Optical Microrheology Using Rotating Laser-Trapped Particles

We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure.

Figure 1

(a) Optical microscope image of a typical vaterite crystal used for viscosity measurements. (b) Scanning electron microscope image of a vaterite crystal.

Figure 2

(a) Signal recorded by the linear polarization measurement apparatus during rotation of a vaterite crystal. The frequency of rotation is $(94.0\pm 0.7)\mathrm{Hz}$. (b) Signal traces from the circular polarization measurement apparatus during rotation of a vaterite crystal.

Figure 3

Variation in viscosity with trapping power for a vaterite crystal of diameter $2.41\mu \mathrm{m}$. The viscosity is independent of power for powers at the trap focus less than approximately 100 mW.

Figure 4

Free vaterite crystal located inside a hexane-filled lipid-walled vesicle of $16.7\mu \mathrm{m}$ in diameter.