Protorheology: avoiding misinterpretation

Protorheology is the paradigm that any observed flow, even in our daily encounters, is an experiment to infer quantitative rheological properties, such as shear viscosity, extensional viscosity, shear normal stress differences, viscoelasticity, and thixotropy without using a rheometer [1,2]. Although this can enable rapid insight, data validation, high-throughput characterization, and an inclusive approach to rheometry without needing a rheometer [3-5], there is also significant risk of misunderstanding the Newtonian and non-Newtonian fluid physics involved, owing to the potentially complex and uncontrolled nature of the flow that may conflate multiple forcing (e.g. gravity, capillarity) and resistance (e.g. viscous, viscoelastic, inertial). Here we describe several examples of embarrassing data misinterpretations of experimental protorheology flows and identify systematic approaches to avoid misinterpretation. Case studies show how serious the problems can be, e.g. mistaking a liquid as a solid, incorrectly assigning a viscoelastic relaxation time to a viscous flow time, and more. Whether you are making inference from a tilted vial, time-lapse gravity-driven flow, a bounce test, die swell, or any other protorheology observation, the examples here serve as a guide for avoiding bad data in protorheology by carefully understanding the flow physics involved.





[1] Hossain & Ewoldt “Protorheology,” J. Rheol. (2024) https://doi.org/10.1122/8.0000667

[2] Hossain & Ewoldt, “Do-it-yourself rheometry,” Phys. Fl. (2022) https://doi.org/10.1063/5.0085361

[3] Trust But Verify. Nat. Mater. (2024). https://doi.org/10.1038/s41563-023-01790-z

[4] Lessard et al., J. Am. Chem. Soc. (2024) https://doi.org/10.1021/jacs.4c01578

[5] Han et al., Nat. Comm. (2024) https://doi.org/10.1038/s41467-024-47696-5