  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.
