Air fluidized balls in a background of smaller beads

TL;DR

This study investigates the dynamics of large balls fluidized by air in a granular system with smaller beads, revealing novel ballistic motion, modified interactions, and fluctuation-mediated attractions.

Contribution

It demonstrates how a background of smaller beads alters large ball behavior, introducing velocity-dependent forces and fluctuation-mediated attractions not seen in simpler systems.

Findings

Large balls propel ballistically in circular orbits.

Background beads induce a Langevin-like motion with additional forces.

Large balls attract each other through fluctuation-mediated interactions.

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.