Dynamics in Confined Brownian Systems 公开

Abstract

We use experiments and simulations to study two phenomena related to the glass transition: the effects of confinement and the phenomenon of cage breaking. Hard sphere colloidal suspensions are used as model glass formers and are visualized using confocal microscopy. Brownian dynamics simulations are used to study a minimalist system of cage breaking hard disks. We also present computational techniques that accurately track the rotational motion of rigid clusters of colloidal particles.

In experiments, we confine colloidal suspensions within emulsion droplets to probe how properties of the external surrounding medium affect internal dynamics. We find dynamics are sensitive to the viscosity of the confining medium and observe a gradient in dynamics as a function of distance from the confining interface. These results are analogous to previous observations in confined polymers and small molecule glass formers, where dynamical properties strongly depend on the interactions present at the confining interface.

Via simulations, we investigate cage breaking in dense hard disk systems using a model of three Brownian disks confined within a circular corral. The exact free energy landscape for this system can be calculated as a function of system size. We find the average time between cage breaking events follows an Arrhenius scaling when the energy barrier is large. We also discuss some of the consequences of using a one-dimensional representation to understand dynamics in a multi-dimensional space, such as diffusion acquiring spatial dependencies and discontinuities in spatial derivatives of free energy.

Finally, we describe a method of tracking the rotational motion of clusters of colloidal particles. Using rigid body transformations to determine the rotations of a cluster, we extend conventional proven particle tracking techniques in a simple way, thus facilitating the study of rotational dynamics in systems containing or composed of colloidal clusters. We test our method by measuring dynamical properties of simulated Brownian clusters under conditions relevant to microscopy experiments. We then use the technique to track and describe the motions of a real colloidal cluster imaged with confocal microscopy.

Table of Contents

Document Outline

• Distribution agreement
• Approval sheet
• Abstract cover page
• Abstract
• Cover page
• Acknowledgements
• Dedication
• Citations to previously published work
• Table of contents
• List of figures
• List of tables
• Introduction
  • The glass transition
  • Hard sphere colloids as model glass formers
  • Confinement effects
  • Summary of results and overview of dissertation
• Experimental Background
  • Fluorescent microspheres
  • Confocal microscopy
  • Particle tracking
  • Brownian motion and diffusion
• Boundary Effects in Confined Colloidal Suspensions
  • Introduction
  • Methods
    • Sample preparation
    • Sample chambers
    • Visualization of droplets and data collection
    • Removing bulk translational motion
    • Removing bulk droplet rotational motion
    • Measuring droplet size
    • Measuring volume fraction
  • Results
    • Slowing of dynamics in confinement
    • Structure within droplets
    • Particle mobility and distance from interface
  • Ansatz for radial mobility
  • Discussion and conclusions
• A Free Energy Landscape for Cage Breaking of Three Hard Disks
  • Introduction
  • Model system
  • Simulation details
  • Dynamics in 1D
  • Energy landscape
  • Discussion and conclusions
• Tracking Rotational Diffusion of Colloidal Clusters
  • Introduction
  • Calculating rotations
    • Challis' procedure for coordinate transformations
    • Application to colloidal clusters
  • Tests of the prescribed method
  • Analysis of rotational motion
  • Experimental application
  • Discussion and conclusions
• Summary and Outlook
  • Confinement
  • Free energy landscapes
  • Cluster tracking
• Appendices
  • A Free energy landscape for cage breaking of three hard disks
    • Calculating n(h): Case A
    • Case B
    • Case C
    • Case D
    • Behavior of FB(h) as ε → 0
    • Barrier heights in θ
  • Tracking rotational diffusion of colloidal clusters

About this Dissertation

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• Permission granted by the author to include this thesis or dissertation in this repository. All rights reserved by the author. Please contact the author for information regarding the reproduction and use of this thesis or dissertation.

School	Laney Graduate School
Department	Physics
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Physics, General Physics, Condensed Matter
关键词	structured fluids condensed matter fluid dynamics colloid confinement
Committee Chair / Thesis Advisor	Weeks, Eric, Emory University
Committee Members	Behrens, Sven, Georgia Institute of Technology Warncke, Kurt, Emory University Alberto, Fernandez-Nieves, Georgia Institute of Technology Roth, Connie, Emory University

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