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

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 macro- molecular aggregates have 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: All the sheared PNIPAM suspensions exhibit distinct dielectric relaxation processes in the low and high frequency regimes. At a temperature below the lower consolute solution temperature (LCST), the complex permittivities of highly dense PNIPAM suspensions decrease with increase in applied oscilla- tory 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 iden- tical 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. Our work illus- trates that rheo-dielectric studies have enormous potential for providing deep insights into the length scale-dependent dynamical properties of complex systems such as dense suspensions and soft glasses.