'Wiggler-Waggler': Towards optical measurements of the complex shear modulus

Microrheology, the study of flow at the microscopic scale, has benefited immensely from a variety of optical micromanipulation techniques developed over the past two decades. However, very few present procedures allow the rapid measurement of the viscoelastic properties of fluid samples with volumes on the order of tens of picolitres over a wide frequency range. We detail preliminary construction and analysis of an active rotational microrheological method which promises to achieve this. Rotational microrheology was performed by optically trapping a birefringent probe particle in a linearly polarised dual-beam trap and rapidly rotating the polarisation direction through a xed angle. This provides measurements of the low-frequency fluid response, whilst passive monitoring of thermal motion is used to determine high-frequency components. Our method is less sensitive to boundary effects and probe particle asphericity than analogous translational microrheological techniques, so will be ideal for microfluidic applications and analysis of fluids which are generally available in volumes which preclude the use of existing experimental techniques