Granular matter
Sand, grains and powders carry load through a network of contacts that is hard to observe. We used light scattering to follow the movements of grains inside a water-saturated pile that is gently tapped, and found that rearrangements become rarer with each tap, as in ageing glasses (Kabla and Debrégeas, Physical Review Letters 2004). With X-ray imaging, we saw small failures inside a pile well before an avalanche (Kabla et al., Europhysics Letters 2005). Walking on wet sand leaves a dry footprint because the sand expands as it is deformed, an effect known as dilatancy; we measured how it unfolds in slow granular flows (Kabla and Senden, Physical Review Letters 2009). With Mohammad Saadatfar and Tim Senden at the Australian National University, we also developed X-ray tomography to count contacts and map forces between grains in three dimensions (Saadatfar et al., 2005; Journal of the Mechanics and Physics of Solids 2012).