The Glass Transition: Rotational & Translational Decoupling and the Confinement Effect Pubblico

Abstract

We study the microscopic properties of two phenomena related to the glass transition: the decoupling of diffusion from a glass-forming material's viscosity as it is cooled and the effect of confinement on the volume fraction that the glass transition occurs. We use colloidal suspensions of microspheres to physically model the glass transition. Colloids are a good approximation of hard-sphere fluids, where particle concentration effectively models a fluid's temperature. Using a high-speed confocal microscope, we rapidly visualize microscopic structural and dynamical processes in three dimensions.

We probe the colloidal fluid's rotational and translational dynamics with ordered clusters of microspheres. Far from the fluid's glass transition, both rotational and translational motion of the clusters are purely Brownian. However, in the liquid's supercooled regime, we observe a decoupling between the two types of motion: as the glass transition is approached, rotational diffusion slows down even more than translational diffusion. Our observation supports the notion that supercooled fluids are not merely fluids with large viscosities but that diffusion takes place by fundamentally changed mechanisms.

The effect of confinement on a fluid's glass transition temperature is the focus of our other experimental investigation. Confining a fluid to a small volume can either increase or decrease the glass transition temperature; in some cases confinement has no effect at all. The effect is strongly dependent on the properties of the boundaries confining the material. We directly observe the three-dimensional dynamical processes of confined colloidal suspensions of microspheres, while systematically varying the confinement volume and the suspension's concentration. The experiments find that confinement induces glassy behavior in a sample that is a fluid when not confined. Like particles in an unconfined near-glassy system, groups of particles in our confined system move together cooperatively. Normally these groups would be spatially isotropic. However, confinement induces a layering of the particles, which modifies the shape of the mobile groups so that they are more planar. The planar restriction helps to explain the sample's glassiness.

Table of Contents

Abstract Cover Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Cover Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Citations to Previously Published Work . . . . . . . . . . . . . . . . . . . x
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Dedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

1 Introduction 1
1.1 The Glass Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Fragile and Strong . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Hard Sphere Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3.1 Fluid Phases . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.2 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.3 Dynamic Arrest and Cooperative Rearrangements . . . . . . . 5
1.4 Summary of Major Results . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Dissertation Structure Overview . . . . . . . . . . . . . . . . . . . . . 6

I Background 8
2 Suspensions of Fluorescent Colloidal Particles 9
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Synthesis of Colloidal Particles . . . . . . . . . . . . . . . . . . . . . . 10
2.2.1 Core-shell Particles . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2 Synthesis of Colloidal Clusters . . . . . . . . . . . . . . . . . . 16
2.3 Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3 Confocal microscopy 21
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2 High Speed Laser Scanning . . . . . . . . . . . . . . . . . . . . . . . . 23
3.3 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.3.1 Scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.3.2 Light Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . 28
II Methods 32
4 Sample Preparation and Data Acquisition 33
4.1 Colloidal Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1.1 Screening of charges . . . . . . . . . . . . . . . . . . . . . . . 34
4.1.2 Solvent Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.2 Bidisperse Suspensions . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2.1 Density-matching . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.3 Sample Chambers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.3.1 Wedge-Shaped Cells . . . . . . . . . . . . . . . . . . . . . . . 39
4.3.2 Rectangular Capillaries . . . . . . . . . . . . . . . . . . . . . . 41
4.4 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.4.1 Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5 Computational Analysis 46
5.1 Particle Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.2 Tracking Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.2.1 Challis' Procedure for Coordinate Transformations . . . . . . 49
5.2.2 Application to Colloidal Clusters . . . . . . . . . . . . . . . . 50
5.3 Analysis of Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3.1 Mean Square Displacement . . . . . . . . . . . . . . . . . . . . 51
5.3.2 Quantifying Noise . . . . . . . . . . . . . . . . . . . . . . . . . 53
5.3.3 Identification of Cage-Breaking Events . . . . . . . . . . . . . 57
5.3.4 Significance of Cage-Breaking . . . . . . . . . . . . . . . . . . 63
III Experimental Findings 64
6 Confinement 65
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
6.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
6.3 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6.3.1 Stuck Particles . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.4.1 Wall-induced structure . . . . . . . . . . . . . . . . . . . . . . 71
6.4.2 Sample-averaged dynamics . . . . . . . . . . . . . . . . . . . 75
6.4.3 Confinement Length Scale . . . . . . . . . . . . . . . . . . . . 78
6.4.4 Defining cooperatively rearranging regions . . . . . . . . . . 81
6.4.5 Hypothesized Behavior of Cooperatively Rearranging Regions 87
6.4.6 Shapes and Sizes of Cooperatively Rearranging Regions . . . 88
6.4.7 Dynamics Within Cooperatively Rearranging Regions . . . . 92
6.4.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.5 Influence of boundary roughness . . . . . . . . . . . . . . . . . . . . . 97
6.5.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.5.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7 Rotational Diffusion 108
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
7.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.2.1 Rotational and translational diffusion . . . . . . . . . . . . . . 109
7.2.2 Colloidal glass transition . . . . . . . . . . . . . . . . . . . . . 111
7.3 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7.4 Data acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7.5 Tracking cluster trajectories . . . . . . . . . . . . . . . . . . . . . . . 113
7.6 Mean square displacement and diffusion . . . . . . . . . . . . . . . . 114
7.7 Diffusive decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
8 Summary of Work and Outlook 125
8.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
8.1.1 The Confinement Effect . . . . . . . . . . . . . . . . . . . . . 125
8.1.2 Rotational Diffusion . . . . . . . . . . . . . . . . . . . . . . . 126
8.2 Impact and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
8.2.1 The Confinement Effect . . . . . . . . . . . . . . . . . . . . . 126
8.2.2 Rotational Diffusion . . . . . . . . . . . . . . . . . . . . . . . 127

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School	Laney Graduate School
Department	Physics
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Physics, Condensed Matter
Parola chiave	rotation colloid confinement decoupling glass transition diffusion confocal microscopy
Committee Chair / Thesis Advisor	Weeks, Eric, Emory University
Committee Members	Fernandez-Nieves, Alberto, GA Tech Victor, Breedveld, GA Tech Roth, Connie, Emory University Boettcher, Stefan, Emory University

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