235 research outputs found
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
Differential Dynamic Microscopy of Bacterial Motility
We demonstrate 'differential dynamic microscopy' (DDM) for the fast, high
throughput characterization of the dynamics of active particles. Specifically,
we characterize the swimming speed distribution and the fraction of motile
cells in suspensions of Escherichia coli bacteria. By averaging over ~10^4
cells, our results are highly accurate compared to conventional tracking. The
diffusivity of non-motile cells is enhanced by an amount proportional to the
concentration of motile cells.Comment: 4 pages, 4 figures. In this updated version we have added simulations
to support our interpretation, and changed the model for the swimming speed
probability distribution from log-normal to a Schulz distribution. Neither
modification significantly changes our conclusion
Characterization and Control of the Run-and-Tumble Dynamics of {\it Escherichia Coli}
We characterize the full spatiotemporal gait of populations of swimming {\it
Escherichia coli} using renewal processes to analyze the measurements of
intermediate scattering functions. This allows us to demonstrate quantitatively
how the persistence length of an engineered strain can be controlled by a
chemical inducer and to report a controlled transition from perpetual tumbling
to smooth swimming. For wild-type {\it E.~coli}, we measure simultaneously the
microscopic motility parameters and the large-scale effective diffusivity,
hence quantitatively bridging for the first time small-scale directed swimming
and macroscopic diffusion
Enhanced diffusion of nonswimmers in a three-dimensional bath of motile bacteria
We show, using differential dynamic microscopy, that the diffusivity of
non-motile cells in a three-dimensional (3D) population of motile E. coli is
enhanced by an amount proportional to the active cell flux. While non-motile
mutants without flagella and mutants with paralysed flagella have quite
different thermal diffusivities and therefore hydrodynamic radii, their
diffusivities are enhanced to the same extent by swimmers in the regime of cell
densities explored here. Integrating the advective motion of non-swimmers
caused by swimmers with finite persistence-length trajectories predicts our
observations to within 2%, indicating that fluid entrainment is not relevant
for diffusion enhancement in 3D.Comment: 5 pages, 3 figure
Enhanced gas-liquid mass transfer of an oscillatory constricted-tubular reactor
The mass transfer performance has been tested for gas-liquid flow in a new tubular reactor system, the oscillating mesotube (OMT), which features the oscillatory movement of fluid across a series of smooth constrictions located periodically along the vertical 4.4 mm internal diameter tube. The effect of the fluid oscillations (frequency,f, and center-to-peak amplitude, x(0), in the range of 0-20 s(-1) and 0-3 mm, respectively) on the overall volumetric mass transfer coefficient (k(L)a) has been tested by measuring the oxygen saturation levels with a fiber-optical microprobe (oxygen micro-optrode), and a mathematical model has been produced to describe the oxygen mass transport in the OMT. The oxygen mass transfer rates were about I order of magnitude higher (k(L)a values up to 0.16 s(-1)) than those values reported for gas-liquid contacting in a 50 mm internal diameter oscillatory flow reactor (OFR), for the same peak fluid oscillatory velocity, i.e., 2 pi fx(0). This represents remarkable oxygen transfer efficiencies, especially when considering the very low mean superficial gas velocity involved in this work (0.37 mm s(-1)). The narrower constrictions helped to increase the gas fraction (holdup) by reducing the rise velocity of the bubbles. However, the extent of radial mixing and the detachment of vortex rings from the surface of the periodic constrictions are actually the main causes of bubbles retention and effective gas-liquid contacting and are, thus, responsible for the enhancement of k(L)a in the OMT.N.R. thanks the Portuguese Foundation for Science and Technology (FCT) for financial support of his work (SFRH/BD/6954/2001)
Characterization and Control of the Run-and-Tumble Dynamics of Escherichia Coli
We characterize the full spatiotemporal gait of populations of swimming Escherichia coli using renewal processes to analyze the measurements of intermediate scattering functions. This allows us to demonstrate quantitatively how the persistence length of an engineered strain can be controlledby a chemical inducer and to report a controlled transition from perpetual tumbling to smooth swimming. For wild-type E. coli, we measure simultaneously the microscopic motility parameters and the large-scale effective diffusivity, hence quantitatively bridging for the first time small-scale directed swimming and macroscopic diffusion
Quantitative characterization of run-and-tumble statistics in bulk bacterial suspensions
We introduce a numerical method to extract the parameters of run-and-tumble
dynamics from experimental measurements of the intermediate scattering
function. We show that proceeding in Laplace space is unpractical and employ
instead renewal processes to work directly in real time. We first validate our
approach against data produced using agent-based simulations. This allows us to
identify the length and time scales required for an accurate measurement of the
motility parameters, including tumbling frequency and swim speed. We compare
different models for the run-and-tumble dynamics by accounting for speed
variability at the single-cell and population level, respectively. Finally, we
apply our approach to experimental data on wild-type Escherichia coli obtained
using differential dynamic microscopy.Comment: 10 pages, 5 figure
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