Improved Rheometry of Yield Stress Fluids Using Bespoke Fractal 3D Printed Vanes

TL;DR

This paper introduces fractal-like 3D printed vane fixtures for rheometry, improving measurement accuracy and homogeneity in yield stress fluids, and demonstrates their effectiveness across various complex fluids.

Contribution

The study presents a novel 3D printed fractal vane design that enhances rheological measurements of yield stress fluids, with validated accuracy and adaptability for diverse materials.

Findings

Viscosity measurements with error <5% using Newtonian oils.

Fractal vane agrees within 1% of cone-and-plate measurements for Carbopol.

Effective probing of complex fluid responses like thixo-elasto-visco-plastic behavior.

Abstract

To enable robust rheological measurements of the properties of yield stress fluids, we introduce a class of modified vane fixtures with fractal-like 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 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 3D printed cup, which attaches to a standard rheometer base. We discuss general design criteria for 3D printed 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-arm 3D printed fractal vane agrees within 1% of reference measurements made with a roughened cone-and-plate geometry. Last, we demonstrate use of the 24-arm fractal vane to probe the thixo-elasto-visco-plastic (TEVP) response of a Carbopol-based hair gel, a jammed emulsion (mayonnaise), and a strongly alkaline carbon black-based battery slurry.