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Achieving good mixing is rarely trivial, especially in industries using non-Newtonian fluids, such as polymer manufacturing, mining, oilsands, and pulp and paper. These fluids do not follow Newton’s law of viscosity (constant viscosity with shear rate), and they can be shear-thinning (decreasing viscosity with shear rate, e.g. ketchup) or shear-thickening (increasing viscosity with shear rate, e.g. starch suspensions). In some cases, the viscosity also changes as a function time. Some fluids have a yield stress that must be overcome before the fluid can move, behaving like a rigid body at low shear, and a viscous fluid at high shear (e.g. toothpaste).

In this video, a yield stress fluid (Carbopol solution) is mixed using a standard impeller. A “cavern” forms near the impeller, behaving like a moving fluid. Outside of this cavern, the surroundings behave like a rigid body. Dye highlights the cavern boundary, which is a function of the material yield stress and the impeller characteristics. As impeller speed increases, the cavern grows larger. Fluid properties, impeller type, speed, and system geometry are critical factors to consider to ensure adequate mixing of non-Newtonian fluids, even in such simple scenarios.