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Microscopic particles suspended in liquids are the prime example of anoverdamped system because viscous forces dominate over inertial effects. Apartfrom their use as model systems, they receive considerable attention assensitive probes from which forces on molecular scales can be inferred. Theinterpretation of such experiments rests on the assumption, that, even if theparticles are driven, the liquid remains in equilibrium, and all modes areoverdamped. Here, we experimentally demonstrate that this is no longer validwhen a particle is forced through a viscoelastic fluid. Even at small drivingvelocities where Stokes law remains valid, we observe particle oscillationswith periods up to several tens of seconds. We attribute these tonon-equilibrium fluctuations of the fluid, which are excited by the particle'smotion. The observed oscillatory dynamics is in quantitative agreement with anoverdamped Langevin equation with negative friction-memory term and which isequivalent to the motion of a stochastically driven underdamped oscillator.This fundamentally new oscillatory mode will largely expand the variety ofmodel systems but has also considerable implications on how molecular forcesare determined by colloidal probe particles under natural viscoelasticconditions.

Oscillating Modes of Driven Colloids in Overdamped Systems