Randy H. Ewoldt

University of Illinois Urbana-Champaign

Mechanical Engineer

Fellow, Elected 2025

Professor Randy Ewoldt and his research group study rheology, non-Newtonian fluid mechanics, and design involving soft matter and complex fluids. His work involves experiments and mathematical modeling. His overarching vision is that new engineering designs will result from a deeper fundamental understanding of rheologically-complex materials. Recent motivating applications include direct-write 3D printing, wildland fire suppressants, hydraulic fluid power, stiffness-changing fibrous materials, medical task simulation, and flow batteries for grid-scale energy storage. His scientific and Society of Rheology contributions include the following categories:

• Weakly-Nonlinear Rheology (a.k.a. MAOS: medium-amplitude oscillatory shear). Made the first complete measurement of weakly-nonlinear oscillatory rheology, which had been theoretically anticipated for over 50 years, and combined this with new theoretical modeling to infer molecular architecture of complex fluids. Significantly advanced the theoretical understanding and experimental practice of these methods.
• Experimental Rheometry Advances. Identified a wide range of common experimental limitations of rotational rheometers and created methods to identify and avoid bad data; the subject of a recorded webinar with >15,000 views. Separately, self-published a fullydocumented and free software to calculate rheological properties from oscillatory rheology signals; requested by over 500 corporate, government, and academic research groups across the world, and incorporated into commercial instrument software.
• Yield-Stress Fluids. Discovered the importance of extensibility with viscoplastic yieldstress fluids; formulated new model materials to study the behavior which led to creation of new direct-write 3D printing inks with unprecedented printing capabilities, including a demonstration of printing from a smaller nozzle diameter than reported in any prior work. Separately, conceived a new dimensionless group that reveals a simplified understanding of viscoplastic yield-stress fluid droplet impact and splashing, motivated by applications in fire-suppression and spray-coating manufacturing.
• Design of, and with, Rheologically-Complex Materials. Advancing the interdisciplinary practice of rheological design with fundamental contributions to inverse problems of 'design with' and 'design of' materials, covering a range of constitutive responses including linear viscoelastic, nonlinear viscoelastic, and yield-stress fluid behavior.
• Contributions to the Society of Rheology-As Technical Program Co-Chair for the 2017 meeting in Denver, he and Anne Grillet created the Gallery of Rheology as a venue for visually striking images produced by our community. This is now a regular session at each SOR Annual Meeting. Chairing the Constitution Review Committee in 2023 he led an effort that produced significant and important amendments to the Constitution and Rules. These were overwhelmingly approved by vote of the members last year.