The work presented in this doctoral thesis employs colloidal suspensions to explore key open problems in condensed matter physics. Colloidal suspensions, along with gels, polymers, emulsions and liquid crystals belong to a family of materials that are collectively labelled as soft matter. Compositionally, colloidal suspensions consist of particles whose size ranges from a few nanometers to a few microns, dispersed in a solvent. A hallmark feature of these systems is that they exhibit Brownian motion, which makes them suitable for investigating statistical mechanical phenomena. Over the last fifteen years or so, colloids have been used extensively as model systems to shed light on a wide array of such phenomena typically observed in atomic systems. The chief reason why colloids are good mimics of atomic systems is their large size and slow dynamics. Unlike atomic systems, the dynamics of colloids can be probed in real time with single-particle resolution, which allows one to establish the link between macroscopic behavior and the microscopic processes that give rise to it. Yet another important feature is that colloidal systems exhibit various phases of matter such as crystals, liquids and glasses, which makes them versatile model systems that can probe a broad class of condensed matter physics problems. The work described in this thesis takes advantage of these lucrative features of colloidal suspensions to gain deeper insights into the physics of glass formation as well as shear-induced anisotropic grain growth in polycrystalline materials. The thesis is organized into two preliminary chapters, four work chapters and a concluding chapter, as follows. Chapter 1 provides an introduction to colloidal suspensions and reviews the chief theo-retical concepts regarding glass formation and grain boundary dynamics that form an integral part of subsequent chapters. Chapter 2 describes the experimental methods used for performing the work presented in the thesis and consists of two parts. The first part describes the protocols followed for synthesizing the size-tunable poly (N-isoprolypacrylamide) (PNIPAm) particles used in our study of shear-induced grain growth. The second part describes the instrumentation and techniques, such as holographic optical tweezers, confocal microscopy, rheology and Bragg diffraction microscopy, used to perform the measurements described in the thesis. Chapter 3 deals with our work on the dynamical facilitation (DF) theory of glass forma-tion. Despite decades of research, it remains to be established whether the transformation of a liquid into a glass is fundamentally thermodynamic or dynamic in origin. While obser-vations of growing length scales are consistent with thermodynamic perspectives, the purely dynamic approach of the DF theory has thus far lacked experimental support. Further, for glass transitions induced by randomly freezing a subset of particles in the liquid phase, theory and simulations support the existence of an underlying thermodynamic phase transi-tion, whereas the DF theory remains unexplored. In Chapter 3, using video microscopy and holographic optical tweezers, we show that dynamical facilitation in a colloidal glass-forming liquid grows with density as well as the fraction of pinned particles. In addition, we observe that heterogeneous dynamics in the form of string-like cooperative motion, which is consid-ered to be consistent with thermodynamic theories, can also emerge naturally within the framework of facilitation. These findings suggest that a deeper understanding of the glass transition necessitates an amalgamation of existing theoretical approaches. In Chapter 4, we further explore the question of whether glass formation is an intrinsi-cally thermodynamic or dynamic phenomenon. A major obstacle in answering this question lies in determining whether relaxation close to the glass transition is dominated by activated hopping, as espoused by various thermodynamic theories, or by the correlated motion of localized excitations, as proposed in the Dynamical Facilitation (DF) approach. In Chapter 4, we surmount this central challenge by developing a scheme based on real space micro-scopic analysis of particle dynamics and applying it to ascertain the relative importance of hopping and facilitation in a colloidal glass-former. By analysing the spatial organization of excitations within cooperatively rearranging regions (CRRs) and examining their parti-tioning into shell-like and core-like regions, we establish the existence of a crossover from a facilitation-dominated regime at low area fractions to a hopping-dominated one close to the glass transition. Remarkably, this crossover coincides with the change in morphology of CRRs predicted by the Random First-Order Transition theory (RFOT), a prominent ther-modynamic framework. Further, we analyse the variation of the concentration of excitations with distance from an amorphous wall and find that the evolution of these concentration profiles with area fraction is consistent with the presence of a crossover in the relaxation mechanism. By identifying regimes dominated by distinct dynamical processes, our study offers microscopic insights into the nature of structural relaxation close to the glass transi-tion. In Chapter 5, we extend our investigation of the glass transition to systems composed of anisotropic particles. The primary motivation for this is to bridge a long-standing di-vide between theories and simulations on one hand, and experiments on molecular liquids on the other. In particular, theories and simulations predominantly focus on simple glass-formers composed of spherical particles interacting via isotropic interactions. Indeed, even the prominent theory of Dynamical Facilitation has not even been formulated to account for anisotropic shapes or interactions. On the other hand, an overwhelming majority of liquids possess considerable anisotropy, both in particle shape as well as interactions. In Chapter 5, we mitigate this situation by developing the DF theory further and applying it to systems with orientational degrees of freedom as well as anisotropic attractive interactions. By analyzing data from experiments on colloidal ellipsoids, we show that facilitation plays a pivotal role in translational as well as orientational relaxation. Further, we demonstrate that the introduction of attractive interactions leads to spatial decoupling of translational and rotational facilitation, which subsequently results in the decoupling of dynamical het-erogeneities. Most strikingly, the DF theory can predict the existence of reentrant glass transitions based on the statistics of localized dynamical events, called excitations, whose duration is substantially smaller than the structural relaxation time. Our findings pave the way for systematically testing the DF approach in complex glass-formers and also establish the significance of facilitation in governing structural relaxation in supercooled liquids. In Chapter 6, we turn our attention away from the glass transition and address the problem of grain growth in sheared polycrystalline materials. The fabrication of functional materials via grain growth engineering implicitly relies on altering the mobilities of grain boundaries (GBs) by applying external fields. While computer simulations have alluded to kinetic roughening as a potential mechanism for modifying GB mobilities, its implications for grain growth have remained largely unexplored owing to difficulties in bridging the disparate length and time scales involved. In Chapter 6, by imaging GB particle dynamics as well as grain network evolution under shear, we present direct evidence for kinetic roughening of GBs and unravel its connection to grain growth in driven colloidal polycrystals. The capillary fluctuation method allows us to quantitatively extract shear-dependent effective mobilities. Remarkably, our experiments reveal that for sufficiently large strains, GBs with normals parallel to shear undergo preferential kinetic roughening resulting in anisotropic enhancement of effective mobilities and hence directional grain growth. Single-particle level analysis shows that the anisotropy in mobility emerges from strain-induced directional enhancement of activated particle hops normal to the GB plane. Finally, in Chapter 7, we present our conclusions and discuss possible future directions.

Gold nanoparticles have received significant interest recently due to their utility in biomedical imaging and therapy. Nanoparticles which exhibit intense extinction in the near infrared (NIR) region, where blood and tissue absorb light weakly, are of great interest as contrast agents for biomedical imaging applications. While strong NIR extinction often requires sizes greater than ~20-30 nm, effective clearance from the body to avoid toxic accumulation necessitates sizes below ~6 nm. Moreover, effective clearance depends upon lack of adsorption of serum proteins in the bloodstream onto the particles. Herein, this conflict is addressed by assembling sub-5 nm gold nanoparticles into clusters with controlled size and morphology, in order to provide intense NIR extinction. Furthermore, the surfaces of the primary gold nanoparticles are designed such that the particles avoid the adsorption of any serum proteins. Binary ligand monolayers of anionic citrate and appropriate amounts of either cationic lysine or zwitterionic cysteine are synthesized to completely prevent serum protein adsorption from undiluted fetal bovine serum. A mechanism is proposed whereby the zwitterionic tips which are present on both the lysine and cysteine ligands limit the interactions between serum proteins and the "buried" charges on the nanoparticle surfaces. These primary nanoparticles are subsequently assembled into biodegradable nanoclusters via "quenched assembly", wherein nanoclusters are assembled and subsequently quenched by the adsorption of a biodegradable polymer on the cluster surface. The sizes of completely reversible "quenched equilibrium" nanoclusters formed from gold nanoparticles coated with a mixture of lysine and citrate are tuned from 20 nm to 40 nm, and nanocluster size is semi-quantitatively predicted by a free-energy model. Additional control over nanocluster size and extinction is demonstrated by adding NaCl, which is shown to decrease the polymer adsorption on the clusters and thus decrease polymer bridging interactions. This nanocluster formation platform is extended to nanospheres capped with citrate and the thiolated, zwitterionic cysteine ligand. A general paradigm is presented whereby the sizes and optical properties of biodegradable gold nanoclusters formed from nanospheres which do not adsorb any serum proteins are tuned via control over van der Waals, electrostatic, depletion, and polymer bridging colloidal interactions. / text

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Early intervention in cancer and other illnesses is highly desired. The evolution of the proteomics field has benefited the possibility of getting to this goal by allowing researchers to look at different biomarkers. However, the high complexity of biological samples and the low levels at which biomarkers are found in these fluids make the analyses even more complicated.Protein microarrays have arisen as alternatives to traditional methods to look at multiple protein levels simultaneously with the benefit of high specificity, low limits of detection and the requirement of small samples. In this work, a significant improvement in net signals obtained with fluorescent detection (using three-dimensional scaffolds based on silica colloidal crystals -SCC-) is presented in contrast to commercially available flat substrates.A novel approach to extract trace proteins in solution and more complex matrices by using sub-micrometer silica particles as support for antibodies in affinity capture experiments is presented. Bovine Serum Albumin, Ephrin-receptor A2, Alpha-fetoprotein, and Prostate Specific Antigen have been used as model proteins. Recoveries of 90% or more are obtained with this method and reusability of the particles was achieved. MALDI-MS detection was successfully performed with the protein extracts which opens up the opportunity of further analysis such as determining post-translational modifications which is relevant when dealing with biomarker candidates.Last we present the use of our substrates as alternatives to conventional targets in mass spectrometry (MS). Traditional Matrix-assisted Laser Desorption Ionization MS (MALDI-MS) of proteins presents the problem of adduct formation with clusters from the matrix used in the process. Those adducts can affect the accurate determination of the molecular weight for a given protein and when could potentially mask slight differences in molecular weight of very similar proteins in mixtures. We present the alternative of using SCC on silicon wafers as a target for MALDI-MS samples. Our peak widths are extremely narrow and approach the one of the isotopic envelops. At the same time, porosity of our material seems to prevent the formation of adducts, which enables the differentiation of proteins with small molecular weight differences like mutants or same proteins from different sources.

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The present research is focused on investigating particle interactions between valuable and gangue materials, and the effect of these interactions on selectivity in flotation. This is a very important issue to operations at several mines across the world (e.g., at Century Mine operated by Zinifex Ltd in Australia). Particle interactions between valuable and gangue minerals with subsequent aggregation have significant impact on flotation performance. Valuable minerals may be depressed if heavily covered with hydrophilic gangue minerals and/or gangue minerals may misreport to the concentrate.

The behavior of dense colloidal fluids near surfaces can now be probed in great detail with experimental techniques like video and confocal microscopy. In fact we are approaching a point where quantitative comparisons of experiments with particle-level theory, such as classical density functional theory (DFT), are appropriate. In a forward sense, we may use a known surface potential to predict a particle density distribution function from DFT; in an inverse sense, we may use an experimentally measured particle density distribution function to predict the underlying surface potential from DFT. In this dissertation, we tested the ability of the closure-based DFT to perform forward and inverse calculations on potential models commonly employed for colloidal particles and surface under different surface topographies. To reduce sources of uncertainty in this initial study, Monte Carlo simulation results played the role of experimental data. The accuracy of the predictions depended on the bulk particle density, potential well depth and the choice of DFT closure relationships. For a reasonable range of choices of the density, temperature, potential parameters, and surface features, the inversion procedure yielded particle-surface potentials to an accuracy on the order of 0.1 kBT. Our results demonstrated that DFT is a valuable numerical tool for microcopy experiments to image three-dimensional surface energetic landscape accurately and rapidly. B

Electric field guided assembly of colloids is a new area of research in colloidal science where sub-micrometer particles, or colloids, are assembled using patterned electrodes. The design of these devices is often limited by an inability to characterize accurately forces and fluxes with linearized electrokinetic theory. The research presented in this dissertation describes an application of the finite element method to the nonlinear electrokinetic equations. The finite element model thus developed is then used to describe the nonlinear electrophoretic mobility of a dilute colloidal dispersion, investigate hydrodynamic and electric particle-particle interactions, and characterize particle-surface interactions. The effect of Stern layer conduction on the electrophoretic mobility and dielectric response is included using the generalized dynamic Stern layer model. The electrokinetic force is calculated using the Maxwell stress tensor method rather than the effective dipole method as it is more consistent with nonlinear electrokinetic theory. Significant results of this dissertation demonstrate the effect of nonlinear electrokinetic phenomena and extend the present electrokinetic theory. The calculation of nonlinear electrophoretic mobility of a dilute colloidal dispersion, which is valid for arbitrary particle surface charge or zeta potential, applied (AC) electric field strength, and applied AC electric field frequency. Also, the adsorption isotherm used by the generalized dynamic Stern layer theory is extended to include non-equilibrium reaction kinetics. This results in a model for Stern layer conduction which is valid for frequencies above 1 MHz. The utilization of the Maxwell stress tensor method results in a finite element model which is valid for arbitrary electric field strength and includes the effects of traveling-wave dielectrophoresis a nonlinear electrokinetic phenomena resulting from non-uniform electric field phase.

<p>Pattern formation is a mysterious phenomenon occurring at all scales in nature. The beauty of the resulting structures and myriad of resulting properties occurring in naturally forming patterns have attracted great interest from scientists and engineers. One of the most convenient experimental models for studying pattern formation are colloidal particle suspensions, which can be used both to explore condensed matter phenomena and as a powerful fabrication technique for forming advanced materials. In my thesis, I have focused on the study of colloidal patterns, which can be conveniently tracked in an optical microscope yet can also be thermally equilibrated on experimentally relevant time scales, allowing for ground states and transitions between them to be studied with optical tracking algorithms. </p><p>In particular, I have focused on systems that spontaneously organize due to particle-surface and particle-particle interactions, paying close attention to systems that can be dynamically adjusted with an externally applied magnetic or acoustic field. In the early stages of my doctoral studies, I developed a magnetic field manipulation technique to quantify the adhesion force between particles and surfaces. This manipulation technique is based on the magnetic dipolar interactions between colloidal particles and their "image dipoles" that appear within planar substrate. Since the particles interact with their own images, this system enables massively parallel surface force measurements (>100 measurements) in a single experiment, and allows statistical properties of particle-surface adhesion energies to be extracted as a function of loading rate. With this approach, I was able to probe sub-picoNewton surface interactions between colloidal particles and several substrates at the lowest force loading rates ever achieved. </p><p>In the later stages of my doctoral studies, I focused on studying patterns formed from particle-particle interaction, which serve as an experimental model of phase transitions in condensed matter systems that can be tracked with single particle resolution. Compared with other research on colloidal crystal formation, my research has focused on multi-component colloidal systems of magnetic and non-magnetic colloids immersed in a ferrofluid. Initially, I studied the types of patterns that form as a function of the concentrations of the different particles and ferrofluid, and I discovered a wide variety of chains, rings and crystals forming in bi-component and tri-component systems. Based on these results, I narrowed my focus to one specific crystal structure (checkerboard lattice) as a model of phase transformations in alloy. Liquid/solid phase transitions were studied by slowly adjusting the magnetic field strength, which serves to control particle-particle interactions in a manner similar to controlling the physical temperature of the fluid. These studies were used to determine the optimal conditions for forming large single crystal structures, and paved the way for my later work on solid/solid phase transitions when the angle of the external field was shifted away from the normal direction. The magnetostriction coefficient of these crystals was measured in low tilt angle of the applied field. At high tilt angles, I observed a variety of martensitic transformations, which followed different pathways depending on the crystal direction relative to the in-plane field. </p><p>In the last part of my doctoral studies, I investigated colloidal patterns formed in a superimposed acoustic and magnetic field. In this approach, the magnetic field mimics "temperature", while the acoustic field mimics "pressure". The ability to simultaneously tune both temperature and pressure allows for more efficient exploration of phase space. With this technique I demonstrated a large class of particle structures ranging from discrete molecule-like clusters to well ordered crystal phases. Additionally, I demonstrated a crosslinking strategy based on photoacids, which stabilized the structures after the external field was removed. This approach has potential applications in the fabrication of advanced materials. </p><p> My thesis is arranged as follows. In Chapter 1, I present a brief background of general pattern formation and why I chose to investigate patterns formed in colloidal systems. I also provide a brief review of field-assisted manipulation techniques in order to motivate why I selected magnetic and acoustic field to study colloidal patterns. In chapter 2, I present the theoretical background of magnetic manipulation, which is the main technique used in my research. In this chapter, I will introduce the basic knowledge on magnetic materials and theories behind magnetic manipulation. The underlining thermodynamic mechanisms and theoretical/computational approaches in colloidal pattern formation are also briefly reviewed. In Chapter 3, I focus on using these concepts to study adhesion forces between particle and surfaces. In Chapter 4, I focus on exploring the ground states of colloidal patterns formed from the anti-ferromagnetic interactions of mixtures of particles, as a function of the particle volume fractions. In Chapter 5, I discuss my research on phase transformations of the well-ordered checkerboard phase formed from the equimolar mixture of magnetic and non-magnetic beads in ferrofluid, and I focus mainly on phase transformations in a slowly varying magnetic field. In Chapter 6, I discuss my work on the superimposed magnetic and acoustic field to study patterns formed from monocomponent colloidal suspensions under vertical confinement. Finally, I conclude my thesis in Chapter 7 and discuss future directions and open questions that can be explored in magnetic field directed self-organization in colloidal systems.</p> / Dissertation