Pattern Formation and flow to Fracture Transitions in Granular and Sheer Thickening Materials

The conditions under which a mixture of water and grains will fracture like a solid, rather than flow like a liquid, is the subject of this thesis. Flow to fracture transitions in saturated granular materials are relevant to numerous geological and engineering environments, in-cluding magma cavern activity, methane venting on seabeds, carbon dioxide storage, food processing, and innovations in body armour. To examine the flow to fracture transition, two systems are considered. The first is gas-driven fracturing of settled granular media, a slow creeping process that forms labyrinthine patterns. The second is gas-driven fractur-ing of suspended cornstarch particles, a system which exhibits fascinating “discontinuous shear thickening” behaviour, a topic of much debate in literature. Both systems are sub-ject to experiments within a Hele-Shaw cell, which enables the visualisation of pseudo-2D invasion flow or fracture patterns. Image analysis performed on these patterns led to the application of theories that can predict their behaviours. Fracture formation is found to be a friction dominated process. The invading pressure pushes on the local grains while surface tension of the receding water pulls on them until frictional forces become strong enough to maintain a front, forcing the pressure to disturb grains elsewhere, and in do-ing so extend and branch the fractures forming a patterned network. Various parameter studies are performed to uncover the variables that determine why a mixture might flow or fracture. Interestingly, the first system is found to transition from fracturing to flowing with increasing pressures, whilst the second system is found to do the opposite.