We experimentally demonstrate that the flow rate of granular material through an aperture is controlled by the exit velocity imposed to the particles and not by the pressure at the base, contrary to what is often assumed in previous works. This result is achieved by studying the discharge process of a dense packing of monosized disks through an orifice. The flow is driven by a conveyor belt. This two-dimensional horizontal setup allows to uncouple pressure and velocity and, therefore, to independently control the velocity at which the disks escape the horizontal silo and the pressure in the vicinity of the aperture. The flow rate is found to be directly proportional to the belt velocity, independent of the amount of disks in the container and, thus, independent of the pressure in the outlet region. In addition, this specific experimental configuration makes it possible to get information on the system dynamics from a single image of the disks that rest on the conveyor belt after the discharge.

In this work, a digital imaging technique is used to study the superficial fluctuations observed when a granular packing is slowly driven to the threshold of instability. The experimental results show the presence of three types of events. Small superficial rearrangements of grains are observed during all the experiments. They present a power-law behavior although the system is not in a critical state as predicted by self-organized criticality models. In thick granular piles, large rearrangements are detected at regular angular intervals. They are related to the threshold of instability of the contact network that relaxes to stable configurations producing internal rearrangements of the grains. Finally, an avalanche is triggered when the superficial beads that are set in motion acquire enough momentum to destabilize grains from layers below.

We study the dewetting of liquid films deposited inside nonwettable and wettable capillaries. Two processes compete: (i) Rayleigh instability (i.e., amplification of thickness fluctuations) and (ii) dewetting by nucleation and growth of a dry zone limited by a rim collecting the liquid. At times shorter than the characteristic time τM of the growth of the Rayleigh instability, we expect two regimes: (i) annular rims and drying at constant velocities and (ii) columnar rims with drying velocities decreasing versus time. For wettable capillaries, in a certain regime of thin thicknesses, the Rayleigh instability is absent and dewetting is the only process to remove the film.