Flow and Slip of Pastes

Concentrated dispersions like pastes, slurries and emulsions have intriguing flow properties: they behave like weak elastic solids at low stresses, whereas they flow like viscous liquids at large stresses. An everyday example is toothpaste, which readily flows when the tube is pressed , yet keeps its shape afterwards. In many industrial processes, it is important to understand how pastes flow. Their flow is often strongly effected by the nature of the surfaces that confine them: rough surfaces are difficult to flow over, whereas pastes can 'slip' over suitably smooth surfaces. We have studied the flow of concentrated suspensions of microgel particles ('deformable spheres') using rheology and flow visualization, for both rough and smooth surfaces. At present we are trying to understand how the paste slip depends on the properties of the particular paste, like the elastic modulus for example.