Current Research
Colloids in Electric Fields: Tunable colloidal phase transitions
Soft matter is characterized by the importance of fluctuations and the ease with which one can control structure with external fields. We use the sensitive interplay of soft matter to external fields to control the interaction forces in colloidal systems. Static structure in three dimensions is obtained via fluorescence laser scanning confocal microscopy, and analyzed quantitatively using image processing software. We also study real-time dynamics and phase transition kinetics in micron-scale colloidal systems.
Diffusion and Dynamics
Pulsed-field-gradient NMR spectroscopy yields ensemble-averaged local information about dynamics in diverse systems. Our goal is to create model systems (colloids, vesicles...) that can be simultaneously studied by diffusion NMR and confocal microscopy. The former gives fantastic ensemble averages and works in opaque systems or systems where the building blocks are nanoscale. Confocal microscopy gives insights into single-particle motions. (More...)
Patterned Materials: Pattern Engineering with and in Colloidal Crystals
We are interested in the properties of materials made from colloidal templates of varying degrees of crystallinity. Photonics Applications: Micron-scale materials with a spatial modulation of the refractive index exhibit a dramatic interaction with incident light when the periodicity of the modulation is commensurate with the wavelength of light. The viability of photonic crystals from colloidal self-assembly requires enormous advances in the control of impurities and defects. Magnetics Applications: Micron-scale and nanoscale structured magnetic materials also have great potential in memory storage applications. (More...)
Lipid Vesicles and Colloid--Liquid Crystal Mixtures
Our goal is to expand study into colloidal building blocks that are less symmetric than microspheres. To this end we are trying to devise model monodisperse lipid vesicle system that we can study with microscopy, rheology and NMR.
One way to achieve greater control over defects in colloidal phases is to use anisotropic and spatially-structured fields. We are studying structure formation in colloid-liquid crystal composites.
Past Research Contributions
Tunable Colloids
This body of work demonstrates unprecedented control over strength and anisotropy of inter-particle colloidal interactions. My recent invited review places my work in the context of other work in the field. We observed several novel colloidal phases and phase transitions which result from the competition of controllable colloidal interactions and entropy. This tunability of phase behaviour provides previously unavailable control variables for the study of structure formation kinetics. Controlling colloidal crystal formation via electric fields, we fabricated large mm-scale patterned photonic materials.
NMR studies of surfactant mesophases
Surfactant mesophases have complex memory effects arising from varying degrees of partial ordering making the data on solvent and surfactant diffusion in different phases (diffusion coefficients as a function of surfactant molar fraction) very noisy. We devised a method to look at well-aligned samples of non-ionic surfactant mesophases that allowed clean characterization of the phase diagram via diffusion coefficients, allowing us to make quantitative statements about the hexagonal and cubic phases. In another work, we tracked memory effects across phase transitions and thereby placed concrete limits on the structure of the lamellar phase.
The nematic-smectic phase transition in liquid crystals
This body of experimental and theoretical work on the nematic-smectic-A phase transition, including an invited review article (book chapter) demonstrated experimental studies that quantified mean-field order parameters and all prefactors for the first time via NMR spectroscopy and established the existence of a fluctuation-induced first-order phase transition via a new optical technique. The existence of a nematic-fluctuation-induced first order phase transition has been contested for decades, but was observed in our experiments. However, magnetic-field and concentration dependencies in our experiments suggested the need to include smectic fluctuations in the theory to account for discrepancies.
