Two Microspheres in an External Flow: a Dance of Cause and Effect

TL;DR

This study investigates how two microspheres in a low Reynolds number environment exhibit reciprocal cause-and-effect behavior through hydrodynamic interactions, revealing that external flows induce coupled motions mimicking imposed flow patterns.

Contribution

The paper demonstrates that in a controlled low Reynolds number system, external flows induce reciprocal coupled motions between microspheres, highlighting a physical cause-and-effect symmetry.

Findings

External flow induces non-trivial coupled microsphere motions.

Microsphere fluctuations mimic imposed flow symmetry.

Reciprocity observed in low Reynolds number hydrodynamic interactions.

Abstract

In low Reynolds number swimming and pumping, differently to everyday experience, a net motion (or flow) can be achieved only if the constructing parts of the swimmer (or pump) follow a non-trivial pattern of motion, in order to break time reciprocity. The case of a driven fan, which spins to create a flow of air, but conversely rotates when turned off and subjected to a strong external flow, is a familiar example of reciprocal connection between physical cause and effect. We explore here in a well controlled low Reynolds number system whether such an exchange of the cause and effect also holds in the low Reynolds number regime. As a case study we investigate the motion of two microspheres which interact hydrodynamically through their surrounding fluid. Each sphere is constrained in a fixed optical trap potential, allowing local fluctuations around an equilibrium position. An external flow is shown to induce non-trivial coupled motion. We find a signature of reciprocity: the nonequilibrium sphere fluctuations mimic the symmetry of the motions that one would impose in order for them to produce a constant flow.