Collective Motion of Vibrated Polar Disks

We experimentally study a monolayer of vibrated disks with a built-in polar asymmetry which enables them to move quasi-balistically on a large persistence length. Alignment occurs during collisions as a result of self-propulsion and hard core repulsion. Varying the amplitude of the vibration, we observe the onset of large-scale collective motion and the existence of giant number fluctuations with a scaling exponent in agreement with the predicted theoretical value.Comment: 4 pages, 4 figure

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

Vibrated polar disks: spontaneous motion, binary collisions, and collective dynamics

We study the spontaneous motion, binary collisions, and collective dynamics of "polar disks", i.e. purpose-built particles which, when vibrated between two horizontal plates, move coherently along a direction strongly correlated to their intrinsic polarity. The motion of our particles, although nominally three-dimensional and complicated, is well accounted for by a two-dimensional persistent random walk. Their binary collisions are spatiotemporally extended events during which multiple actual collisions happen, yielding a weak average effective alignment. We show that this well-controlled, "dry active matter" system can display collective motion with orientationally-ordered regions of the order of the system size. We provide evidence of strong number density in the most ordered regimes observed. These results are discussed in the light of the limitations of our system, notably those due to the inevitable presence of walls.Comment: 13 pages, 10 figures, 4 movie

Dispersion of magnetic nanoparticles in a nematic liquid crystal host: Phase diagram, Fredericks transition and deformation of droplets

During the seventies when the main properties of magnetic fluids were first understood, (superparamagnetism, birefringence, hydrodynamics instabilities...) theoreticians [1] have imagined the possible advantages of a ferrofluid with a thermotropic nematic liquid crystal (LC) as a solvent. Such a “ferronematic” would indeed combine the properties of two systems which become optically anisotropic (birefringent) under electrical and magnetic fields. The today widely used liquid crystals displays (LCDs) are based on the transition between transparent and opaque state of LCs, controlled by electric fields. For certain applications, magnetic fields could be used instead if we could lower down the threshold magnetic field intensity Hc of the so called Fredericks transition arising from the competition between alignment of LC molecules by surfaces and by an applied magnetic field. This idea motivated our experimental study of dispersion of nanoparticles made of maghemite iron oxide (Γ-Fe2O3) and 5-CB, one of the most standard nematic LCs which is convenient due to its nematic-isotropic temperature (TN-I=35°C) slightly above room temperature. However, we found that a true (monophasic) ferrofluid with 5-CB as solvent can be obtained only in the isotropic phase (above TN-I), whereas in the nematic state, the system separates between two phases: one the one hand magnetic microdroplets made of a high concentration of magnetic nanoparticles (about 18 vol% from SAXS measurements) in isotropic 5-CB and on the other hand a non magnetic 5-CB nematic matrix [2]. This phenomenon was explained by the thermodynamic laws for a ternary system (nanoparticles – LC – surfactant). Two aspect of these highly magnetic droplets in a LC host matrix where studied : i) their influence on the threshold field Hc of the Fredericks transition of a 5-CB layer sandwitched between two plates with homeotropic alignment conditions; ii) their strong ellipsoidal deformation under a magnetic field of low intensity, which – by analogy with ferrofluid droplets in a non magnetic liquid – provides an experimental measurement of the interfacial tension and tentatively of the anchoring energy of LC molecules onto nanoparticles [3]. ___________________________________________________ [1] F. Brochard, P. G. de Gennes, J. Phys. (Paris), 1970, 31, 691. [2] C. Da Cruz, O. Sandre, V. Cabuil, Journal of Phyical Chemistry B (2005) 109, 14292. [3] J. Deseigne, report of ESPCI engineering school short training period (March 2006)

Stochastic model for nucleosome sliding in the presence of DNA ligands

Heat-induced mobility of nucleosomes along DNA is an experimentally well-studied phenomenon. A recent experiment shows that the repositioning is modified in the presence of minor-groove binding DNA ligands. We present here a stochastic three-state model for the diffusion of a nucleosome along DNA in the presence of such ligands. It allows us to describe the dynamics and the steady state of such a motion analytically. The analytical results are in excellent agreement with numerical simulations of this stochastic process.With this model, we study the response of a nucleosome to an external force and how it is affected by the presence of ligands.Comment: 10 pages, 8 figures, submitted to Eur. Phys. J.

Long-Range Ordering of Vibrated Polar Disks

Vibrated polar disks have been used experimentally to investigate collective motion of driven particles, where fully-ordered asymptotic regimes could not be reached. Here we present a model reproducing quantitatively the single, binary and collective properties of this granular system. Using system sizes not accessible in the laboratory, we show in silico that true long-range order is possible in the experimental system. Exploring the model's parameter space, we find a phase diagram qualitatively different from that of dilute or point-like particle systems.Comment: 5 pages, 4 figure

Traffic Instabilities in Self-Organized Pedestrian Crowds

In human crowds as well as in many animal societies, local interactions among individuals often give rise to self-organized collective organizations that offer functional benefits to the group. For instance, flows of pedestrians moving in opposite directions spontaneously segregate into lanes of uniform walking directions. This phenomenon is often referred to as a smart collective pattern, as it increases the traffic efficiency with no need of external control. However, the functional benefits of this emergent organization have never been experimentally measured, and the underlying behavioral mechanisms are poorly understood. In this work, we have studied this phenomenon under controlled laboratory conditions. We found that the traffic segregation exhibits structural instabilities characterized by the alternation of organized and disorganized states, where the lifetime of well-organized clusters of pedestrians follow a stretched exponential relaxation process. Further analysis show that the inter-pedestrian variability of comfortable walking speeds is a key variable at the origin of the observed traffic perturbations. We show that the collective benefit of the emerging pattern is maximized when all pedestrians walk at the average speed of the group. In practice, however, local interactions between slow- and fast-walking pedestrians trigger global breakdowns of organization, which reduce the collective and the individual payoff provided by the traffic segregation. This work is a step ahead toward the understanding of traffic self-organization in crowds, which turns out to be modulated by complex behavioral mechanisms that do not always maximize the group's benefits. The quantitative understanding of crowd behaviors opens the way for designing bottom-up management strategies bound to promote the emergence of efficient collective behaviors in crowds.Comment: Article published in PLoS Computational biology. Freely available here: http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.100244

Active colloids in complex fluids

We review recent work on active colloids or swimmers, such as self-propelled microorganisms, phoretic colloidal particles, and artificial micro-robotic systems, moving in fluid-like environments. These environments can be water-like and Newtonian but can frequently contain macromolecules, flexible polymers, soft cells, or hard particles, which impart complex, nonlinear rheological features to the fluid. While significant progress has been made on understanding how active colloids move and interact in Newtonian fluids, little is known on how active colloids behave in complex and non-Newtonian fluids. An emerging literature is starting to show how fluid rheology can dramatically change the gaits and speeds of individual swimmers. Simultaneously, a moving swimmer induces time dependent, three dimensional fluid flows, that can modify the medium (fluid) rheological properties. This two-way, non-linear coupling at microscopic scales has profound implications at meso- and macro-scales: steady state suspension properties, emergent collective behavior, and transport of passive tracer particles. Recent exciting theoretical results and current debate on quantifying these complex active fluids highlight the need for conceptually simple experiments to guide our understanding.Comment: 6 figure

Collective Motion and Phase Transitions of Symmetric Camphor Boats

The motion of several self-propelled boats in a narrow channel displays spontaneous pattern formation and kinetic phase transitions. In contrast with previous studies on self-propelled particles, this model does not require stochastic fluctuations and it is experimentally accessible. By varying the viscosity in the system, it is possible to form either a stationary state, correlated or uncorrelated oscillations, or unidirectional flow. Here, we describe and analyze these self organized patterns and their transitions.Comment: 6 pages, 6 figure