Rolling ferrofluid drop on the surface of a liquid

TL;DR

This study demonstrates controlled magnetic manipulation of ferrofluid drops on liquid surfaces, providing a theoretical model that accurately predicts their speed, with potential applications in microfluidics.

Contribution

The paper introduces a simplified theoretical model for ferrofluid drop propulsion under rotating magnetic fields, matching experimental data without fitting parameters.

Findings

Drop speed depends on magnetic field amplitude, frequency, and drop volume.

Theoretical models accurately predict experimental drop speeds.

Potential microfluidic applications are discussed.

Abstract

We report on the controlled transport of drops of magnetic liquid, which are swimming on top of a non-magnetic liquid layer. A magnetic field which is rotating in a vertical plane creates a torque on the drop. Due to surface stresses within the immiscible liquid beneath, the drop is propelled forward. We measure the drop speed for different field amplitudes, field frequencies and drop volumes. Simplifying theoretical models describe the drop either as a solid sphere with a Navier slip boundary condition, or as a liquid half-sphere. An analytical expression for the drop speed is obtained which is free of any fitting parameters and is well in accordance with the experimental measurements. Possible microfluidic applications of the rolling drop are also discussed.