Glassy and Jammed Systems: Structures and Dynamics 公开
Cao, Cong (Fall 2020)
Abstract
Soft materials are usually composed of basic units much larger than atomic scale. For example, colloids are 10 nm to 10 μm solid particles dispersed in a liquid phase; emulsions consist of two immiscible liquids, with one dispersed in another as a droplet form. On one hand, these systems possess similarities with atomic glassy systems; on the other hand, thermal fluctuations and gravity could both play a role in a system's structure and dynamics depending on their components' scale. In this dissertation, we will explore structural and dynamical characteristics of various soft material systems: from ~μm colloidal suspensions to ~mm granular materials; from spherical particles to rods with different aspect ratios. We are curious about the unique properties in each system, while at the same time eager to find connections between different soft materials.
In chapter 2, we use confocal microscopy to study the aging of a bidisperse colloidal glass near rough and smooth boundaries. Near smooth boundaries, the particles form layers, and particle motion is dramatically slower near the boundary as compared to the bulk. Near rough boundaries, the layers nearly vanish, and particle motion is nearly identical to that of the bulk. The gradient in dynamics near the boundaries is demonstrated to be a function of the gradient in structure for both types of boundaries. Our observations show that wall-induced layer structures strongly influence aging.
In chapter 3, we conduct x-ray tomography experiments and study the boundary effect of 3D rod packing (rods packed in a finite cylindrical container). We then compare our tomography results with traditional protocols and simulation results. In all cases, rods pack randomly in cylindrical containers whose smallest dimension is larger than the rod length. Packings in smaller containers have lower volume fractions than those in larger containers, demonstrating the influence of the boundaries. X-ray tomography experiments show that the boundary effects depend on the orientation of the boundary, indicating a strong influence of gravity, whereas the simulation finds boundary effects that are purely geometric. In all cases, the boundary influence extends approximately half a particle length into the interior of the container.
In chapter 4, we study the rheology of monodisperse and bidisperse emulsions with various droplet sizes (1 μm -- 2 μm diameter). Above a critical volume fraction, these systems exhibit solid-like behavior and possess a yield stress. Previous experiments suggest that for small thermal particles, rheology will see a glass transition at 0.58; for large athermal systems, rheology will see a jamming transition at 0.64. However, simulations point out that at the crossover of thermal and athermal regimes, the glass and jamming transitions may both be observed in the same sample. Here we conduct an experiment by shearing four oil-in-water emulsions with a rheometer. We observe both a glass and a jamming transition for our smaller diameter droplets, and only a jamming transition for our larger diameter droplets. The bidisperse sample behaves similarly to the small droplet sample, with two transitions observed. We fit our data with both Herschel-Bulkley model and Three-Component model. Based on the fitting parameters, our raw rheological data would not collapse into a master curve. Our results suggest that liquid-solid transitions may not be universal, but depends on particle type.
Table of Contents
1 Introduction 1
1.1 Soft materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Packing and soft materials . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Jamming and glass transition . . . . . . . . . . . . . . . . . . . . . . 6
2 Aging Near Rough and Smooth Boundaries in Colloidal Glass 9
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3 Random packing of rods in small containers: X-ray tomography
experiments 26
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.1.1 Random close packing . . . . . . . . . . . . . . . . . . . . . . 26
3.1.2 Recent studies on long thin packing . . . . . . . . . . . . . . . 27
3.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.1 Rods and CT scanners . . . . . . . . . . . . . . . . . . . . . . 30
3.2.2 Image processing . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 3D rods results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.3.1 Tomography results . . . . . . . . . . . . . . . . . . . . . . . . 33
3.3.2 Implications of tomography data on bulk parameters . . . . . 37
3.3.3 Comparison with simulation data . . . . . . . . . . . . . . . . 39
3.3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4 2D and Sandpaper . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4.1 2D rods packing results . . . . . . . . . . . . . . . . . . . . . . 41
3.4.2 Sandpaper results . . . . . . . . . . . . . . . . . . . . . . . . . 43
4 Rheology of jammed and glassy materials 47
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2.1 Sample synthesis . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2.2 Measuring volume fraction . . . . . . . . . . . . . . . . . . . . 54
4.2.3 Measuring particles' size . . . . . . . . . . . . . . . . . . . . . 55
4.2.4 Rheological details . . . . . . . . . . . . . . . . . . . . . . . . 56
4.3 Analysis and results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5 Conclusion 69
Bibliography 72
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