Direct observation of closed-loop ferrohydrodynamic pumping under traveling magnetic fields

Abstract

Ferrofluid-based liquid manipulation schemes typically actuate an immiscible liquid via a ferrofluid plug, using high magnetic flux (\ensuremath{\sim}1 T) densities and strong field gradients created with bulky permanent magnets. They rely on surface tension effects to maintain the cohesion of the ferrofluid plug, necessitating miniature channels and slow (\ensuremath{\sim}1 \ensuremath{\mu}l/min) flow speeds. Here, we demonstrate direct ferrohydrodynamic pumping using traveling magnetic fields at controllable speeds in a simple, closed-loop geometry without any mechanically actuated components. The pumping approach is compact, scalable, and practical. Using moderate field amplitudes (\ensuremath{\sim}10 mT), we obtained a maximum volumetric flow rate of 0.69 ml/s using a readily available commercial ferrofluid. Our closed-loop pumping approach could lead to integrated and efficient liquid manipulation and cooling schemes based on ferrofluids.