Diving into Yield Stress Fluids

Many foods, cosmetics, building materials and even soil/snow can be categorized as yield stress fluids, showing time-dependent rheological behavior that strongly affects the way they flow. In applications, it is often necessary to have a quick and reliable way of measuring the "consistency" of these semi-solid products without a rheometer. The standard method across industries makes use of a "penetrometer": a device that releases a cone of given weight on top of the material and measures the total penetration depth after the cone reaches equilibrium. Inspired by such simple device, in this talk we will explore the dynamics of an object diving into yield stress fluids. By resolving the displacement evolution over time for different forcing weights and object shapes, we will show how the material rheological behavior affects the fluid dynamics during diving. We will focus on simple and thixotropic yield stress fluids across industries, e.g., paraffin gels, hair gels, shaving creams and butter. Combining experimental observations with constitutive relations and fluid modeling, we will elucidate how time-dependent rheological parameters, such as plastic viscosity and yield stress, can affect the transient flow before the diving object reaches its final equilibrium position.