Fifty shades of yield stress fluids: rheological challenges and engineering perspectives

While they were still marginal around 30 years ago and even the very existence of the yield stress was still under debate, research work involving yield stress fluids has exploded over the last 20 years in rheology and physics, even to the point of sometimes appearing hackneyed. Yield stress fluids are now fully recognized as a specific state of matter by physicists and widely studied for this reason. They are also used for their remarkable mechanical behavior in a rapidly growing range of applications, notably in additive manufacturing or 3D printing in bioengineering, civil engineering, food processing, etc. This review first discusses the areas in which a sufficient knowledge might be considered acquired to be used in yield stress fluid engineering. This in particular includes the characterization of materials, through practical tests or sophisticated approaches, the use of simplistic constitutive equations or more complex models including various subtleties of behavior in view of flow simulations, a basic rheophysical framework for predicting the behavior of yielding dispersions or aggregated systems, but also for the widespread practical case of suspensions in yield stress fluids. However, there also appear large areas of major impact for which a comprehensive knowledge seems still lacking. This is in particular the case of: a relevant 3D formulation of the constitutive equation to describe the complex flows encountered in numerous applications, the physical and mechanical characteristics of the solid–liquid transition, the characterization and description of thixotropy, the transition to pasty materials, at the very frontier of “pure” solids.