W. G. Pritchard Lab Seminar: 3:30-4:30 PM, 103 Osmond Laboratory

	          **Monday September 30 2002**
	
Rotational Brownian motion of optically trapped microdisks

Thomas G. Mason

Corporate Strategic Research
ExxonMobil Research and Engineering Co.

Abstract:

We observe that thin micron-sized wax disks suspended in water can be
stably trapped in three dimensions, with light torques causing the disk's
axis of symmetry to orient perpendicular to the laser beam's axis. These
disks, created through an emulsification and solidification process, have
diameter to thickness aspect ratios of about five and resemble short
right circular cylinders. Because the disks are birefringent, we are also
able to control the in-plane orientation of the trapped disk's symmetry
axis by controlling the laser polarization. The interaction of the light
with the trapped microdisk creates a symmetric light streak that we
observe and track using a high speed digital camera. Linear polarization
results in restoring light torques that give rise to a harmonic
rotational trap, whereas circular polarization creates a constant torque
that drives a continuous rotation, producing a "colloidal lighthouse".
Using this new method of light streak tracking, we have made the first
measurement of a rotational harmonically bound Brownian particle over a
wide range of time scales. We also show that light streak tracking can be
used to perform rotational diffusion microrheology measurements of a
polymer solution.

This work was performed in collaboration with Z. Cheng and P.M. Chaikin.