Microrheology is the study of the deformation and flow of complex fluids at the microscale. As a graduate student, T.G. Mason invented an approach in 1993 for deducing local and macroscopic viscoelastic shear moduli of complex fluids by measuring the thermal fluctuations of colloidal particles introduced as probes. This approach, later published in Physical Review Letters in 1995 with his advisor D.A. Weitz (PRL 74 1250), sparked the modern field of microrheology. We continue research on glasses using thermal microrheology; our group also uses laser tweezers to manipulate dielectric probe microparticles such as polystyrene spheres and wax disks in order to investigate non-linear rheological properties.

Our research focuses on using advanced optical methods for measuring and controlling the motion of colloidal probe particles in complex fluids. We typically measure nanoscale displacements of microscale particles. These optical methods allow us to explore the high frequency rheological properties of complex fluids that are not accessible using mechanical rheometers that are limited by inertia. They also enable us to probe rheological properties in very small volumes. An exciting emerging frontier is the use of microrheology of naturally-occuring organelles to examine the mechanical properties at different places within living cells.

In prior work by a group member, Z. Cheng (now an assistant professor at Texas A&M), we measured the linear viscoelastic moduli of a polymer solution by probing the rotational diffusion of the axis of symmetry of a microdisk that has been oriented 'on edge' by linearly polarized laser tweezers. Applying circularly polarized light to a birefringent disk in water causes it to spin, as shown by the time sequence (frames are separated by 1/6 s).

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