Levich Institute Seminar Announcement, 03/21/2006
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Professor Victor Breedveld
Georgia Institute of Technology
School of Chemical ∓ Biomolecular Engineering
"Studying Microstructural Dynamics of Complex Fluids with Particle Tracking Microrheology "
[This is a CCNY/Columbia NSF-IGERT Soft Materials seminar]
Over the last decade, particle tracking microrheology has matured as a new tool for complex fluids research. The main advantages of microrheology over traditional macroscopic rheometry are: the required sample size is extremely small (< 1 microliter), local viscoelastic properties can be probed with high spatial resolution (~1 10 micrometer), and the sample is not disturbed by moving rheometer parts. I will present two examples of recent work in my group that highlight how these characteristics can be exploited to acquire unique information about the dynamics of complex fluids.
First, we have developed a dialysis cell for microrheology for studying microstructural rearrangements and rheological responses due to changes in solvent composition. With macroscopic rheometry, it is virtually impossible to change the solvent composition in a sample during an experiment and monitor the resulting rheological response. By integrating microfluidics and microrheology, we have overcome this barrier: microfluidic devices have sufficiently small dimensions to achieve rapid and reversible changes in sample composition via diffusive mass transport. The dialysis cell enables well-defined experiments with solvent-sensitive materials. In this seminar, results will be presented for solutions of polyelectrolytes and self-assembled block copolypeptide hydrogels, both of which are highly sensitive to the ionic strength of the solvent.
In a second project, we have employed microrheology to monitor the progress of photopolymerization reactions with high spatial and temporal resolution. By integrating the experimental video microscopy set-up of particle tracking microrheology with an external illumination source, we have been able to study photopolymerization of UV curable acrylate resins and hydrogels. Microrheology enabled a detailed study of three-dimensional gelation profiles; other experimental parameters that were varied include photoinitiator concentration, inhibitor concentration, monomer composition, and light intensity. Significant changes in gelation time were observed across 100 micrometer thick samples. The experimental results were used to show the limitations of the energy threshold model, which is often used to empirically predict the outcome of photopolymerization reactions in practical applications, and provide a solid basis for validation and refinement of more involved models.
BRIEF ACADEMIC/EMPLOYMENT BACKGROUND:
Dr. Breedveld's research theme is "Structure and Rheology of Complex Fluids", investigating the structure and mechanical strength of materials that are neither simple Newtonian fluids nor elastic solids. Complex fluids encompass a large variety of materials: food products, polymer melts and solutions, coatings, personal care products, biological fluids and gels, etc. The mechanical properties (visco-elasticity, shear viscosity) are controlled by the microscopic molecular structure, which can be tuned by changing the interactions between molecules. The interplay between molecular structure and rheology is the focus of his research.