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We report on the energy exchanges between two granular gases of different densities coupled electromechanically by immersed blades attached to dc motors. Zeroing the energy flux between the two subsystems, we demonstrate that an immersed blade is a convenient way to assess the properties of the granular gases, provided that the dissipation in the motor is properly taken into account. In addition, when the two gases have different densities, the fluctuations of the energy flux are asymmetric, very intermittent, and with most probable zero flux. We show that, for weak coupling, the main features of the energy exchanges can be explained considering the fluctuations of the two subsystems.

A ball bouncing repeatedly on a vertically vibrating surface constitutes the famous "bouncing ball" problem, a nonlinear system used in the 1980s, and still in use nowadays, to illustrate the route to chaos by period doubling. In experiments, in order to avoid the ball escape that would be inevitable with a flat surface, a concave lens is often used to limit the horizontal motion. However, we observe experimentally that the system is not stable. The ball departs from the system axis and exhibits a pendular motion in the permanent regime. We propose theoretical arguments to account for the decrease of the growth rate and of the asymptotic-size of the trajectory when the frequency of the vibration is increased. The instability is very sensitive to the physics of the contacts, which makes it a potentially interesting way to study the collisions rules, or to test the laws used in numerical studies of granular matter.

We investigate ordering of granular rods in a container subject to vibrations in a gravitational field as a function of number density of the rods. We study rods with three different length to diameter aspect ratios A(r)= 5, 10, and 15. The measurements are performed in three dimensions using x-ray computer tomography to visualize the rods in the entire container. We first discuss a method to extract the position and orientation of the rods from the scans which enables us to obtain statistical measures of the degree of order in the packing. We find that the rods with A(r)=5 phase separate into domains with vertical and horizontal orientation as the number density of the rods is increased, whereas, for A(r)=10 and 15 the rods are predominately oriented vertically in layers. By calculating two-point spatial correlation functions, we further show that long range hexagonal order occurs within a layer when the rods are oriented along the vertical axis. Thus, our experiments find that long range order increases rapidly in granular rods with growing anisotropy.