Viscoelastic Fluids

Viscoelastic fluids are of great importance in biological systems and in medical and industrial applications. Their flow properties have been extensively studied by bulk rheology [1] and more recently by microrheology [2] using embedded colloidal probes. For instance,in passive microrheology the fluctuation-dissipation theorem is used to determine such properties by measuring the thermal fluctuations of the particle position [3]. This is only valid provided that the fluid and the particle are in thermal equilibrium. However, this assumption is not trivially satisfied, if the microstructure of the fluid is driven far from equilibrium, e.g. by inducing a local deformation by means of the particle [4,5]. Unlike to Newtonian fluids, viscoelastic solutions reveal long stress relaxation times similar to that of the colloidal motion[1]. Even a humble shearing excites a non-equilibrium state, since the bath cannot relax rapidly like in Newtonian fluids. As recently studied, the bath leaves its non-linear fingerprints already in equilibrium measurements [6], e. g. for a freely diffusing probe particle. Further, experimental evidence has been found that that the coupling of a non-equilibrium environment to the fluctuations of a colloidal particle leads to a shear-rate dependent rotational diffusion coefficient [7]. In our research, we explore the fluctuations of a colloidal particle driven by optical tweezers through a viscoelastic medium. Especially the range of gentle driving velocities opens a variety of novel and astonishing phenomena, like a new oscillatory state appearing on a timescale, which is a multiple of the structural relaxation time of the fluid [8].