Dichotomous behavior of stress and dielectric relaxations in dense suspensions of swollen thermoreversible hydrogel microparticles
| When |
May 03, 2022
from 11:00 to 11:30 |
|---|---|
| Where | Salle des thèses |
| Attendees |
Chandeshwar Misra |
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While the mechanical disruption of microscopic structures in complex fluids by large oscillatory strains has been studied extensively, the effects of applied strains on the dielectric properties of macromolecular aggregates has received far less attention. Using a precision impedance analyzer, we study the dielectric behavior of aqueous suspensions of densely and loosely-packed thermoreversible hydrogel poly(N-isopropylacrylamide) (PNIPAM) particles under controlled oscillatory strains at different temperatures. Below the lower consolute solution temperatures (LCST) the dielectric permittivities of the densely-packed suspensions exhibit distinct relaxation processes in the low and high-frequency regimes, with the real and imaginary parts of the complex dielectric constant both decreasing with increase in applied oscillatory strain amplitudes. Moreover, we note a counter-intuitive slowdown of the dielectric relaxation processes in both relaxation regimes. In contrast, the dielectric responses are insensitive to the applied strain for the densely-packed suspensions at temperatures above the LCST and for the loosely-packed suspensions at all experimental temperatures. Interestingly, our bulk rheology experiments performed below the LCST reveal shear-thinning of densely-packed PNIPAM suspensions with increasing strain amplitudes. We propose the shear-induced formation of fragile clusters of swollen PNIPAM particles to explain our observations. Our rheo-dielectric study uncovers interesting new aspects of hydrogels suspension dynamics over length scales and time scales spanning decades and has important implications in our understanding of the electrical energy storage devices.