Improved rheometry of yield stress fluids using bespoke fractal 3D printed vanes

To enable robust rheological measurements of the properties of yield stress fluids, we introduce a class of modified vane fixtures with fractallike cross-sectional structures. A greater number of outer contact edges leads to increased kinematic homogeneity at the point of yielding and beyond. The vanes are 3D printed (3DP) using a desktop stereolithography machine, making them inexpensive (disposable), chemically compatible with a wide range of solvents, and readily adaptable as a base for further design innovations. To complete the tooling set, we introduce a textured 3DP cup, which attaches to a standard rheometer base. We discuss general design criteria for 3DP rheometer vanes, including consideration of sample volume displaced by the vanes, stress homogeneity, and secondary flows that constrain the parameter space of potential designs. We also develop a conversion from machine torque to material shear stress for vanes with an arbitrary number of arms. We compare a family of vane designs by measuring the viscosity of Newtonian calibration oils with error <5% relative to reference measurements made with a cone-and-plate geometry. We measure the flow curve of a simple Carbopol yield stress fluid and show that a 24-armed 3DP fractal vane agrees within 1% of reference measurements made with a roughened cone-and-plate geometry. Last, we demonstrate use of the 24-armed fractal vane to probe the thixo-elastoviscoplastic response of a Carbopol-based hair gel, a jammed emulsion (mayonnaise), and a strongly alkaline carbon black-based battery slurry. (c) 2020 The Society of Rheology.