Optically driven nonlinear microrheology of gelatin.

We demonstrate the microscopic equivalent of a step-stress rheological measurement. An optical torque is applied to a birefringent wax microdisk embedded in gelatin, a highly entangled viscoelastic biopolymer, using circularly polarized laser tweezers. By increasing the laser power and measuring the angular displacement of the disk, we explore the microscopic rheological response of presheared gelatin from the linear to the nonlinear regime and observe yielding at the microscale. The shape of the microscopic torque-angle relationship matches the stress-strain relationship from a macroscopic measurement of presheared gelatin; from this, we extract an applied stress and deduce the effective strain induced by the rotating disk.