Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions

Ashesh Ghosh, Gaurav Chaudhary, Jin Gu Kang, Paul V Braun, Randy H Ewoldt, Kenneth S Schweizer

Research output: Contribution to journalArticle

Abstract

We present an integrated experimental and quantitative theoretical study of the mechanics of self-crosslinked, slightly charged, repulsive pNIPAM microgel suspensions over a very wide range of concentrations (c) that span the fluid, glassy and putative "soft jammed" regimes. In the glassy regime we measure a linear elastic dynamic shear modulus over 3 decades which follows an apparent power law concentration dependence G′ ∼ c 5.64 , a variation that appears distinct from prior studies of crosslinked ionic microgel suspensions. At very high concentrations there is a sharp crossover to a nearly linear growth of the modulus. To theoretically understand these observations, we formulate an approach to address all three regimes within a single conceptual Brownian dynamics framework. A minimalist single particle description is constructed that allows microgel size to vary with concentration due to steric de-swelling effects. Using a Hertzian repulsion interparticle potential and a suite of statistical mechanical theories, quantitative predictions under quiescent conditions of microgel collective structure, dynamic localization length, elastic modulus, and the structural relaxation time are made. Based on a constant inter-particle repulsion strength parameter which is determined by requiring the theory to reproduce the linear elastic shear modulus over the entire concentration regime, we demonstrate good agreement between theory and experiment. Testable predictions are then made. We also measured nonlinear rheological properties with a focus on the yield stress and strain. A theoretical analysis with no adjustable parameters predicts how the quiescent structural relaxation time changes under deformation, and how the yield stress and strain change as a function of concentration. Reasonable agreement with our observations is obtained. To the best of our knowledge, this is the first attempt to quantitatively understand structure, quiescent relaxation and shear elasticity, and nonlinear yielding of dense microgel suspensions using microscopic force based theoretical methods that include activated hopping processes. We expect our approach will be useful for other soft polymeric particle suspensions in the core-shell family.

Original languageEnglish (US)
Pages (from-to)1038-1052
Number of pages15
JournalSoft Matter
Volume15
Issue number5
DOIs
StatePublished - Jan 1 2019

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Structural relaxation
Rheology
rheology
Suspensions
Elastic moduli
Relaxation time
Yield stress
shear
relaxation time
Swelling
Elasticity
Mechanics
predictions
swelling
Fluids
modulus of elasticity
crossovers
elastic properties
fluids
Experiments

ASJC Scopus subject areas

  • Chemistry(all)
  • Condensed Matter Physics

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Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions. / Ghosh, Ashesh; Chaudhary, Gaurav; Kang, Jin Gu; Braun, Paul V; Ewoldt, Randy H; Schweizer, Kenneth S.

In: Soft Matter, Vol. 15, No. 5, 01.01.2019, p. 1038-1052.

Research output: Contribution to journalArticle

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