Oscillating modes of driven colloids in overdamped systems

Abstract

Microscopic colloidal particles suspended in liquids are a prominent example of an overdamped system where viscous forces dominate over inertial effects. Frequently, colloids are used as sensitive probes, e.g., in biophysical applications from which molecular forces are inferred. The interpretation of such experiments rests on the assumption that, even when the particles are driven, the liquid remains in equilibrium. Here we experimentally demonstrate that this is not valid for particles in viscoelastic fluids. Even at small driving forces, we observe particle oscillations with several tens of seconds. They are attributed to non-equilibrium fluctuations of the fluid being excited by the particle’s motion. The oscillatory dynamics is in quantitative agreement with an overdamped Langevin equation with negative friction-memory term being equivalent to a stochastically driven underdamped oscillator. Such oscillatory modes are expected to widen the use of colloids as model systems but must also be considered in colloidal probe experiments.

Properties of a nonlinear bath: experiments, theory, and a stochastic Prandtl–Tomlinson model

“…) for even smaller k-values (currently not resolvable) as a comparison with the value obtained from a fit of the mean-square displacement seems to suggest [46].…”

“…It is thus important to develop basic understanding of such systems, and the studies performed here provide a first step towards systematic investigation and modeling. With the given findings at hand, future work will address non-equilibrium cases, and investigate how the equilibrium observations and modeling will determine nonlinear responses and far from equilibrium properties, such as those found in [45,46]. figure A1 and is negligibly small even for small values of s provided that t max is a multiple of the longest relaxation time of the system.…”

“…After overnight mixing, worm-like micelles form and deform dynamically in such solvents [48]. They build a highly dynamical entangled viscoelastic network which exhibits a comparatively large structural relaxation time of t =  2.5 0.2 s s determined by a recoil experiment [45] and macrorheological measurements, thereby giving rise to highly non-Newtonian properties [45,46]. The length of wormlike micelles is typically found between 100 and 1000 nm [49], and the characteristic mesh size is on the order of 30 nm [50].…”

“…In this paper, we experimentally and theoretically investigate the clean and rich system of a Brownian particle suspended in a complex, viscoelastic bath, a system which has in various forms been addressed before [5,[41][42][43][44], and which indeed shows unexplained non-equilibrium properties [45,46]. For example, the (transient) dynamics of a probe particle that has strongly perturbed its surrounding requires a theoretical description to account correctly for the stress release in the strained fluid, which is not avaliable.…”

Active particles in geometrically confined viscoelastic fluids

“…Understanding the behavior of these artificial agents in realistic complex environments is of great significance [16], since they can be employed in many biomedical applications such as drug delivery [17], cargo transport [18,19], bio-sensing [20] etc. Besides, an active particle (AP) in a viscoelastic fluid represents an example of a random walker in a nonequilibrium thermal bath, being of fundamental relevance for non-equilibrium statistical physics [21]. Despite holding such immense potential, theoretical studies involving the dynamics of self-propelled particles in complex fluids are rather scarce [22][23][24][25][26][27][28][29][30][31].…”