Measurements of viscoelasticity in the microscopic regime are of interest in polymer solutions as well as in microscopic structures such as cells. Viscoelasticity can be studied using a localized microrheometer based on optical tweezers. We rotate a birefringent micron-sized calcium carbonate sphere crystallized in a vaterite structure. By applying a time-dependent torque or using the time-dependent thermal torque, viscoelasticity can be measured. The torque can be measured purely optically, by measuring the polarization state of the trapping beam after passing through the particle. We control the torque by controlling the relative amplitudes of two orthogonally circularly polarized components of the trapping beam with two acousto-optic modulators. This allows a wide range of oscillation frequencies to be used. We demonstrate applications of the methods on several systems.

Date Published: 6 February 2009
PDF: 9 pages
Proc. SPIE 7227, Complex Light and Optical Forces III, 72270D (6 February 2009); doi: 10.1117/12.814357

Published in SPIE Proceedings Vol. 7227:
Complex Light and Optical Forces III
Enrique J. Galvez; David L. Andrews; Jesper Glückstad, Editor(s)