Characterization of azimuthal and radial velocity fields induced by rotors in flows with a low Reynolds number

TL;DR

This paper combines theoretical modeling and experimental measurements to analyze the complex azimuthal and radial velocity fields generated by low Reynolds number rotors, revealing significant deviations from simple models and implications for microfluidic transport.

Contribution

It introduces a superposition of two rotlets model for non-axisymmetric microrotor flows and validates it with experimental data, highlighting radial flow components.

Findings

Radial velocity oscillations at twice the rotor frequency observed.

Deviations from single sphere azimuthal flow patterns reported.

Model accurately predicts complex flow fields in microfluidic rotors.

Abstract

We theoretically and experimentally investigate the flow field that emerges from a rod-like microrotor rotating about its center in a non-axisymmetric manner. A simple theoretical model is proposed that uses a superposition of two rotlets as a fundamental solution to the Stokes equation. The predictions of this model are compared to measurements of the azimuthal and radial microfluidic velocity field components that are induced by a rotor composed of fused microscopic spheres. The rotor is driven magnetically and the fluid flow is measured with the help of a probe particle fixed by an optical tweezer. We find considerable deviations of the mere azimuthal flow pattern induced by a single rotating sphere as it has been reported by Di Leonardo \textit{et al.} [Phys. Rev. Lett. 96, 134502 (2006)]. Notably, the presence of a radial velocity component that manifests itself by an oscillation of the probe particle with twice the rotor frequency is observed. These findings open up a way to discuss possible radial transport in microfluidic devices.