Compared to Newtonian liquids, viscoleastic fluids are distinguished by rather long relaxation times which can be on the order of seconds. Accordingly, they can be easily brought out of equilibrium already by small shear flows, i.e. far below values where shear thinning or shear thickening occurs. As a consequence, a simple Markovian description is no longer valid. We have experimentally demonstrated that the coupling of a non-equilibrium environment to the fluctuations of a colloidal particle leads to a shear-rate dependent rotational diffusion coefficient. In our studies we have observed rotational diffusion coefficients up to 250 times larger than the corresponding Stokes value, both for active but also for sedimenting passive particles. At the same time, the translational diffusion coefficient is enlarged by about a factor of 10. Our findings demonstrate, that even in the linear response regime where the viscosity remains constant, a viscoelastic fluid can not be considered as a thermal reservoir but that its properties are strongly altered by the presence of (self) driven objects.
|Dynamics of self-propelled Janus particles in viscoelastic fluids|
|Juan Ruben Gomez-Solano, Alex Blokhuis, and Clemens Bechinger|
Phys. Rev. Lett. 116, 138301 (2016)