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

Swimming in circles: Motion of bacteria near solid boundaries

Near a solid boundary, E. coli swims in clockwise circular motion. We provide a hydrodynamic model for this behavior. We show that circular trajectories are natural consequences of force-free and torque-free swimming, and the hydrodynamic interactions with the boundary, which also leads to a hydrodynamic trapping of the cells close to the surface. We compare the results of the model with experimental data and obtain reasonable agreement. In particular, we show that the radius of curvature of the trajectory increases with the length of the bacterium body.Comment: Also available at http://people.deas.harvard.edu/~lauga

Novel Methods for Analysing Bacterial Tracks Reveal Persistence in Rhodobacter sphaeroides

Tracking bacteria using video microscopy is a powerful experimental approach to probe their motile behaviour. The trajectories obtained contain much information relating to the complex patterns of bacterial motility. However, methods for the quantitative analysis of such data are limited. Most swimming bacteria move in approximately straight lines, interspersed with random reorientation phases. It is therefore necessary to segment observed tracks into swimming and reorientation phases to extract useful statistics. We present novel robust analysis tools to discern these two phases in tracks. Our methods comprise a simple and effective protocol for removing spurious tracks from tracking datasets, followed by analysis based on a two-state hidden Markov model, taking advantage of the availability of mutant strains that exhibit swimming-only or reorientating-only motion to generate an empirical prior distribution. Using simulated tracks with varying levels of added noise, we validate our methods and compare them with an existing heuristic method. To our knowledge this is the first example of a systematic assessment of analysis methods in this field. The new methods are substantially more robust to noise and introduce less systematic bias than the heuristic method. We apply our methods to tracks obtained from the bacterial species Rhodobacter sphaeroides and Escherichia coli. Our results demonstrate that R. sphaeroides exhibits persistence over the course of a tumbling event, which is a novel result with important implications in the study of this and similar species

Relations bilatérales entre aéroports secondaires et compagnies à bas coût : Quelles justifications concurrentielles sur la base des gains d’efficacité ?

International audienc

Eutectic mixture of Protic Ionic Liquids as an Electrolyte for Activated Carbon-Based Supercapacitors

International audienceOne of the drawbacks of ionic liquids is that some of them are solid or very viscous at room temperature; this prevents their use as electrolytes in energy storage systems. One solution consists of a binary mixture of pure, solid salts with the formation of a eutectic, allowing it to be used at room temperature. This work describes, for the first time, the formulation and use of a binary mixture of protic ionic liquids (PILs) based on the same H-bond donor, pyrrolidinium cation, with nitrate ([Pyrr][NO3]) and bis(trifluoromethanesulfonyl) imide ([Pyrr][TFSI]) anions as an electrolyte for carbon-based supercapacitors. The physico-chemical and thermal properties of mixtures as a function of composition and temperature were fully investigated and related to their electrochemical behavior as an electrolyte. The electrochemical characterization of some selected optimal compositions (x[Pyrr][NO3] = 0.64, 0.72, 0.80) shows, at first, good capacitive behavior (up to 148 F g−1) at a potential voltage of 2.0 V. Moreover, above 2.0 V, anion intercalation was observed, increasing the specific capacitance up to 209 F g−1. At 2.5 V during cycling, the characteristic peak of anion intercalation/deintercalation decreases, caused by saturation of the positive electrode by anions. Finally, accelerated aging tests showed good capacity retention (> 80%) at 2.0 V during 110 hours for x[Pyrr][NO3] = 0.72, at 25 °C. According to these results, the use of eutectic PIL binary mixtures is a promising way to design adapted electrolytes according to the material electrode for energy devices at room temperature