21 research outputs found
Creep and fluidity of a real granular packing near jamming
We study the internal dynamical processes taking place in a granular packing
below yield stress. At all packing fractions and down to vanishingly low
applied shear, a logarithmic creep is evidenced. The experiments are analyzed
under the scope of a visco-elastic model introducing an internal "fluidity"
variable. For all experiments, the creep dynamics can be rescaled onto a unique
curve which displays jamming at the random-close-packing limit. At each packing
fraction, a stress value is evidenced, corresponding to the onset of internal
granular reorganisation leading to a slowing down the creep dynamics before the
final yield
Mechanical fluctuations suppress the threshold of soft-glassy solids : the secular drift scenario
We propose a dynamical mechanism leading to the fluidization of soft-glassy
amorphous mate-rial driven below the yield-stress by external mechanical
fluctuations. The model is based on the combination of memory effect and
non-linearity, leading to an accumulation of tiny effects over a long-term. We
test this scenario on a granular packing driven mechanically below the Coulomb
threshold. We bring evidences for an effective viscous response directly
related to small stress modulations in agreement with the theoretical
prediction of a generic secular drift
Sedimentation of a single soluble particle at low Reynolds and high P\'eclet numbers
We investigate experimentally the dissolution of an almost spherical
butyramide particle during its sedimentation, in the low Reynolds high P\'eclet
regime. The particle sediments in a quiescent aqueous solution, and its shape
and position are measured simultaneously by a camera attached to a translation
stage. The particle is tracked in real time, and the translation stage moves
accordingly to keep the particle in the field of the camera. The measurements
from the particle image show that the radius shrinking rate is constant with
time, and independent of the initial radius of the particle. We explain this
with a simple model, based on the sedimentation law in the Stokes' regime and
the mass transfer rate at low Reynolds and high P\'eclet numbers. The
theoretical and experimental results are consistent within . We introduce
two correction factors to take into account the non-sphericity of the particle
and the inclusions of air bubbles inside the particle, and reach quantitative
agreement. With these corrections, the indirect measurement of the radius
shrinking rate deduced from the position measurement is also in agreement with
the model. We discuss other correction factors, and explain why there are
negligible in the present experiment. We also compute the effective Sherwood
number as a function of an effective P\'eclet number
3D spatial exploration by E. coli echoes motor temporal variability
Unraveling bacterial strategies for spatial exploration is crucial for
understanding the complexity in the organization of life. Bacterial motility
determines the spatio-temporal structure of microbial communities, controls
infection spreading and the microbiota organization in guts or in soils. Most
theoretical approaches for modeling bacterial transport rely on their
run-and-tumble motion. For Escherichia coli, the run time distribution was
reported to follow a Poisson process with a single characteristic time related
to the rotational switching of the flagellar motors. However, direct
measurements on flagellar motors show heavy-tailed distributions of rotation
times stemming from the intrinsic noise in the chemotactic mechanism.
Currently, there is no direct experimental evidence that the stochasticity in
the chemotactic machinery affect the macroscopic motility of bacteria. In stark
contrast with the accepted vision of run-and-tumble, here we report a large
behavioral variability of wild-type \emph{E. coli}, revealed in their
three-dimensional trajectories. At short observation times, a large
distribution of run times is measured on a population and attributed to the
slow fluctuations of a signaling protein triggering the flagellar motor
reversal. Over long times, individual bacteria undergo significant changes in
motility. We demonstrate that such a large distribution of run times introduces
measurement biases in most practical situations. Our results reconcile the
notorious conundrum between run time observations and motor switching
statistics. We finally propose that statistical modeling of transport
properties currently undertaken in the emerging framework of active matter
studies, should be reconsidered under the scope of this large variability of
motility features.Comment: 12 pages, 7 figures, Supplementary information include
Scénario dynamique de fluidisation d'un solide granulaire
Nous présentons des résultats démontrant la fluidisation d'un solide granulaire loin de son seuil d'écoulement. L'étude d'un modèle visco-élastique générique, prenant en compte les effets de vieillissement et de rajeunissement du matériau nous permet de proposer un scénario original de liquéfaction des solides amorphes sous leur seuil dynamique. Une étude expérimentale d'un solide granulaire cisaillé, soumis à de très faibles fluctuations de contrainte autour d'une valeur imposée, nous permet de mettre en évidence ce phénomène dans le cas d'un système amorphe modèle
Enhanced diffusion due to active swimmers at a solid surface
We consider two systems of active swimmers moving close to a solid surface,
one being a living population of wild-type \textit{E. coli} and the other being
an assembly of self-propelled Au-Pt rods. In both situations, we have
identified two different types of motion at the surface and evaluated the
fraction of the population that displayed ballistic trajectories (active
swimmers) with respect to those showing random-like behavior. We studied the
effect of this complex swimming activity on the diffusivity of passive tracers
also present at the surface. We found that the tracer diffusivity is enhanced
with respect to standard Brownian motion and increases linearly with the
activity of the fluid, defined as the product of the fraction of active
swimmers and their mean velocity. This result can be understood in terms of
series of elementary encounters between the active swimmers and the tracers.Comment: 4 pages, 2 figures in color, Physical Review Letters (in production
Enhanced diffusion due to active swimmers at a solid surface
We consider two systems of active swimmers moving close to a solid surface, one being a living population of wild-type E. coli and the other being an assembly of self-propelled Au-Pt rods. In both situations, we have identified two different types of motion at the surface and evaluated the fraction of the population that displayed ballistic trajectories (active swimmers) with respect to those showing diffusive-like behavior. We studied the effect of this complex swimming activity on the diffusivity of passive tracers also present at the surface. We found that the tracer diffusivity is enhanced with respect to standard Brownian motion and increases linearly with the activity of the fluid, defined as the product of the fraction of active swimmers and their mean velocity. This result can be understood in terms of series of elementary encounters between the active swimmers and the tracers
Run-to-tumble variability controls the surface residence times of E. coli bacteria
Motile bacteria are known to accumulate at surfaces, eventually leading to
changes in bacterial motility and bio-film formation. We use a novel
two-colour, three-dimensional Lagrangian tracking technique, to follow
simultaneously the body and the flagella of a wild-type . We observe long surface residence times and surface escape
corresponding mostly to immediately antecedent tumbling. A motility model
accounting for a large behavioural variability in run-time duration, reproduces
all experimental findings and gives new insights into surface trapping
efficiency
Optimised hyperbolic microchannels for the mechanical characterisation of bio-particles
The transport of bio-particles in viscous flows exhibits a rich variety of dynamical behaviour, such as morphological transitions, complex orientation dynamics or deformations. Characterising such complex behaviour under well controlled flows is key to understanding the microscopic mechanical properties of biological particles as well as the rheological properties of their suspensions. While generating regions of simple shear flow in microfluidic devices is relatively straightforward, generating straining flows in which the strain rate is maintained constant for a sufficiently long time to observe the objects' morphologic evolution is far from trivial. In this work, we propose an innovative approach based on optimised design of microfluidic converging–diverging channels coupled with a microscope-based tracking method to characterise the dynamic behaviour of individual bio-particles under homogeneous straining flow. The tracking algorithm, combining a motorised stage and a microscopy imaging system controlled by external signals, allows us to follow individual bio-particles transported over long-distances with high-quality images. We demonstrate experimentally the ability of the numerically optimised microchannels to provide linear velocity streamwise gradients along the centreline of the device, allowing for extended consecutive regions of homogeneous elongation and compression. We selected three test cases (DNA, actin filaments and protein aggregates) to highlight the ability of our approach for investigating dynamics of objects with a wide range of sizes, characteristics and behaviours of relevance in the biological world