Title:

Particle Motion in Colloids: Microviscosity,

Speaker:

Ms. Roseanna Zia

Affiliation:

California Institute of Technology, Pasadena, California

When:

Thursday, March 3, 2011 at 11:00:00 AM   Add to Calendar

Where:

Love Building, Room 311

Host:

Dr. Minami Yoda
minami.yoda@me.gatech.edu
404-894-6838

Abstract

Colloidal dispersions play an important role in nearly every aspect of life { from blood to biofuels to nano-therapeutics. In the study of complex uids, a connection is sought between macroscopic material properties and the micromechanics of the suspended par- ticles. Such properties include viscosity, dusivity, and the osmotic pressure. But complex uids are much more than particle dispersions: they encompass such diverse systems as biolms, hydrogels, and the interior of the cell. Many such systems are them- selves only microns in size; recent years have thus seen a dramatic growth in demand for exploring microscale systems whose dynamic response properties must be measured at a much smaller length scale than can be probed with conventional macroscopic techniques. Microrheology is one approach to such microscale interrogation; here, one studies the motion of a single Brownian `probe' particle as it travels through a complex medium. At equilibrium, the probe undergoes thermal displacements which can be connected to the material properties of the suspending medium by the well-known Stokes-Einstein relation. This so-called `passive' microrheology reveals near-equilibrium properties, but by its very nature is limited in scope to the linear response of the material. Alterna- tively, in nonlinear microrheology the probe is actively forced through the medium, and dynamic response behavior studied. As the probe is driven through the suspension, it deforms the spatial conguration of background particles; in turn, the shape of the distorted microstructure in uences the mean and uctuating motion of the probe. The reduction in mean probe speed due to hindrance of the microstructure gives the eec- tive material viscosity. The velocity of the probe also uctuates due to collisions with the suspended particles, causing the probe to undergo a random walk process, which gives rise to anisotropic force-induced diusion, or `microdiusivity'. Strong forcing produces a tremendous enhancement to probe diusivity. Diusion has previously been related to stress in equilibrium systems; exploring this idea further, we discovered a new relation between particle ux and stress gradients in non-equilibrium suspensions. A simple analytical expression was derived, in which the stress tensor is given in terms of the probe viscosity and diusivity { its mean and mean-square motion. This theo- retical prediction was compared to Brownian dynamics simulation. Agreement between the two was excellent, indicating that the viscosity, diusivity, and full stress tensor may be determined by simply monitoring the mean and mean-square displacement of a single particle. Results are compared to the viscosity, normal stress dierences, and the osmotic pressure predicted by traditional shear (macro)rheology theory; the two are in excellent agreement. These results open up an important class of materials for mechanical exploration, and have important applications in nano-scale technology.

Biography

Roseanna N. Zia is a doctoral candidate and NDSEG Fellow in the Division of Engineer- ing and Applied Science at the California Institute of Technology. She will receive her PhD in mechanical engineering, with a minor in chemical engineering, in May 2011 under the supervision of Professor John Brady, where her thesis work has focused on the dynamical behavior of active microscale particles in complex media, and more generally in the area of colloid physics and suspension dynamics. She received her BS in mechanical engineering at the University of Missouri, and subsequently worked as a mechanical engineer in the auto- motive industry in Detroit, Michigan at General Motors Corporation. During this time she also received a Master of Engineering degree at the University of Michigan in Ann Arbor under the supervision of Professor Liwei Lin. Her ongoing and future research interests are in complex media and low-Reynolds number uid mechanics, particularly in problems at the interface of continuum mechanics and statistical mechanics { e.g. soft matter such as colloidal dispersions, biophysical systems, and gels. Her research approach combines a theoretical framework based in microhydrodynamics, colloid physics and non-equilibrium statistical mechanics; and computational techniques such as Brownian dynamics and Stoke- sian dynamics.

Notes

Reception will follow.