ABSTRACT
Granular avalanches are presented as a paradigm of self-organized criticality (SOC) which seeks to explain why the dynamics of many complex and dissipative systems lack characteristic time and length scales, exhibiting fluctuations that can be fitted to a power law statistics (1/f α with α≈ 1). The work of Jaeger et al. (1989) demonstrated however that avalanches of large sized spherical beads (particle size dp >∼ 100 µm) in a partially filled drum slowly rotated around its horizontal axis display a quasiperiodic oscillation and have a well-defined size incompatible with SOC. Other researches showed that by adjusting the mechanism of energy dissipation granular avalanches could indeed self-organize to a critical state characterized by longrange spatial and temporal correlations typical of SOC. The first experimental evidence of SOC behavior was provided by Frette et al. (1996) who investigated sandpile avalanches driven by the addition of grains at a low rate from above the pile. Only in the case of elongated grains the distribution of avalanche sizes could be fitted to a power law as predicted by SOC. It was claimed that in order to observe SOC it is necessary to suppress inertial effects: sliding friction ruled the dynamics of long shaped grains therefore inducing a higher effective friction that minimized inertia. Frette et al. (1996) suggested that an additional reason for the SOC behavior of elongated rice grains could be the rugged profile of the pile. Rough surface profiles can be achieved in practice introducing cohesiveness into the system (Quintanilla et al. 2001; Tegzes et al. 2003). Cohesion also reduces inertia effects. Experiments on the avalanching behavior of cohesive large beads (particle size dp >∼ 100 µm) were performed where interparticle cohesion was tuned by modifying the bead surface properties, either by adhering strongly attractive microparticles to it (Quintanilla et al. 2001)
or by moisturizing it (Tegzes et al. 2003). In both cases the statistic analysis of avalanches was inconsistent with SOC, showing instead a regular succession of small precursors and large relaxation events. We will focus on the effect of decreasing particle size on granular avalanches by analyzing the statistics of avalanche size as well as the temporal evolution of the average angle of the slope.
