Research
The research carried out by my group concentrates on understanding the relationships between surface physical chemistry and the material properties of colloidal suspensions. Particular attention is paid to methods of manipulating interparticle forces to alter particle and suspension properties.
Anisotropic Particle Suspensions
Traditionally colloids research laboratories have focused on studying spherically shaped particles. It is well known that spherical particles order into cubic arrangements, and while cubic structures represent an important class of materials, we are interested in creating more complex arrangements to see how such a change in microstructure influences macroscopic properties. Some applications of these new particle arrangments include impact modifiers, sensors, and photonic band gap materials which may be used in solar cells, telecommunications, microelectronics and a variety of other industries. Using novel synthesis techniques we control the anisotropy of particles, with the goal of studying how anisotropy affects microstructural arrangements.
Structure is characterized using dynamic light scattering, scanning electron microscopy, and ultra-small angle x-ray scattering. Furthermore, small angle neutron scattering is used to characterize dense suspensions of anisotropic particles under shear and these experiments are coupled with rheological data to relate microstructure to macroscopic properties. Currently we are interested in studying phase transitions in suspensions of particles as anisotropy is varied, as well as rheological differences between spherical and anisotropic particle suspensions. Likewise, we plan to exploit chemical anisotropy to create more classes of complex particle arrangements and perform similar studies on these structures.
Depletion Interactions
When non-adsorbing polymers are added to stable colloidal suspensions the particles experience an induced depletion attraction due to an unbalanced osmotic pressure arising from the exclusion of polymer molecules from the region between colloids. The strength of these attractions is controlled by the polymer concentration, while the range of attractions depends on the colloid to polymer size ratio, R/Rg. Here, R is the colloid radius and Rg is the polymer radius of gyration.
Depending on the strength and range of these attractions, suspensions undergo phase transitions resulting in a variety of microstructures such as gels, dilute and dense liquids or crystals. Control of these phase changes is important in achieving desired material properties suited for particular applications where suspension microstructure and mechanical properties are important. We investigate the influence of polymer induced depletion interactions on solution thermodynamics and phase behavior of colloidal suspensions. Our studies involve the entire range of colloid to polymer size ratio, from R/Rg >> 1 (colloid limit) to R/Rg ~ 1 (protein/nanoparticle limit). Our combined experimental and theoretical study is relevant to understanding fundamental aspects of colloid-polymer systems of interest in organic and inorganic materials science.
Filled Polymer Melts
We explore the effects of polymer molecular weight and particle volume fraction on particle interactions and particle suspension microstructure when nanoparticles are suspended in polymer melts. These studies were conducted as part of an effort to understand the role of particle/polymer segment interactions on the degree of aggregation in filled polymer systems. These systems are an important class of materials where colloidal particulate matter is dispersed in a polymer matrix. They have considerable commercial importance in the manufacture of rubber, nanocomposites, biomaterials, insulating and conductive polymers, and in coatings. Fillers are used in polymers to improve and alter mechanical, electrical, rheological, optical, and/or thermal properties of the composite.
The phase behavior of filled polymers will be governed by enthalpic and entropic contributions. A variety of phases are expected as particle volume fraction, polymer molecular weight, and segment-surface interactions are varied: homogeneous fluid, phase separation, or nonequilibrium gel. The competition between enthalpic and entropic contributions will drive the organization of the particles in the polymer matrix. By developing experiments that systematically vary key parameters, segment/particle size ratio, particle volume fraction, and the strength and range of attraction between polymer segments and the particle surface, we are attempting to elicit information about filled polymer phase behavior.
Colloidal Nucleation
Controlling the kinetics of crystallization is important in industries such as pharmaceuticals, foods and cosmetics, and in more recent applications such as the manufacture of photonic crystals and the determination of protein structures. Our work aims at identifying optimal conditions for colloidal crystallization. We have developed kinetic approaches for modelling crystal nucleation in hard sphere suspensions and in attractive systems that mimic globular protein solutions. A more rigorous population balance model is currently being developed to understand the formation of critical clusters and establish links between predicted nucleation rates and measurable induction times. The kinetics of crystal nucleation is experimentally characterized through nucleation induction time measurements performed using a micro-scintillation light scattering setup. Our theoretical and experimental investigations aim at determining the links between nucleation kinetics and strength of interactions.
Protein Crystallization : Art or Science?
Proteins are difficult to crystallize and the current methods rely on high throughput screening methods, in which hundreds to thousands of solution conditions are screened to locate and optimize the solution conditions where X-ray diffraction quality crystals are formed. We have developed a method that ensures the formation of a solid phase in each experiment and, as a result, each trial yields information on crystal solubility, and crystal nucleation/growth rates. This method relies on evaporating the solvent from a drop at a predetermined rate set by the dimensions of the crystallization platform. Once solution conditions resulting in good crystals are found, monitoring the crystal size during the experiment provides a way to extract physico-chemical parameters of importance to crystal growth. The population balance model which has been widely used to model and predict crystal size distributions in mixed suspensions, batch crystallizers on lab scale, and industrial crystallizers, is successfully adapted to model the nucleation and growth kinetics in the experiments performed using the evaporation based crystallization platform.
Current area of research includes experimental study and model development to increase understanding of phase behavior of solutions used for membrane protein crystallization. Light and X-ray scattering studies will be performed to unravel the phase behavior; in particular the kinetics involved in phase changes, of lipid.
Responsive Polymers
Stimuli-responsive nanoparticle systems have received increasing attention in various applications like bio-separations, drug delivery systems and as rheological property modifiers. These polymeric systems are known to exhibit unique, rapid and reversible property changes with external stimuli. This work involves understanding this response in the case of thermosensitive microgel suspensions. Each microgel particle is composed of chemically cross-linked polymer strands that undergo structural changes eventually leading to the unique temperature responsive behavior. Rheological, thermodynamic and microstructural changes in these microgel particles are investigated with a view to find underlying links between pair interaction potentials and bulk mechanical properties.
Structured Particle Suspensions
Fumed silica represents one particular class of colloids with a complex geometry where primary particles are fused into approximately spherical agglomerates. A question fundamental to the application of these materials revolves around the relative roles of structure and surface chemistry in controlling the mechanical properties of fumed silica suspensions. One of the key advances in this work is the investigation of structured particles that interact only with hydrodynamic and Brownian forces. Working with different grades of fumed silica, particle structure is characterized by X-ray and light scattering and suspension flow properties are characterized as a function of particle concentration. The study of the surface chemistry is undertaken by examining the macroscopic properties of native fumed silica particles in a series of solvents. By changing the solvent, the mechanics of the suspension range from that of a liquid to a gel. Not only does the initial state of the suspension change, but there are also long time thixotropic changes. Time based dynamic light scattering measurements capture these changes and can be used to describe the rate of aggregation of the particles along with the strength of the gelled suspension.