Optically Driven Nonlinear Microrheology Thomas G. Mason, Department of Chemistry and Biochemistry, Dept. of Physics and Astronomy, & California NanoSystems Institute University of California- Los Angeles, Los Angeles, CA 90095 Circularly-Polarized Laser Tweezers exert a torque on a birefringent wax microdisk in a viscoelastic complex fluid. For aqueous b-gelatin solutions at 1.5 wt%, we measure the rotation angle  as a function of time t after opening a shutter for a series of different laser powers P up to 1.5 W. The estimated stress () departs from linear behavior near  ≈ 60 deg, so the yield stress is y = 250 dyn/cm2. Yield Stress Linear B-Type Gelatin 1.5% wt Circularly Polarized Focused Laser Light Measurements by UCLA Graduate Student J.N. Wilking