From the formation of animal flocks to the emergence of coordinate motion in bacterial swarms, at all scales, populations of motile organisms display coherent collective motion.This consistent behavior strongly contrasts with the difference in communication abilities between the individuals. Guided by this universal feature, physicists have proposed that solely alignment rules at the individual level could account for the emergence of unidirectional motion at the group level [1][2][3][4] . This hypothesis has been supported by agent-based simulations 1,5,6 . However, more complex collective behaviors have been systematically found in experiments including the formation of vortices 7-9 , fluctuating swarms 7, 10 , clustering 11,12 , and swirling [13][14][15][16] . All these (living and man-made) model systems (bacteria 9,10, 16 , biofilaments and molecular motors 7,8,13 , shaken grains 14, 15 and reactive colloids 11,12 ) predominantly rely on actual collisions to display collective motion. As a result, the potential local alignment rules are entangled with more complex, and often unknown, interactions. The large-scale behaviour of the populations therefore strongly depends on these uncontrolled microscopic couplings that are extremely challenging to measure and describe theoretically.Here, we demonstrate a new phase of active matter. We reveal that dilute populations of millions of colloidal rollers self-organize to achieve coherent motion along a unique direction, with very few density and velocity fluctuations. Identifying, quantitatively, the microscopic interactions between the rollers allows a theoretical description of this polar-liquid state. Comparison of the theory with experiment suggests that hydrodynamic interactions promote the emergence of collective motion either in the form of a single macroscopic flock at low densities, or in that of a homogenous polar phase at higher densities. Furthermore, hydrodynamics protects the polar-liquid state from the giant density fluctuations, which were hitherto considered as the hallmark of populations of self-propelled particles 2, 3, 17 . Our experiments demonstrate that genuine physical interactions at the individual level are sufficient to set homogeneous active populations into stable directed motion.Our system consists of large populations of colloids capable of self-propulsion and of sensing the orientation of their neighbors solely by means of physical mechanisms. We take advantage of an overlooked electrohydrodynamic phenomenon referred to as the Quincke rotation 18,19 (Fig. 1a).When an electric field E 0 is applied to an insulating sphere immersed in a conducting fluid, above a critical field amplitude E Q , the charge distribution at the sphere's surface is unstable to infinitesimal fluctuations. This spontaneous symmetry breaking results in a net electrostatic torque, which causes the sphere to rotate at a constant speed around a random direction transverse to E 0 18 . We 2 exploit this instability to engineer self-propelled colloidal rollers. We use ...

We experimentally study a monolayer of vibrated disks with a built-in polar asymmetry which enables them to move quasibalistically 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.

We report spontaneous motion in a fully bio-compatible system consisting of pure water droplets in an oil-surfactant medium of squalane and monoolein. Water from the droplet is solubilized by the reverse micellar solution, creating a concentration gradient of swollen reverse micelles around each droplet. The strong advection and weak diffusion conditions allow for the first experimental realization of spontaneous motion in a system of isotropic particles at sufficiently large Péclet number according to a straightforward generalization of a recently proposed mechanism [1, 2] Experiments with a highly concentrated solution of salt instead of water, and tetradecane instead of squalane, confirm the above mechanism. The present swimming droplets are able to carry external bodies such as large colloids, salt crystals, and even cells.

The dynamics of a bidimensional dense granular packing under cyclic shear is experimentally investigated close to the jamming transition. Measurement of multipoint correlation functions are produced. The self-intermediate scattering function, displaying slower than exponential relaxation, suggests dynamic heterogeneity. Further analysis of four point correlation functions reveal that the grain relaxations are strongly correlated and spatially heterogeneous, especially at the time scale of the collective rearrangements. Finally, a dynamical correlation length is extracted from a spatiotemporal pattern of mobility. Our experimental results open the way to a systematic study of dynamic correlation functions in granular materials.

Investigations of counter-rotating Taylor-Couette flow (TCF) in the narrow gap limit are conducted in a very large aspect ratio apparatus. The phase diagram is presented and compared to that obtained by Andereck et al. [1]. The spiral turbulence regime is studied by varying both internal and external Reynolds numbers. Spiral turbulence is shown to emerge from the fully turbulent regime via a continuous transition appearing first as a modulated turbulent state, which eventually relaxes locally to the laminar flow. The connection with the intermittent regimes of the plane Couette flow (pCf) is discussed. 47.20.Ft 47.20.Ky 47.54.+r, 47.27.Cn The "barber pole structure of turbulence" [2] between two counter-rotating cylinders, also called spiral turbulence, is commonly described as alternating helical stripes of laminar and turbulent flow. There are few quantitative studies of this puzzling regime, where long range order coexists with small scale turbulence. In early studies Coles and Van Atta [3][4][5] [6,7] described spiral turbulence within the framework of phase dynamics. All these studies were limited by their relatively small size. Only one helical turbulent stripe, winding no more than twice along the cylinder axis, could be observed. Altogether, the origin of this flow pattern remains unknown.Performing measurements in large aspect ratio TaylorCouette flow, we show that the spiral turbulence bifurcates continuously from the turbulent flow, appearing as a modulated turbulent state. After a rapid description of the experimental set up, we present the phase diagram and compare it to the one obtained by Andereck et al. [1] with a different cylinder radius ratio. Then we describe the successive steps leading to the fully turbulent flow before discussing the origin of spiral turbulence. Finally, we examine its breakdown into a spatio-temporal disordered regime similar to the laminar-turbulent coexistence dynamics observed in plane Couette flow [8,9]. We visualize the flow by a "fluorescent lighting "technique [10] developed for this study. The water flow is seeded with Kalliroscope AQ 1000 (6 × 30 × 0.07µm platelets). The inner cylinder is covered by a fluorescent film and the entire apparatus is UV-lighted. The fluorescent film re-emits a uniform visible lighting, transmitted through the fluid layer: the more turbulent the flow, the brighter it appears. As the gap is very thin, the Kalliroscope concentration is increased up to 25% by volume to enhance the contrast. A rheological study has shown that the fluid remains Newtonian, so that the only impact is a viscosity increase up to ν = 1.13 10 −6 m 2 /s at 20 • C. The flow is thermalized by water circulation inside the inner cylinder. At thermal equilibrium the temperature is uniform in space up to 0.1 • C and does not vary more than 0.1 • C/hour. Images and spatio-temporal diagrams (temporal recording of one line along the cylinder axis) are recorded by a CCD camera. Two plane mirrors reflect the two thirds of the flow hidden to the camera so that the whole cy...

This paper provides a prescription for the turbulent viscosity in rotating shear flows for use e.g. in geophysical and astrophysical contexts. This prescription is the result of the detailed analysis of the experimental data obtained in several studies of the transition to turbulence and turbulent transport in Taylor-Couette flow. We first introduce a new set of control parameters, based on dynamical rather than geometrical considerations, so that the analysis applies more naturally to rotating shear flows in general and not only to Taylor-Couette flow. We then investigate the transition thresholds in the supercritical and the subcritical regime in order to extract their general dependencies on the control parameters. The inspection of the mean profiles provides us with some general hints on the mean to laminar shear ratio. Then the examination of the torque data allows us to propose a decomposition of the torque dependence on the control parameters in two terms, one completely given by measurements in the case where the outer cylinder is at rest, the other one being a universal function provided here from experimental fits. As a result, we obtain a general expression for the turbulent viscosity and compare it to existing prescription in the literature. Finally, throughout all the paper we discuss the influence of additional effects such as stratification or magnetic fields.

Abstract. -The dynamical properties of a dense horizontally vibrated bidisperse granular monolayer are experimentally investigated. The quench protocol produces states with a frozen structure of the assembly, but the remaining degrees of freedom associated with contact dynamics control the appearance of macroscopic rigidity. We provide decisive experimental evidence that this transition is a critical phenomenon, with increasingly collective and heterogeneous rearrangements occurring at length scales much smaller than the grains' diameter, presumably reflecting the contact force network fluctuations. Dynamical correlation time and length scales soar on both sides of the transition, as the volume fraction varies over a remarkably tiny range (δφ/φ ∼ 10 −3 ). We characterize the motion of individual grains, which becomes super-diffusive at the jamming transition φJ , signaling long-ranged temporal correlations. Correspondingly, the system exhibits long-ranged four-point dynamical correlations in space that obey critical scaling at the transition density.Introduction. -As the volume fraction of hard grains is increased beyond a certain point, the system jams and is able to support mechanical stresses. It has been argued that this rigidity transition is akin to phase transitions in thermal systems, and that the jamming density is a genuine critical point [1]. Despite recent efforts [1][2][3][4][5][6][7][8][9][10][11], the mechanisms underlying the jamming transition remain an open problem. "Jamming" appears to be associated with two different notions, not always well distinguished in the literature. On the one hand, the glass/jamming transition reflects the freezing of structural degrees of freedom associated to topological neighborhoods and the divergence of the structural relaxation time τ α , as observed in glass-forming liquids, colloidal suspensions and granular assemblies. The common mechanism for the divergence of τ α appears to be increasingly collective rearrangements [12][13][14][15], supporting the idea that the transition is a critical phenomenon -albeit of a new kind. On the other hand, the rigidity/jamming transition, is the appearance of mechanical rigidity [1,3,4,19] for which there are only a few direct experimental evidence of the mechanisms at

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