8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Tue 611:23-14:00P9.29Active microrheology to probe directionalviscoelasticity: applicability and limitationsManas Khan 1 and A. K. Sood 21 Fachbereich Physik, Universität Konstanz, Fachbereich Physik, LS Maret, UniversitätKonstanz, D-78457 78457, Konstanz, Germany2 Department of Physics, Indian Institute of Science, Bangalore, IndiaShear induced alignment causes shear thinning in many wormlike micellar systems. The asymmetricorientations of the microstructures bring in anisotropy in the viscoelastic properties of thesystem. An optical tweezers based active microrheology technique can be employed to probethis directional viscoelasticity under shear, through simultaneous measurement of flow propertiesalong two orthogonal directions - the direction of applied shear and perpendicular to it. Whilethe displacements of a trapped bead in response to active drag force display conventional shearthinning, its spontaneous position fluctuations along the perpendicular direction are analyzedthrough fluctuation dissipation theorem to calculate the rheological properties along that direction,which shows an orthogonal shear thickening [1]. However, the applicability of this techniquehas critical dependence on the relaxation time of the system. In systems with slower relaxation,at moderately high shear rates where the micella start getting aligned, the probe bead’s positionfluctuations along the direction perpendicular to the applied shear cannot be considered asspontaneous thermal fluctuations. In this scenario, this technique cannot probe the shear thickeningalong the orthogonal direction to the applied shear, instead shows a deceptive shear thinning.[1] Manas Khan and A. K. Sood, Europhys. Lett. 92 (2010) 48001.29