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

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 agreementmore » 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. Here, we expect our approach will be useful for other soft polymeric particle suspensions in the core–shell family.« less

Authors:

Ghosh, Ashesh  ^[1];

• Search DOE PAGES for author "Ghosh, Ashesh"
• Search DOE PAGES for ORCID "0000-0002-3312-6107"

Chaudhary, Gaurav  ^[1];

• Search DOE PAGES for author "Chaudhary, Gaurav"
• Search DOE PAGES for ORCID "0000-0002-5428-5094"

Kang, Jin Gu ^[1]; Braun, Paul V.  ^[1];

• Search DOE PAGES for author "Braun, Paul V."
• Search DOE PAGES for ORCID "0000-0003-4079-8160"

Ewoldt, Randy H.  ^[1];

• Search DOE PAGES for author "Ewoldt, Randy H."
• Search DOE PAGES for ORCID "0000-0003-2720-9712"

Schweizer, Kenneth S. ^[1]

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+ Show Author Affiliations

1. Univ. of Illinois at Urbana-Champaign (United States)

Publication Date:

Wed Dec 26 00:00:00 EST 2018

Research Org.:

Univ. of Illinois at Urbana-Champaign, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1634191

Alternate Identifier(s):

OSTI ID: 1491277

Grant/Contract Number:  

FG02-07ER46471

Resource Type:

Accepted Manuscript

Journal Name:

Soft Matter

Additional Journal Information:

Journal Volume: 15; Journal Issue: 5; Journal ID: ISSN 1744-683X

Publisher:

Royal Society of Chemistry

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE