The rheology of jamming

Traffic jams are a common phenomenon on highways; when there are too many cars on the road the traffic gets stuck. A similar jamming phenomenon also occurs in yield-stress fluids that consist of a dispersion of a material in a liquid, such as suspensions of particles or polymers, foams or emulsions. At high concentrations, these materials behave like solids (like in traffic jams there is no flow), and they only start to flow when enough stress is applied. For example, toothpaste behaves like a solid at rest but it starts to flow when you squeeze it out of the tube. This threshold stress that is needed to initiate flow is called the yield stress, hence the name yield-stress material. It is important to understand these kinds of properties and the flow behavior (rheology) of these materials since they are widely applied in the cosmetic, oil and food industry. We seek to understand the transition from mechanically solid-like to fluid-like behavior on a fundamental level. The associated jamming transition between solid and liquid "states" has similarities to classical phase transitions like those between solid, liquid, gas and plasma phases. However it is not completely clear how general the jamming description is, and to what extent the mechanical behavior of jammed materials can be fully described by considering the jamming transition to be analogous to a classical phase transition. In this thesis we study the flow behavior (the rheology) of a variety of yield-stress materials in the aim of describing, understanding and predicting the rheology of jamming.