Coaxing particles to prepare Vortices Using Magnetic Fields

In a recent study, scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory prepared small swirling vortices out of magnetic particles, offering insight into the behaviour that governs such systems, which opens up novel opportunities for substances and devices with novel properties.

Argonne researcher Alexey Snezhko and his team members tipped a pile of small magnetic microparticles, each about as big as the diameter of a human hair, into a dish of liquid with a concave bottom. Then they applied an oscillating magnetic field and tinkered with the parameters, observing the behaviour of the particles as they began to roll. At just the right settings, the particles spontaneously coalesced into a swirling vortex.

“It is a bit as though you randomly tossed a bunch of balls onto a pool table, and they instigate to swirl in a circle as they rolled,” says Snezhko. “Our study involves mapping this active system and its behaviours that could inspire novel devices and materials with unique capabilities.”

The research is of interest in the growing field called ‘active matter’ or ‘active systems’, in which groups of individual agents use energy from their environments to form organized systems. This illustrates the behaviour of flock of birds, schools of fish and even the way our cells construct their internal structures. Similar principles govern the behaviour of these systems despite their very distinct makeup and origins, and researchers want to harness such principles to build novel active substances with unique properties, like materials that could heal themselves, alter their properties in response to external stimuli or offer novel functionalities.

For instance, as stated by Snezhko, the vortices could be employed to transport cargo in small microfluidic devices, like biochips or labs-on-a-chip, or to prepare swirling fluid motion around them to mix components at the microscale. Snezhko and his group members also found that the noise from the particles surface imperfections was often a key to trigger the flocking behaviour.

Unlike primary studies into similar systems at Argonne that discovered, for example, how to prepare moving snakes out of microparticles, which can only be organized at the surface of a liquid, these vortices exist in bulk liquid, at the solid bottom of a container full of liquid. This offers excellent versatility to those looking to prepare small machinery using such principles.