Understanding the yielding transition in soft materials

Kamani, Krutarth

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https://hdl.handle.net/2142/125510 Copy

Description

Title

Understanding the yielding transition in soft materials

Author(s)

Kamani, Krutarth

Issue Date

2024-05-23

Director of Research (if dissertation) or Advisor (if thesis)

Rogers, Simon A.

Doctoral Committee Chair(s)

Rogers, Simon A.

Committee Member(s)

• Higdon, Jonathan J. L.
• Kong, Hyunjoon
• Ewoldt, Randy H.

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Yielding
• Modeling
• Brittle yielding
• ductile yielding
• Memory
• Rheo-XPCS
• Yield stress fluids
• Design
• Structure-property-processing relations
• Recovery tests

Abstract

Yield stress fluids represent a subset of soft materials that undergo a transition from solid-like to liquid-like behavior under applied load or deformation, a phenomenon known as yielding. Understanding the physics behind this transition is of great interest to the behavior of biological, environmental, and industrial materials, including those used as inks in additive manufacturing. Studying their behavior under various rheological protocols reveals fascinating phenomena that challenge the conventional notions of solid and liquid properties. Understanding the macroscopic origin of the yielding transition and relating them to the microstructural properties would enable the design of YSFs from fundamental principles compared to the current method of trial and error. To this end, in this thesis, we first aim to develop constitutive relations that accurately capture the key observations related to the yielding behavior. Subsequently, we use rheo-X-ray photon correlation spectroscopy (rheo-XPCS) to understand the relation between macroscale properties and microstructural properties under deformation. The rheology of YSFs has traditionally relied on the Oldroyd-Prager formalism that makes a piecewise distinction between the unyielded solid behavior and the yielded fluid response. While such approaches are useful for describing quasi-static case, they are not appropriate for applications where transient conditions are important. We recently proposed a model that shows that the rheology of YSFs can be described in a continuous manner by acknowledging that the recoverable elastic deformations can influence plastic flow and that the relaxation time of the system is dependent on the shear rate. This model has been shown to explain the rheology of simple YSFs that are composed of suspensions of jammed hydrogels that have identical bulk and local moduli, which can be said to be compositionally homogeneous. Even though all YSFs show yielding transition, they vary in how this transition occurs. For some materials, the yielding transition is gradual, while others yield abruptly. We refer to these behaviors as being ductile and brittle. The key rheological signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a steep increase in the loss modulus during oscillatory amplitude sweeps. We show how this spectrum of yielding behaviors may be accounted for in a continuum model for yield stress materials by introducing a new parameter we call the brittility factor. Physically, an increased brittility decreases the contribution of recoverable deformation to plastic deformation, which impacts the rate at which yielding occurs. The model predictions are successfully compared to the results of different rheological protocols from a number of real yield stress fluids with different microstructures, indicating the general applicability of the phenomenon of brittility. Our study shows that the brittility of soft materials plays a critical role in determining the rate of the yielding transition, and provides a simple tool for understanding its effects under various loading conditions. Linking the macroscopic flow properties and nanoscopic structure is a fundamental challenge to understanding, predicting, and designing the behavior of disordered soft materials. We connect the transient structure and rheological memory of a colloidal gel under cyclic shearing across a range of amplitudes via a generalized memory function using rheo-X-ray photon correlation spectroscopy (rheo-XPCS). Our rheo-XPCS data show that the nanometer scale aggregate-level structure recorrelates whenever the change in recoverable strain over some interval is zero. The macroscopic recoverable strain is therefore a measure of the nano-scale structural memory. We further show that yielding in disordered colloidal materials is strongly heterogeneous and that memories of prior deformation can exist even after the material has been subjected to flow. Building on the concept that the flow viscosity in the proposed model can adopt any form based on the steady shear flow curve, we use the cross-form of flow viscosity to explore the behavior of soft materials under various rheological protocols. During oscillatory shear experiments at different frequencies, we observe variations in the sizes and shapes of the overshoot in loss modulus. Despite similar overall strain responses, analyzing component moduli enables a more accurate interpretation of material behavior. We also study the effect of incorporating a brittility factor on the model predictions. Finally, we exploit the recovery rheology ideas to test the existing constitutive relations rigorously. The work in this thesis advances the understanding of yielding in a time-resolved sense, by demonstrating new ways of putting together constitutive relations for soft materials that are more accurate. Using rheo-XPCS, we identify the importance of the fundamental recoverable strain, that is closely linked to microstructural evolution.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125510

Copyright and License Information

Copyright 2024 Krutarth Kamani

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