Using single-beam optical tweezers for the passive microrheology of complex fluids

Abstract

One important aspect of the complete physical characterization of novel viscoelastic materials is the assessment of their response on short timescales. Optical tweezers, equipped with a fast quadrant photodiode, aid in fulfilling this task by providing high-frequency viscoelastic information about the sample. In passive microrheology, this is normally achieved by extracting rheological information from the thermal motion of an optically trapped bead embedded in a test fluid. Here we present the calibration and use of optical tweezers to study the formation of thermally reversible DNA hydrogels. We complement our results with rheological data from dynamic light scattering, video microscopy and conventional bulk rheology. Merging experimental data from different techniques allows us to study the viscoelastic behavior of these DNA networks over a wide frequency-band and the scaling of the complex viscoelastic modulus at the two frequency extremes. By analyzing the high-frequency behavior of our transient network, we prove the semi-flexible polymer nature of DNA and provide an estimate of its persistence length.