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Magnetically controlled droplets rock, rotate, and roll

Droplets with localized iron oxide nanoparticles move in response to a magnetic field and could have applications in optics

by XiaoZhi Lim, special to C&EN
July 31, 2020

Credit: ACS Cent. Sci.
Multiphase droplets with precisely placed magnetic nanoparticles on their surfaces move in concert with an external magnetic field.

Liquid droplets bearing precisely located magnetic nanoparticles on their surfaces can be manipulated with magnets in surprising ways (ACS Cent. Sci. 2020, DOI: 10.1021/acscentsci.0c00686). Such magnetic liquids have potential for optical applications such as sensing or image capture by serving as lenses, the authors say.

Magnetic iron oxide nanoparticles (MNPs) have been widely used to direct the movement of materials, in applications such as guiding drugs to a target or recovering catalysts. More recently, researchers have begun trapping MNPs at the interfaces between liquids to produce magnetic liquid droplets. Last year, a team led by Thomas P. Russell at the University of Massachusetts Amherst prepared such droplets by confining MNPs at each droplet’s surface with surfactants. Each droplet behaved like a solid, nanoscale magnet and retained its magnetism even after removal of the magnetic field (Science 2019, DOI: 10.1126/science.aaw8719).

In the new work, Timothy M. Swager of the Massachusetts Institute of Technology and colleagues built on this approach to make more complex droplets with a wider repertoire of behaviors under magnetic fields, albeit with weaker magnetism. “The entire area of structured liquids is completely new,” says Russell, who was not involved in this study. “It’s a very rich open ground right now.” Swager’s strategy takes this droplet-building approach to the next level, he adds.

Graduate student Cassandra A. Zentner and postdoctoral researcher Alberto Concellón from Swager’s group made Janus, or two-sided, droplets and confined MNPs to one hemisphere’s surface. They warmed a liquid fluorocarbon with a liquid hydrocarbon and then dispersed the mixture in water with two types of surfactants. Upon cooling, the two oily liquids separated to form the two-sided droplets, each side stabilized with a surfactant. The researchers then added MNPs by reacting a fluorocarbon-aldehyde compound in the fluorocarbon side of the droplet with amine-labeled MNPs.

Left alone, the Janus droplets sit with their denser, MNP-decorated fluorocarbon side facing down. But when a magnet is held close, the Janus droplets roll against gravity to align themselves with the magnetic field. This creates a change in the optics of the collective droplets that could be useful for sensing applications. In this way, researchers can redirect light that goes through the droplets, Swager says.

Zentner and Concellón made another type of Janus droplet by replacing the hydrocarbon phase with liquid crystals and decorating that side of the droplet with MNPs. Liquid-crystal molecules are long and tend to align themselves in neat rows.

In one arrangement, the liquid-crystal molecules lined up vertically and forced all the MNPs into a single point on top of the droplet, as if it had a north pole. When the researchers moved a magnet around these droplets, the droplets themselves didn’t rotate or move. Instead, the group of MNPs at the pole slid down the side of the droplet to follow the magnet, effectively reorienting the alignment of the liquid crystals, Swager says. This could serve as another way to control liquid crystals, which is key for optical applications like displays in which images are created by liquid crystals reorienting themselves in response to changing electric fields.


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