Microdynamics of Complex Fluids: Yield Stress in Chitosan Solutions and Active Colloids
Gasbarro, Nina
2020
Abstract
In this dissertation, we study the role of microstructure and microdynamics in determining the functional properties of soft matter using rheology, dynamic light scattering (DLS), and particle-tracking video microscopy. This role is considered in two systems: aqueous solutions of chitosan and active Janus microspheres. The microdyanmical characterization of complex solutions is of broad scientific interest as understanding solution microstructure enables the prediction and manipulation of macroscopic properties. For chitosan, solution rheology is explored to probe the existence of an apparent yield stress. The possibility of shear-banded flow is investigated using a rheo-optical setup, and the quiescent microstructure of concentrated solutions is characterized using DLS. For active Janus microspheres, the microdynamics are characterized using DLS as a novel investigative technique. The macroscopic rheology of chitosan solutions features shear thinning at low shear rates consistent with the existence of an apparent yield stress. At shear rates above yielding, a constant viscosity plateau is observed with concentration-dependent scaling consistent with existing models of entangled and associating polymers below the gel point. The concentration-dependent scaling of the apparent yield stress, σ0(c) ~ c2.8±0.2 in the concentration range c = 7.50-65.0 mg/mL, is consistent with a solution microstructure of fractal clusters of fractal dimension df = 1.6±0.2. The addition of urea, a hydrogen bond and hydrophobic interaction disrupter, did not change the reported concentration-dependent scaling of the apparent yield stress or the plateau viscosity but did weaken the apparent yield stress magnitude by ~30% on average. The microdynamics of a concentrated chitosan solution and a weak chitosan gel were characterized by DLS. The extracted slow microdynamics are consistent with the presence of a ¬¬¬structured network or glassy fluid. To evaluate the possibility of shear banded flow of chitosan solutions in the shear rate range of the apparent yield stress, a rheo-optical instrument was used to measure the velocity profile in the gap between parallel plates. Silicone oil was applied around the gap to prevent rapid sample evaporation but resulted in image distortion as well as new secondary flows. To validate our modified experimental conditions, we carefully evaluate lower stress and shear rate limits using standard solutions. We find that in the experimentally valid shear rate range of γ ̇ = 0.01-0.056 1/s chitosan solutions do not exhibit shear banding. The shear rate range of characterization, although significantly lower than most literature measurements, does not completely rule out the possibility of shear banding in self-associating chitosan solutions. The microdynamics of active colloids were characterized using DLS as a novel investigative tool. In the presence of hydrogen peroxide, Janus microspheres of platinum and polystyrene undergo active motion by means of self-diffusiophoresis. The dynamic structure factor decays more steeply with increasing hydrogen peroxide concentration, suggestive of non-diffusive microdynamics and propulsion. A model that combines active motion with passive diffusion is developed to extract v* from the dynamic structure factor. For hydrogen peroxide concentrations of 1-10 vol.%., particles of d = 0.7 μm and d = 1.2 μm have v* = 4-16 μm/s and v* = 2-13 μm/s respectively. Comparison with v* from direct measurements using confocal microscopy and particle tracking was limited by anomalies in the optical microscopy and particle tracking data. However, external measurements of v* from confocal microscopy and particle tracking agree with v* determined from DLS measurements.Subjects
Microdynamics of soft matter Chitosan Active motion Rheology Dynamic light scattering
Types
Thesis
Metadata
Show full item recordCollections
Remediation of Harmful Language
The University of Michigan Library aims to describe library materials in a way that respects the people and communities who create, use, and are represented in our collections. Report harmful or offensive language in catalog records, finding aids, or elsewhere in our collections anonymously through our metadata feedback form. More information at Remediation of Harmful Language.
Accessibility
If you are unable to use this file in its current format, please select the Contact Us link and we can modify it to make it more accessible to you.