The underlying theme to my research is to use the inherent Brownian motion of complex media to measure linear dynamical properties of the sample.  This field is often called microrheology which is in reference to the traditional field of rheology, literally translated as the study of flow. Thus, microrheology is the study of flow at micron and submicron length-scales.

     The observation of Brownian motion can be traced back a couple a hundred years ago when Robert Brown looked under his microscope and observed the irregular motion of particles contained within pollen vacuoles. Later it was recognized this motion is due to the constant molecular bombardment of the surrounding fluid against the embedded particle. Today we stand on the foundation of many great scientists such as Bachelier, Einstein, Langévin, and others to build a theory that not only explains how molecular motion manifests itself into observable properties such as viscosity, but one that also accounts for the unusual behavior associated with complex fluids. Specifically, complex fluids not only have the ability to dissipate energy via viscosity but also to store energy through the internal structure of the fluid, a property known as viscoelasticity.

     The experiments that we carry out are similar in character to that of Robert Brown. We embed particles (usually polystyrene spheres) into a sample and observe their motion. The motion of these particles provides a descriptor of the fluid which is characteristic of its properties on a macroscopic length scale. In the van Zanten laboratory we use two techniques to measure the Brownian motion of the probe particles. The first is a dynamic light scattering technique known as Diffusing Wave Spectroscopy (DWS) and the second is Video Particle Tracking (VPT). DWS offers the advantage of quantifying the molecular motion of individual probe particles at extremely small length and time scales (on the order of angstroms and microseconds, respectively). VPT allows for the direct observation of the probe particles, obtaining not only single particle information but also how two particles interact, so-called two body interactions. Both techniques have their advantages but are common in purpose: To measure the motion of probe particles embedded in a complex fluid.

     One family of complex fluids of interest are aqueous solutions of PEO-PPO-PEO triblock copolymers known as Pluronics^TM over a wide range of block lengths, composition, temperature and pressure.  The latter variation is most exciting since we have observed a rich dynamical landscape that is largely unexplored. We then compare the dynamical behavior with structural information obtained from neutron and static light scattering to hypothesize possible mechanisms. In the end, we hope to draw conclusions about how the microscopic structure of aqueous Pluronics^TM lead to the dynamical behavior we observe.