Two step micro-rheological behavior in a viscoelastic fluid

Abstract

We perform micro-rheological experiments with a colloidal bead driven through a viscoelastic worm-like micellar fluid and observe two distinctive shear thinning regimes, each of them displaying a Newtonian-like plateau. The shear thinning behavior at larger velocities is in qualitative agreement with macroscopic rheological experiments. The second process, observed at Weissenberg numbers as small as a few percent, appears to have no analog in macro-rheological findings. A simple model introduced earlier captured the observed behavior and implied that the two shear thinning processes correspond to two different length scales in the fluid. This model also reproduces oscillations, which have been observed in this system previously. While the system under macro-shear seems to be near equilibrium for shear rates in the regime of the intermediate Newtonian-like plateau, the one under micro-shear is thus still far from it. The analysis suggests the existence of a length scale of a few micrometres, the nature of which remains elusive.

Recoil experiments determine the eigenmodes of viscoelastic fluids

“…Since typical relaxation times of the microstructure of viscoelastic materials can be on the order of several seconds, this leads to non-trivial behaviors for microscopic probes when sheared through such systems. Typically, probe particles are driven either by constant or oscillating external forces which then allows to obtain insights into the non-linear mechanical response of such systems [1][2][3][4][5][6]. However, so far hardly any theoretical description exists for driving conditions which may be relevant in non-steady flows within microfluidic devices or the intermittent motion of living organisms.…”

Memory-induced oscillations of a driven particle in a dissipative correlated medium

“…In this respect, a notable example of such complex media is provided by viscoelastic fluids [3]: their non-Newtonian behavior originates from the storage and dissipation of energy within their complex microstructure, which translate into a macroscopically long stress-relaxation time. Dragging a colloidal particle through such a fluid -as it is typically done in active microrheology experiments [4][5][6][7][8][9] -drives the medium out of equilibrium. In turn, this affects the statistics of the particle position [2].…”

How are mobility and friction related in viscoelastic fluids?

“…An example are wormlike micellar solutions, for which local flow curves, i.e. nonlinear force-velocity relations 5,6 , particle oscillations during shearing [6][7][8][9] and transient particle motion 10,11 were investigated. The existence of a number of different relaxation channels for the probe motion was recorded, which could be considered an intrinsic property of the system of viscoelastic fluid plus immersed colloidal particle.…”