Recoil experiments determine the eigenmodes of viscoelastic fluids

Abstract

We experimentally investigate the recoil dynamics of a colloidal probe particle after shearing it with constant velocity through a viscoelastic fluid. The recoil displays two distinct timescales which are in excellent agreement with a microscopic model built on a particle being linked to two bath particles by harmonic springs. This model yields analytical expressions which reproduce all experimental protocols, including additional waiting periods before particle release. Notably, two sets of timescales appear, corresponding to reciprocal and nonreciprocal eigenmodes of the model.

How are mobility and friction related in viscoelastic fluids?

“…For further details regarding the experimental recoil setup, we refer to the recent work of Ginot et. al 11 and its Supplementary Material.…”

“…These models have complementary benefits. We start with a linear two-bath particle model which has been previously proposed for a quantitative understanding of recoils 11 . It is very generic and can be analyzed in complete analytical detail 37 .…”

“…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.…”

“…Recent microrheology experiments of colloidal probes in worm-like micellar solutions indicated that recoil spectra could provide crucial insights 10,11 . It is known that such micellar solutions in the semi-dilute regime are well described by a Maxwell model with a single timescale [25][26][27][28] ; at higher concentrations more complex relaxation behavior can occur 29,30 .…”

“…The Maxwell model captures the memory on macroscopic scales such as macrorheological measurements. Moreover, previous works could systematically fit the equilibrium Brownian motion, for example the mean squared displacement, of immersed colloids to Maxwell's model with a single relaxation time 10,11,[31][32][33][34] . Yet, 'recoil' measurements, which test the back-motion of the colloidal probe when released after it was moved with optical tweezers, recorded a very different temporal evolution.…”