Microrheology and the fluctuation theorem in dense colloids

¹ COSMIC and School of Physics & Astronomy, The University of Edinburgh, Kings Buildings Mayfield Road, Edinburgh EH9 3JZ, UK, EU
² Group of Complex Fluids Physics, Department of Applied Physics, University of Almeria 04120 Almería, Andalucía, Spain, EU

^a apuertas@ual.es

Received: 12 November 2010
Accepted: 14 February 2011

Abstract

We present experiments and computer simulations of a “tracer” (or “probe”) particle trapped with optical tweezers and dragged at constant speed through a bath of effectively hard colloids with approximately the same size as the probe. The results are analyzed taking the single-particle case and assuming effective parameters for the bath. The effective microscopic friction coefficient and effective temperature of the tracer are obtained. At high probe velocities, the experimental microviscosity compares well with the viscosity from bulk rheology, whereas a correction due to hydrodynamic interactions (absent in the simulations) is necessary to collapse the simulation data. Surprisingly, agreement is found without any need of hydrodynamic corrections at small probe velocities. The dynamics of the tracer inside the trap shows, both in the simulations and experiments, a fast relaxation due to solvent friction and a slow one caused by the collisions with other particles. The latter is less effective in dissipating the energy introduced by the moving trap and causes increasing fluctuations in the tracer motion, reflected as higher effective temperature.