereby enabling reconfiguration and assembly to achieve a desired property
Phase separation and rotor self-assembly in active particle suspensions
Adding a non-adsorbing polymer to passive colloids induces an attraction
between the particles via the `depletion' mechanism. High enough polymer
concentrations lead to phase separation. We combine experiments, theory and
simulations to demonstrate that using active colloids (such as motile bacteria)
dramatically changes the physics of such mixtures. First, significantly
stronger inter-particle attraction is needed to cause phase separation.
Secondly, the finite size aggregates formed at lower inter-particle attraction
show unidirectional rotation. These micro-rotors demonstrate the self assembly
of functional structures using active particles. The angular speed of the
rotating clusters scales approximately as the inverse of their size, which may
be understood theoretically by assuming that the torques exerted by the
outermost bacteria in a cluster add up randomly. Our simulations suggest that
both the suppression of phase separation and the self assembly of rotors are
generic features of aggregating swimmers, and should therefore occur in a
variety of biological and synthetic active particle systems.Comment: Main text: 6 pages, 5 figures. Supplementary information: 5 pages, 4
figures. Supplementary movies available from
httP://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116334109/-/DCSupplementa
Dynamic stereo microscopy for studying particle sedimentation
We demonstrate a new method for measuring the sedimentation
of a single colloidal bead by using a combination of optical tweezers and a stereo microscope based on a spatial light modulator. We use optical tweezers to raise a micron-sized silica bead to a fixed height and then release it to observe its 3D motion while it sediments under gravity. This experimental procedure provides two independent measurements of bead diameter and a measure of Faxén’s correction, where the motion changes due to presence of the boundary
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