Recoil experiments determine the eigenmodes of viscoelastic fluids

Abstract

We experimentally investigate the transient recoil dynamics of a colloidal probe particle in a viscoelastic fluid after the driving force acting on the probe is suddenly removed. The corresponding recoil displays two distinct timescales which are in excellent agreement with a microscopic model which considers the probe particle to be coupled to two bath particles via harmonic springs. Notably, this model exhibits two sets of eigenmodes which correspond to reciprocal and non-reciprocal force conditions and which can be experimentally confirmed in our experiments. We expect our findings to be relevant under conditions where particles are exposed to non-steady shear forces as this is encountered e.g. in microfluidic sorting devices or the intermittent motion of motile bacteria within their natural viscoelastic surrounding.

Memory-induced alignment of colloidal dumbbells

“…In the following, we will use δ x for the distance between the initial and final positions of the dumbbell's center of mass (COM), θ 0 for the angle just before release, and δ θ for the amplitude of the orientational alignment motion. As a first observation, the center of mass of the dumbbell exhibits a bi-exponential recoil motion, similar to what was previously reported for simple spherical particles 12 . Surprisingly, depending on their position relative to the COM, the particles exhibit different trajectory shapes during the recoil (see Fig.…”

“…3 (see Eq. ( 5)), the translational component (inset) increases linearly 12,13 , for v ≲ 0.2 µm s −1 . The temporal behavior of recoils are shown in SM.…”

“…As motion is prescribed, this description is useful in the presence of a (strong) optical trap. The connection between dynamics with trap on and trap off is non-trivial 12,13,21 , and we do not discuss it here for simplicity.…”

“…Therefore, pronounced memory effects (non-Markovian behavior) can be expected in such systems due to the slow relaxation of the fluid's mesoscopic microstructure and which has been confirmed using microrheological techniques [2][3][4][5][6][7][8][9] . In particular, so-called recoil experiments, where first a driving force is applied to a colloidal particle which is then suddenly removed, provide a useful method to explore how the motion of particles is modified when coupled to a slowly relaxing environment [10][11][12][13] . Previous studies have revealed rather general double-exponential recoil dynamics which can be quantitatively described by so-called bath particle models, where the response of the fluid is mimicked by harmonically coupled fictitious bath particles 12,14 .…”

“…In particular, so-called recoil experiments, where first a driving force is applied to a colloidal particle which is then suddenly removed, provide a useful method to explore how the motion of particles is modified when coupled to a slowly relaxing environment [10][11][12][13] . Previous studies have revealed rather general double-exponential recoil dynamics which can be quantitatively described by so-called bath particle models, where the response of the fluid is mimicked by harmonically coupled fictitious bath particles 12,14 . So far, such studies have been only conducted with spherical colloidal particles which produce only axially-symmetric strain fields in the fluid upon shear.…”