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Identifier

000425013

Title

An investigation of dynamic transitions in crowded model soft colloids

Author

Gury Leo

Thesis advisor

Vlassopoulos, Dimitris
Cloitre Michel

Abstract

At high packing fractions, colloidal suspensions are yield stress materials that are characterized by a solid behavior at rest and an ability to flow under a sufficient applied stress. While the formation of an organized crystalline phase may occur under certain conditions of concentration, preparation and monodipersity, most systems usually form disorganized glasses. The solid regime of hard colloids is theoretically limited by their close packing fraction (74% in the case of spheres), but the experimentally accessible limit is often the random close packing (64% for spheres) and therefore the solid regime only exist in a small range of packing fraction. The ability of soft colloids to deform and accommodate contacts (deswelling, faceting, interpenetration) extends the range of concentration at which they form a solid phase, but also complicate the evolution of the dynamic properties in this regime. As such, the systematic study of these properties requires the use of well-characterized model systems. In this work, we report on a careful examination of the rheological properties of the two main experimental model soft colloids, namely star polymers and microgel particles. In particular, we focus here on a very dense star polymer with nearly 900 short arms (5.8 kg.mol-1 per arm). We perform a series of rheological tests both in the linear and nonlinear regime. The differences between the two systems may be attributed to the main microstructural difference, the presence of dangling chains in star polymers, which are absent in microgels. We also find striking similarities, not only between the two investigated systems, but also in the data of the available literature on soft colloids, which suggests a possible universal behavior. In particular, we find that a change in the evolution of dynamic properties inside the solid regime seems ubiquitous, and we call it jamming. Several arguments point towards a transition to a regime where elasticity becomes dominated by contacts when the packing fraction reaches values very close to 1. If proven correct, this may provide a useful link between the microstructural properties (chemical composition, softness) of soft colloids and their macroscopic rheological properties (elasticity, yielding, and behavior under flow). The objective of this work is therefore to provide and disseminate reliable data to help create solid bases for the design of novel soft materials with increased performances.

Language

English

Subject

Polymers

Rheology

Soft matter

jamming

Issue date

2019-11-29

Collection

School/Department--School of Sciences and Engineering--Department of Materials Science and Technology--Doctoral theses

Type of Work--Doctoral theses