In this two‑part webinar, we illustrate how modern broadband rheometric techniques can be used to measure the material functions of what may broadly be referred to as mutating materials (or changing materials), with rheological properties that are both time‑ and frequency‑dependent.

Numerous approaches have been proposed in the past for developing time‑resolved rheometric protocols capable of providing both time‑and frequency‑resolved measurements on aging, curing, or crosslinking gel systems. These include multi‑wave techniques, random/white noise sequences, short‑time Fourier transforms, and repeated step‑strain pulses.

In Part I of this webinar series, we show how a common audio signal processing sequence—the exponential chirp—can be adapted to rheometry, and explain why it offers several benefits, including a frequency‑independent amplitude and a continuously varying phase. This enables the linear viscoelastic properties, such as G′ and G″, of a time‑evolving material (e.g., a thermally or chemically crosslinking gel) to be rapidly determined over several decades in frequency within just a few minutes.

However, closer investigation of data on model polymer networks shows that measurement precision is compromised at the highest and lowest test frequencies due to “leakage” of material information into side lobes of the signal’s power spectrum. Taking inspiration from chirp sequences used by bats and dolphins in echolocation, we explain how these inaccuracies can be mitigated by adding a carefully chosen windowing function or envelope to the chirp.

The resulting waveforms can be readily encoded in MATLAB or implemented directly through instrument firmware such as Fast Frequency Chirps, and then used to impose optimal deformations in controlled‑strain or controlled‑stress rheometers. Numerical computations and experimental measurements show that the error magnitude can be reduced exponentially with correct selection of window parameters and careful signal conditioning.

We first demonstrate basic applications of the technique using a semi‑dilute entangled polymer solution, a wormlike micellar fluid, and a time‑evolving crosslinked biopolymer gel. These examples show that the resulting Optimally Windowed Chirp (OWCh) technique can rapidly and accurately extract the entire linear viscoelastic spectrum of a time‑evolving complex material in less than 15 seconds (the time typically required to obtain the complex modulus at a single low frequency).

What you’ll learn:

• Discover how optimally‑windowed exponential chirps (OWCh) deliver rapid, broadband viscoelastic spectra for mutating materials in seconds.
• Learn how to minimize spectral leakage, tune chirp parameters, and implement protocols in MATLAB or instrument firmware for high‑quality, time‑resolved data.

Gareth H. McKinley is a renowned mechanical engineer and professor at the Massachusetts Institute of Technology (MIT), where he serves as the School of Engineering Professor of Teaching Innovation and Professor of Mechanical Engineering.

McKinley is internationally recognized for his expertise in fluid dynamics and rheology—the study of how complex materials flow and deform. His research explores non-Newtonian fluids, microfluidics, hydrogels, and advanced materials, with applications ranging from industrial processes to biotechnology. His work has led to important advances in understanding phenomena such as bubble formation in liquids and the behavior of viscoelastic materials, as well as practical innovations like techniques for harvesting drinking water from fog in arid environments.

He earned his Ph.D. from MIT in 1991 after completing a B.A. and M.Eng. at the University of Cambridge. Over his career, McKinley has published more than 275 scientific papers and has played a significant role in shaping the field of rheology.

His achievements have been widely honored. He is a Fellow of the Royal Society and the American Physical Society and a member of the U.S. National Academy of Engineering. He has also received major awards such as the Bingham Medal and the Gold Medal of the British Society of Rheology.

In addition to his research, McKinley has held several leadership roles at MIT and is known for his dedication to teaching and mentoring, influencing the next generation of engineers and scientists.