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Rotating spheres can be used to measure the microrheology of fluids. We would like to know at what point can the macrorheology be recovered from the microrheology of complex fluids?
Birefringent particles rotate in circularly polarised optical tweezers because they act like microscopic wave-plates, converting circularly polarised light to elliptically polarised light. Angular momentum is lost from the beam during this conversion and is transferred to the trapped particle causing it to spin.
The optical torque applied to the particle depends upon the beam power, the birefringence and size of the particle. By varying the beam power and the material birefringence we obtain a wide range of optical torques (0.05-20 pN.µm) for a 2 micron diameter particle. The optical torque applied to the particle is monitored by measuring the outgoing polarisation from the particle. From the same data we can also extract the rotation frequency of the particle and thus determine the viscous drag experienced by a rotating particle.
We are applying this technique to study a range of media including Newtonian solvents, polymer solutions and colloidal fluids.