Paper Number
GS11 

Session
Gels and Self-Assembled Systems

Title
Measuring the viscoelastic properties of gelling systems using optimal Fourier transform techniques

Presentation Date and Time
October 10, 2017 (Tuesday) 4:10

Track / Room
Track 5 / Crestone B

Authors (Click on name to view author profile)

1. Keshavarz, Bavand (MIT, Mechanical Engineering)
2. Geri, Michela (MIT, Mechanical Engineering)
3. Divoux, Thibaut (MIT-CNRS)
4. McKinley, Gareth H. (Massachusetts Institute of Technology, Department of Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Bavand Keshavarz¹, Michela Geri¹, Thibaut Divoux², and Gareth H. McKinley^1
1Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; ²MIT-CNRS, Cambridge, MA 02139

Speaker / Presenter 
Keshavarz, Bavand

Text of Abstract
We analyze both the advantages and the limitations of a newly proposed amplitude- and frequency-modulated rheological test sequence, that uses exponential chirps. Using this new technique, we study the linear viscoelastic response of a slowly mutating acid-induced protein gel and show that by combining the exponential chirp with a modulated amplitude envelope for the input strain signal we can considerably increase the accuracy of the measured frequency response. This optimally-windowed-chirp (OWCh) signal enables us to rapidly measure the transient evolution of the elastic and loss moduli of the casein acid-induced gels over a broad range of frequencies. The linear viscoelastic response of these gels over two decades of frequency (0.3 = \omega = 30 rad/s) can be accurately measured using a test signal of just 15 seconds in length. We study, the gelation process for casein gels at different mutation numbers and compare the performance of the OWCh signal to other signal processing techniques such as multi-wave and step strain measurements. A simple numerical model of the gelation process shows how the fidelity of the viscoelastic measurements depends on the time-frequency bandwidth of the input waveform and a dimensionless mutation number that characterizes the speed of the gelation process. Finally, we discuss the sources of error in the measurements performed with windowed chirp signals and provide theoretical scalings for the error magnitude as a function of a single dimensionless window length parameter.