We study a quasi-two-dimensional monolayer of granular rods fluidized by a spatially and temporally homogeneous upflow of air. By tracking the position and orientation of the particles, we characterize the dynamics of the system with sufficient resolution to observe ballistic motion at the shortest time scales. Particle anisotropy gives rise to dynamical anisotropy and superdiffusive dynamics parallel to the rod's long axis, causing the parallel and perpendicular mean-square displacements to become diffusive on different time scales. The distributions of free times and free paths between collisions deviate from exponential behavior, underscoring the nonthermal character of the particle motion. The dynamics show evidence of rotationaltranslational coupling similar to that of an anisotropic Brownian particle. We model rotationaltranslational coupling in the single-particle dynamics with a modified Langevin model using nonthermal noise sources. This suggests a phenomenological approach to thinking about collections of self-propelling particles in terms of enhanced memory effects. We study a quasi-two-dimensional monolayer of granular rods fluidized by a spatially and temporally homogeneous upflow of air. By tracking the position and orientation of the particles, we characterize the dynamics of the system with sufficient resolution to observe ballistic motion at the shortest time scales. Particle anisotropy gives rise to dynamical anisotropy and superdiffusive dynamics parallel to the rod's long axis, causing the parallel and perpendicular mean-square displacements to become diffusive on different time scales. The distributions of free times and free paths between collisions deviate from exponential behavior, underscoring the nonthermal character of the particle motion. The dynamics show evidence of rotationaltranslational coupling similar to that of an anisotropic Brownian particle. We model rotational-translational coupling in the single-particle dynamics with a modified Langevin model using nonthermal noise sources. This suggests a phenomenological approach to thinking about collections of self-propelling particles in terms of enhanced memory effects.

We present experiments on a monolayer of air-fluidized beads in which a jamming transition is approached by increasing pressure, increasing packing fraction, and decreasing kinetic energy. This is accomplished, along with a noninvasive measurement of pressure, by tilting the system and examining behavior vs depth. We construct an equation of state and analyze relaxation time vs effective temperature. By making time and effective temperature dimensionless using factors of pressure, bead size, and bead mass, we obtain a good collapse of the data but to a functional form that differs from that of thermal hard-sphere systems. The relaxation time appears to diverge only as the effective temperature to pressure ratio goes to zero. PACS numbers: 64.70.ps, The relaxation time for amorphous liquids can grow unbearably long when the temperature is lowered [1,2]. It can also grow when the pressure is increased, although this is more difficult to study experimentally [3][4][5]. Similarly, the relaxation time for colloidal suspensions can exceed experimentalists' patience when the packing fraction or pressure is increased [6][7][8]. In both the thermal and colloidal glass transitions, the particles appear to develop a fixed set of neighbors and the bulk medium appears to become mechanically rigid. It was recently suggested that these two glass transitions are manifestations of the same phenomenon for the system of thermal hard spheres [9]. In such a system, dimensional analysis suggests that the relaxation time τ , made dimensionless as τ (P σ d−2 /m) 1/2 by pressure P , the sphere diameter σ, and the sphere mass m, must depend only on the dimensionless ratio T /P σ d , where d is the dimensionality and the Boltzmann constant is set to unity. Thus, the dimensionless relaxation time increases in exactly the same way whether T is lowered or P is raised.Although no system behaves exactly like hard spheres, Medina-Noyola and coworkers showed that there should be "dynamical equivalence," so that soft spheres behave as hard spheres with a smaller diameter [10,11]. Indeed, it was found that particles with a variety of finite-ranged repulsive interaction potentials exhibited collapse of dimensionless relaxation time with T /pσ d as long as the pressure was low, so that P ε/σ d , where ε is the interaction energy scale [9]. These results suggest that real systems might exhibit the temperature-pressure scaling expected for hard spheres; however, this has not been tested by experiment. For hard-sphere colloids, this is not possible for a single system because the packing fraction φ, or equivalently the pressure P , is the only control parameter; temperature is bounded by the freezing and boiling points of the solvent and therefore cannot be varied appreciably. For molecular liquids, scaling is not expected to hold where van der Waals attractions are appreciable compared to the pressure; in this regime the scaling must be modified to account for the mean-field effect of the attractions [8].Here we describe experiments on a granular mono...

We report on an observation of propagating compression waves in a quasi-two-dimensional monolayer of apolar granular rods fluidized by an upflow of air. The collective wave speed is an order of magnitude faster than the speed of the particles. This gives rise to anomalously large number fluctuations, ΔN~N 0.72±0.04 , which are greater than ordinary number fluctuations of N 1/2 . We characterize the waves by calculating the spatiotemporal power spectrum of the density. The position of observed peaks, as a function of frequency ω and wave vector k, yields a linear dispersion relationship in the long-time, long-wavelength limit and a wave speed c=ω/k. Repeating this analysis for systems at different densities and air speeds, we observe a linear increase in the wave speed with increasing packing fraction with almost no dependence on the air flow. We also observe that the parallel and perpendicular root-mean-square speeds of the rods are identical when waves are present, but become different at low packing fractions where there are no waves. Based on this apparent exclusivity, we map out the phase behavior for the existence of waves vs speed anisotropy as a function of density and fluidizing air flow. Disciplines Physical Sciences and Mathematics | PhysicsThis journal article is available at ScholarlyCommons: https://repository.upenn.edu/physics_papers/607PHYSICAL REVIEW E 83, 061304 (2011) Propagating waves in a monolayer of gas-fluidized rods We report on an observation of propagating compression waves in a quasi-two-dimensional monolayer of apolar granular rods fluidized by an upflow of air. The collective wave speed is an order of magnitude faster than the speed of the particles. This gives rise to anomalously large number fluctuations, N ∼ N 0.72±0.04 , which are greater than ordinary number fluctuations of N 1/2 . We characterize the waves by calculating the spatiotemporal power spectrum of the density. The position of observed peaks, as a function of frequency ω and wave vector k, yields a linear dispersion relationship in the long-time, long-wavelength limit and a wave speed c = ω/k. Repeating this analysis for systems at different densities and air speeds, we observe a linear increase in the wave speed with increasing packing fraction with almost no dependence on the air flow. We also observe that the parallel and perpendicular root-mean-square speeds of the rods are identical when waves are present, but become different at low packing fractions where there are no waves. Based on this apparent exclusivity, we map out the phase behavior for the existence of waves vs speed anisotropy as a function of density and fluidizing air flow.

We report on quasi-two-dimensional granular systems in which either one or two large balls is fluidized by an upflow of air in the presence of a background of several hundred smaller beads. A single large ball is observed to propel ballistically in nearly circular orbits, in direct contrast to the Brownian behavior of a large ball fluidized in the absence of this background. Further, the large ball motion satisfies a Langevin equation with an additional speed-dependent force acting in the direction of motion. This results in a non-zero average speed of the large ball that is an order of magnitude faster than the root mean square speed of the background balls. Two large balls fluidized in the absence of the small-bead background experience a repulsive force depending only on the separation of the two balls. With the background beads present, by contrast, the ball-ball interaction becomes velocity-dependent and attractive. The attraction is long-ranged and inconsistent with a depletion model; instead, it is mediated by local fluctuations in the density of the background beads which depends on the large balls' motion.