Kenneth Desmond
     5th Year Graduate Student
Rheology     Microrheology     Why Study Microrheology     Ellipsoidal Diffusion    

Up until now the most common micorheology technique is to study the fluctuations of spherical particles suspended in solution. In order to compute the bulk modulus using this technique the radius of the spherical particles must be known. We are extending this technique to ellipsoids to exploid the asymmetry which allows one to measure the bulk modulus without knowing any information about the size of the ellipsoids. The asymmetry allows one to easily measure not only translational fluctuations, but rotational fluctuations as well. By measuring both types of fluctuations the need to know about the size of the ellipsoids is no longer needed to compute the bulk modulus.

We study the diffusion of ellipsoids by first synthesizing the ellipsoids in the lab. Next we inject our ellipsoids into a fluid and use confocal microscopy to image their 3 dimensional motion. Using software we have written, we can track both the center of mass and orientation of the particles with time. To the right is a cartoon movie of the diffusion of dead bacteria. I rendered this cartoon movie using a program called POV-ray. Click here to see a movie for red/blue 3D glasses. Below are two 2D movies of our synthesized ellipsoids diffusing in solution.


Understanding the microrheology of the simplest asymmetric particle, the ellipsoid, will aide in understanding the diffusion of more complex asymmetric particles. Since microrheology requires injecting a sample with tiny tracer particles, this may slightly alter the rheology of the sample. However, many complex solutions may already have micron size particles suspended in them (which are most likely asymmetric). Once we understand the microrheology of asymmetric particles, there will be cases where injecting particles is no longer needed.

In addition to what's been mentioned, there is another reason to study the diffusion of ellipsoids. Currently the hydrodynamic interaction between two ellipsoids is poorly understood. By observing the diffusion of multiple ellipsoids suspended in solution we can characterize these interactions by measuring the correlated motion of pairs of ellipsoids.