Air-fluidized balls in a background of smaller beads

Abstract

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.

Anomalous Diffusion in a Monolayer of Lightweight Spheres Fluidized in Airflow

“…For a vibrated monolayer of steel balls [24], a plateau appears in the MSD at φ ≈ 0.7. It is reported that the caging occurred at φ ≈ 0.7 for fluidized ping-pong balls (ρ = 0.08 g cm −3 ) [13], while for fluidized beads no caging was observed [6]. Fluidized bidispersed steel balls [5] exhibit strong caging at φ = 0.74 but do not show subdiffusion for φ < 0.64.…”

“…This leads to an analogy to the thermal system despite the dissipative and macroscopic dynamics in the many-particle system. Actually many studies in recent years have found that various macroscopic, strongly dissipative (and thus out-of-equilibrium) systems such as particles agitated by turbulent airflow [3][4][5][6][7][8] or by mechanical vibration [9][10][11], can mimic Brownian behavior to an appreciable degree which has long been considered mainly in the context of energetically closed system. Thus far a variety of studies on macroscopic two-dimensional systems have been conducted [7,[12][13][14].…”