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Departmental Colloquium


Rotational Dynamics of Optically Trapped Microdisks

Dr. Thomas G. Mason

Corporate Strategic Research
ExxonMobil Research and Engineering Co.
New Jersey

Time
 
Mon. January 13, 2003     11:30 AM     STIRLING 501

Abstract
 

Understanding how to manipulate anisotropically shaped microparticles in solution will lead to important applications in subjects ranging from biophysics to microengineering. We create an aqueous dispersion of micron-sized disks by emulsifying hot liquid wax into water, solidifying the wax by cooling, and then applying a shape-dependent depletion attraction to separate out the microdisks. Polarized light microscopy reveals that the microdisks are birefringent, and they exhibit translational and rotational Brownian motion. When we apply laser tweezers to a microdisk, the disk becomes optically trapped on edge. Because the disks are birefringent, we can apply optical torques to orient and spin them by controlling the polarization of the laser light. Linear polarization produces a harmonic rotational trap, whereas circular polarization drives a continuous rotation, yielding a "colloidal lighthouse". Laser light emanating from a trapped microdisk creates a streak that can be used as an optical lever to precisely measure the orientation of the disk's symmetry axis. By tracking the light streak using a high speed digital camera, we measure the bounded rotational diffusion of a microdisk in a rotational trap. Furthermore, by embedding a microdisk as a probe in a viscoelastic polymer solution and tracking its orientation, we deduce the frequency-dependent shear modulus of the solution using a rotational form of the generalized Stokes-Einstein relation. This opens up the new approach of rotational diffusion microrheology.

Dr. Thomas Mason is a candidate for the tenure-stream position for Condensed Matter Physics within the Department.

Refreshments will be available in the lounge after the talk.

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