A Low-Cost, Open-Source Cylindrical Couette Rheometer

Rheology, commonly simplified by the property of viscosity, describes the flow of all fluids, from food and plastics, to coatings, adhesives, and 3D printing inks. While viscometers adequately probe Newtonian (constant) viscosity, most fluids have complex viscosity, requiring tests over multiple shear rates, and transient measurements. As a result, rheometers are typically large, expensive, and require additional infrastructure (e.g., gas lines), rendering them inaccessible for regular use by many individuals, small organizations, and educators. Here, we introduce a low-cost (under USD$200 bill of materials) Open Source Rheometer (OSR), constructed entirely from thermoplastic 3D printed components and off-the-shelf electromechanical components. A sample fluid rests in a cup while a microstepping motor rotates a tool inside the cup, applying strain-controlled shear flow. A load cell measures reaction torque exerted on the cup, and viscosity is calculated. To establish the measurement range, the viscosity of four Newtonian samples of 0.1–10 Pa.s were measured with the OSR and compared to benchmark values from a laboratory rheometer, showing under 23% error. Building on this, flow curves of three complex fluids – a microgel (hand sanitizer), foam (Gillette), and biopolymer solution (1% xanthan gum) – were measured with a similar error range. A further stress relaxation test was demonstrated on the biopolymer solution. The OSR cost is ∼1/25th that of commercially available devices with comparable minimum torque (200 µ N.m), and provides a platform for further innovation in open-source rheometry.