Dresden 2014 – wissenschaftliches Programm

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BP: Fachverband Biologische Physik

BP 18: Complex Fluids and Soft Matter (joint DY/CPP/BP)

BP 18.1: Vortrag

Dienstag, 1. April 2014, 09:30–09:45, ZEU 118

Microrheology of shear thinning solutions — •Juan Ruben Gomez-Solano^{1, 2} and Clemens Bechinger^{1, 2} — ¹Universitaet Stuttgart, 2. Physikalisches Institut, Pfaffenwaldring 57, 70569 Stuttgart, Germany — ²Max-Planck-Institute for Intelligent Systems, 70569 Stuttgart, Germany

Colloidal probes embedded in complex fluids have been extensively employed to investigate their rheological response to small stress. However, this approach is not evident for fluids subjected to large stresses, where a variety of non-Newtonian behaviors can occur. One example of such systems are semi-dilute micellar solutions, which consist of surfactant molecules forming worm-like micelles entangled in aqueous solution. In this work, we study the motion of a colloidal probe dragged by an optical trap through a semi-dilute micellar solution of cetylpyridinium chloride. The motion of the probe creates a shear strain, which depends linearly on its mean velocity v. We measure the effective viscous drag on the probe and the fluctuations of its position as a function of v. We find that at small v, the system can be characterized by a constant viscosity, whereas the position fluctuations are statistically the same as in thermal equilibrium. However, above a certain value, the viscosity decreases as a function of v. The fluctuations of the particle position are also affected in the shear-thinning regime, and their power spectral density increases with increasing v. We find that the transition between both regimes typically occurs when the shear rate exceeds the inverse relaxation time of the entangled micelles.