Paper Number
GS10 

Session
Gels and Self-Assembled Systems

Title
Optimal Fourier transform for probing oscillatory rheology of networks: Introduction and application to thermoreversible gels

Presentation Date and Time
October 10, 2017 (Tuesday) 3:45

Track / Room
Track 5 / Crestone B

Authors (Click on name to view author profile)

1. Geri, Michela (MIT, Mechanical Engineering)
2. Keshavarz, Bavand (MIT, Mechanical Engineering)
3. Divoux, Thibaut (MIT-CNRS)
4. Clasen, Christian (KU Leuven)
5. Curtis, Daniel J. (Swansea University)
6. McKinley, Gareth H. (Massachusetts Institute of Technology, Department of Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Michela Geri¹, Bavand Keshavarz¹, Thibaut Divoux², Christian Clasen³, Daniel J. Curtis⁴, and Gareth H. McKinley^1
1Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; ²MIT-CNRS, Cambridge, MA 02139; ³KU Leuven, Heverlee, Belgium; ⁴Swansea University, Swansea, United Kingdom

Speaker / Presenter 
Geri, Michela

Text of Abstract
Measuring linear viscoelastic spectra of time-varying networks requires signals that can provide both time- and frequency-resolution. Several methods have been proposed for developing new and improved protocols, including multi-wave techniques, random/white noise sequences, short-time Fourier transformations and step-strain pulses. In this work, we focus on an alternative technique that has been effectively used in radar and audio signal processing, based on a chirp, an input signal with constant amplitude and continuously modulated frequency. Exponential chirps have been proposed in the past to determine the linear viscoelastic properties of gels and have been claimed to be an optimal Fourier rheometry technique. However, analyses using a model entangled polymeric fluid reveal that spectral leakage in the chirp power spectrum can compromise the precision of the measurements, especially when the signal length is decreased to improve time-resolution. Inspired by the echolocation signals of bats and dolphins, we modulate the amplitude of the chirp with a specific envelope and show that this dramatically reduces the error due to spectral leakage. By using the same semi-dilute polymer solution we optimize the envelope modulation and show that the resulting Optimally Windowed Chirp (OWCh) waveform can be used to extract linear viscoelastic moduli with both improved time- and frequency-resolution and minimum error. The versatility of the OWCh input sequence is demonstrated by studying the gelation of a model thermoreversible gel composed of mineral oil and paraffin wax. Using a series of OWCh signals, we study the evolution of the gel properties under different thermal and shear histories and elucidate the complex nature of the thermogelation process in this material. This new signal processing protocol can be applied to resolve details of gelation dynamics for a wide range of mutating systems.