Alice P. Gast
Current Research

The physical and chemical processes governing the behavior of macromolecular liquids result from an intriguing balance between molecular forces. In our research we aim to understand these processes and their influence on bulk properties through a combination of colloid science, polymer physics, and statistical mechanics.

Macromolecules at Interfaces
We are engaged in a number of studies of the adsorption and association properties of polymers. The association of block copolymers into micelles has been the focus of light, x-ray, and neutron scattering investigations in our group. In these projects we investigate the structure of long polymer chains in micelles, and probe the interactions between micelles at low and high concentrations. We have recently begun to systematically study the behavior of highly branched polymers known as dendrimer-like stars. We have found, through small angle x-ray scattering studies, some structures. We also investigate the behavior of these systems with small angle neutron scattering. We complement our experimental studies with models from statistical mechanics.

Proteins provide interesting albeit complex macromolecules whose interfacial properties are of great significance. We are investigating the unique crystallization properties of proteins tethered to lipid monolayers. Here fluorescence microscopy reveals ordering phenomena of great interest for both biological applications and fundamental physics. Point mutations allow us to alter the protein-protein interactions in a systematic and detailed way and thus to investigate the molecular basis for the ordering behavior.

Disorder-Order Transitions and Wetting in Colloidal Suspensions
The organization of simple spherical particles into cubic arrays is only partly understood. Hard particles will order into face-centered cubic crystals while softer interactions produce body-centered cubic crystals. This behavior appears in atomic systems such as metals, as well as in charged colloidal suspensions and polymeric micelles. In the colloidal and polymeric systems we can tune the interactions from hard to soft and we can add weak or strong interactions. We study such systems with microscopy, light, and x-ray scattering. We combine experimental studies with statistical mechanical analysis of the ordering process and of the solid-liquid interface.

After many years of fundamental study, the time is right for exploitation of the physical properties of colloidal phase transitions for new and interesting applications. In this project, we build on our past experience with the study of colloidal crystals and liquids to investigate the important interfacial properties of such suspensions. The interfacial behavior of suspensions is of central importance in many systems such as inks, paints, and ceramics, where the final product depends on wetting or interaction with a substrate. Our primary goal is to understand how the phase behavior of a suspension influences its wetting and surface tension properties. We then wish to manipulate these interfacial properties via an electric field to produce unique and controllable contact angles and wetting processes. We focus on understanding the interplay between colloidal interactions, structure, and suspension interfacial phenomena. We then manipulate the colloidal suspension wetting processes by applying electric fields.

Magnetorheological Fluids
Magnetorheological (MR) suspensions are composed of paramagnetic colloidal particles that acquire dipole moments when subjected to an external magnetic field. At sufficient field strengths and concentrations, the dipolar particles rapidly aggregate to form long chains. Subsequent lateral cross-linking of the dipolar chains is responsible for a rapid liquid-to-solid-like rheological transition. The unique, magnetically-activated rheological properties of MR suspensions make them ideal for electrical-mechanical transducers. Additionally, the ability to experimentally probe colloidal suspensions interacting through tunable anisotropic potentials is of fundamental interest.

We work toward a microscopic understanding of magnetorheological behavior. Much of the rheological behavior arises from the cross-linked structure caused by defects and Landau-Peierls thermal fluctuations of dipolar chains. We use the light scattering technique, diffusing-wave spectroscopy, to investigate the fluctuations of dipolar chains. We prepare monodisperse neutrally buoyant MR suspensions, allowing us to probe the dynamics of the dipolar chains using light scattering without complications due to gravitational forces and polydispersity. Optical gradient force trapping techniques, or laser tweezers, have become increasingly important tools for studying the microscopic structure, mechanics, and interactions in biological, colloidal, and macromolecular materials. We also study the micromechanical properties of dipolar chains and chain aggregates in a magnetorheological suspension using optical traps. Using dual-trap optical tweezers, we are able to directly measure the deformation of dipolar chains parallel and perpendicular to the applied magnetic field. We observe the field-dependence of chain mechanical properties, such as tensile strain, chain reorganization, defect-annealing, and rupture.

Magnetic Microfluidics
The miniaturization of chemical and biochemical processes for efficient drug screening, and combinatorial chemistry motivates interest in chemical processes on a microscopic scale. This shrinking size scale requires that many analytical and chemical processes must be cleverly scaled down to deal with nanoliter volumes and nanomoles of reactants. Among the chemical processes to be miniaturized, effective reactor mixing and separations are two of the most challenging. Most of the current state of the art relies on electrokinetic processes to transport analytes and products around the device. In this work another control parameter, a magnetic field, is added to the system. This provides a means to manipulate microscopic fluid flows in conjunction with, yet separately from, electrokinetics.

The self-assembly of magnetic colloidal particles into chains forms the heart of this project. When tethered in patterns to surfaces, they provide a magnetically actuated means to manipulate microscopic fluid flow. The fundamental issues we study include: the dynamics and flexibility of chains of magnetic colloidal particles in small channels and tethered to surfaces, the fluid mechanical behavior of arrays of magnetic particle chains, the chaotic advection created by magnetic beads in fluctuating fields, and the filtering and sieving capabilities of magnetic bead arrays. We address these problems through two interesting examples that provide a fertile testing ground for ideas and pose potentially useful applications. The separation process involving sieving or filtration of cells or particles, and the converse issue of effective mixing in a microreactor both provide a platform for fundamental studies and potentially useful technological development.

< back to bio