The yielding transition in soft materials

Many materials exist which span the behavioral gap between solids and liquids. Such materials are commonly referred to as either complex fluids or soft materials, and exhibit varying extents of solid-like deformation or fluid-like flow, depending on the specific microstructure of the material. A frequently observed property in soft materials is yielding: the reversible transition of a material from solid-like deformation to fluid-like flow as the deformation on the material increases past some threshold. While yielding has traditionally been modeled in a constitutive sense as a binary transition at a critical stress (i.e. a “yield stress”) though a well-defined threshold has been difficult to determine in practice. Such a description is also unable to account for many of the transient consequences of the yielding transition seen in macroscopic rheology as well as microstructural scattering, imaging, and velocimetric tests. The tests currently available for characterizing and understanding the yielding transition are varied, but it is unclear which, if any, are the correct ones to use in practice. This thesis focuses on determining and refining the experimental and analytical techniques which can be used to deepen our understanding of the yielding transition. These methods fall into three categories: 1) analytical frameworks for characterizing the yielding transition in a time-resolved manner from common rheological tests, 2) novel measurement techniques to access more detailed rheo-physical information about the progression of yielding, and 3) combined rheo-scattering techniques to assist in the development of structure-property relations for the yielding transition. The characterization studies investigate the yielding and unyielding transitions of both industrial metallization pastes and polymer microgels under large amplitude oscillatory shear (LAOS) by means of the sequence of physical processes (SPP) framework. The SPP approach allows insights to be gained regarding these processes on time scales significantly faster than the period of oscillation, with the temporal resolution limited only by the data acquisition time. Through the use of the SPP framework, the changes in instantaneous material properties during yielding are able to be inferred, and the rate of yielding is characterized for the first time. These studies demonstrate that yielding occurs as an intra-cycle process in LAOS, and that it is a gradual transition with respect to time. To measure the rheo-physics of yielding more directly, a novel rheological protocol, iterative oscillatory shear/recovery, is developed. This protocol combines strain-controlled oscillatory deformation with iterative stress-controlled recovery tests in order to experimentally decompose the recoverable and unrecoverable components of a material’s deformation in a time-resolved manner. These tests are performed on a range of soft materials which exhibit yielding, enable a number of advances in the rheology of yielding materials. They demonstrate that the source of the loss modulus overshoot in yielding materials is the transition from viscoelastic solid-like dissipation to viscoplastic fluid-like flow with increasing deformation. These findings have in turn resulted to the proposal of a continuous, fully-viscoelastic constitutive model for the yielding transition, and have allowed for the explicit experimental determination of the sequence of physical processes which occur in LAOS of yielding materials. To relate the microstructural dynamics during the onset of yielding to corresponding macroscopic deformations, a series of rheological creep/recovery experiments were performed with simultaneous X-ray scattering. By utilizing X-ray photon correlation spectroscopy (XPCS) to analyze the scattering results in a time-resolved manner, the rheological deformations can be connected to the instantaneous dynamics of the material. Through these experiments, a direct relationship between the unrecoverable strain acquired during shear and the microstructural correlation is observed. The work in this thesis advances the understanding of yielding in a time-resolved sense, by demonstrating experimental and analytical techniques which are able to resolve when the yielding transition occurs in time, and how it progresses.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste