Oscillating modes of driven colloids in overdamped systems

Johannes Berner,Matthias Krüger,Clemens Bechinger,Boris Müller,Juan Ruben Gomez-Solano

doi:10.1038/s41467-018-03345-2

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.

Highlights

Microscopic colloidal particles suspended in liquids are a prominent example of an overdamped system where viscous forces dominate over inertial effects
Our experiments are performed in an equimolar solution of surfactant, cetylpyridinium chloride monohydrate (CPyCl) and sodium salicylate (NaSal) in deionised water at a concentration of 7 mM and at temperature T = 298 ± 0.2 K
These mixtures form an entangled viscoelastic network of worm-like micelles[21] with a structural relaxation time τs = 2.5 ± 0.2 s determined by a recoil experiment[19], where the length of the worm-like micelles is typically found in between 100 and 1000 nm[22], and the typical mesh size is on the order of 30 nm[23]

Summary

Introduction

Microscopic colloidal particles suspended in liquids are a prominent example of an overdamped system where viscous forces dominate over inertial effects. 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. In particular for large driving velocities (high shear rates), several experiments reported the occurrence of unsteady particle motion[8,9] and strong deviations from the behaviour in simple viscous liquids[10,11,12,13,14,15,16,17,18,19] These findings originate from the nonlinear rheological properties in viscoelastic fluids (e.g., shear thinning), which is generally observed in micro- and macro-rheological experiments[2,11,17,20]. We believe that the reported oscillations are a generic feature of particles in nonequilibrium baths

Methods

Results

Conclusion