Antoinette Tordesillas Department of Mathematics and Statistics, University of Melbourne Searching for grains of truth Thursday 2nd, March 11:05-11:55pm, Carslaw Building Room 173. In 1987, Per Bak, Chao Taug, and Kurt Wiesenfeld published a now famous paper in Physical Review Letters, proposing the theory of self-organized criticality, in which they used an all-too-common sight - a sand pile - as a paradigm for complex systems out of equilibrium (e.g. an outbreak of a war, a stock market crash, forest fires). Sand, M&Ms, powders, pharmaceutical pills, grains and other granular materials exhibit a vast range of complex phenomena that defy our usual classification of matter into a solid, liquid or gas. Take, for example, vacuum-packed coffee: under pressure, it is a solid-like material as hard as a brick, but when opened and poured into a container, it flows like water. In common with other complex systems, the simplest unit in a granular assembly, i.e. the interaction of two particles in contact, is well understood. The complexity emerges from the collective behaviour of the whole assembly, which ultimately arises from internal mechanisms developed on multiple length scales. Recently, non-invasive experimental studies have given remarkable insight into the evolution of these mechanisms, thereby providing benchmarks and a unique opportunity for the theoretical modelling of these systems. In this talk, challenges in our quest to develop micromechanical constitutive models that can capture these multiscale mechanisms will be presented. Multidisciplinary collaborations with experimentalists and theoreticians from physics, geomechanics and geology constitute an underpinning aspect of this program that will also be discussed.