Studying microstructural dynamics of complex fluids
with particle tracking microrheology

Victor Breedveld

School of  Chemical & Biomolecular Engineering, Georgia Institute of Technology.

Abstract: 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 three examples of recent work in my group that highlight how these characteristics can be exploited to acquire unique information about the microstructure of complex fluids.  

First, we have studied protein unfolding, which has traditionally been studied with spectroscopic techniques (circular dichroism, NMR, fluorescence).  Although viscosity has been listed in textbooks for many years as a suitable technique, few –if any– quantitative rheological studies of unfolding have been reported, mainly due to technical difficulties.  With microrheology, we have been able to quantify the size of folded and unfolded proteins, as well as the Gibbs free energy of unfolding.

Secondly, we have developed new technology for studying the microstructural dynamics of solvent-responsive complex fluids.  With macroscopic rheometry it is virtually impossible to change the solvent composition in a sample and monitor its rheological response.  By integrating microfluidics and microrheology, we have been able to overcome this barrier: the microscopic lengthscales in microfluidic devices shorten the diffusive timescales in a dialysis set-up enough to achieve rapid and reversible changes in sample composition.  Our dialysis cell for microrheology is a unique tool for studying the dynamics of structural and rheological changes induced by solvent composition.

Thirdly, we have employed microrheology to monitor the progress of photopolymerization reactions with high spatial and temporal resolution. We have been able to show that the polymerization reation is highly localized and quantitatively deterine these apatial variations.