Dichotomous behaviors of stress and dielectric relaxations in dense suspensions of swollen thermoreversible hydrogel microparticles

TL;DR

This study investigates how large shear strains affect the dielectric and mechanical properties of dense suspensions of thermoreversible hydrogel particles, revealing complex relaxation behaviors and shear-induced structural changes.

Contribution

It provides new insights into the simultaneous dielectric and rheological responses of thermoresponsive hydrogel suspensions under shear, highlighting shear-induced cluster rupture and relaxation dynamics.

Findings

Dielectric permittivities show distinct low and high frequency relaxations.

Increased strain amplitudes decrease dielectric permittivities and slow relaxation dynamics.

Rheology shows shear-thinning behavior despite dielectric slowdown.

Abstract

Hypothesis: While the mechanical disruption of microscopic structures in complex fluids by large shear flows has been studied extensively, the effects of applied strains on the dielectric properties of macromolecular aggregates has received far less attention. Simultaneous rheology and dielectric experiments can be employed to study the dynamics of sheared colloidal suspensions over spatiotemporal scales spanning several decades. Experiments: Using a precision impedance analyzer, we study the dielectric behavior of strongly sheared aqueous suspensions of thermoreversible hydrogel poly(N-isopropylacrylamide) (PNIPAM) particles at different temperatures. We also perform stress relaxation experiments to uncover the influence of large deformations on the bulk mechanical moduli of these suspensions. Findings: The real parts of the complex dielectric permittivities of all the sheared PNIPAM suspensions exhibit distinct relaxation processes in the low and high frequency regimes. At a temperature below the lower consolute solution temperature (LCST), both real and imaginary parts of the permittivities of highly dense PNIPAM suspensions decrease with increase in applied oscillatory strain amplitudes. Simultaneously, we note a counter-intuitive slowdown of the dielectric relaxation dynamics. Contrary to our rheo-dielectric findings, our bulk rheology experiments, performed under identical conditions, reveal shear-thinning dynamics with increasing strain amplitudes. We propose the shear-induced rupture of fragile clusters of swollen PNIPAM particles to explain our observations. Keywords : Rheo-dielectric; Thermoresponsive hydrogels; Segmental motion; Counterion polarization; Interfacial polarization; Dense suspensions; Rheology; Shear-thinning.