Abstract
We report on quasi-two-dimensional granular systems in which either one or two large ballsare fluidized by an upflow of air in the presence of a background of several hundred smallerbeads. A single large ball is observed to propel ballistically in nearly circular orbits, indirect contrast to the Brownian behavior of a large ball fluidized in the absence of thisbackground. Further, the large ball motion satisfies a Langevin equation with anadditional speed-dependent force acting in the direction of motion. This results in anon-zero average speed of the large ball that is an order of magnitude faster than theroot-mean-square speed of the background balls. Two large balls fluidized in the absence ofthe small-bead background experience a repulsive force depending only on theseparation of the two balls. With the background beads present, in contrast, theball–ball interaction becomes velocity-dependent and attractive. The attraction islong-ranged and inconsistent with a depletion model; instead, it is mediated by localfluctuations in the density of the background beads which depends on the large balls’motion.
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