Researchers have shown that external magnetic fields can be used to remotely manipulate geometrically identical nanostructures in different directions, allowing them to be positioned at arbitrary locations close to a solid surface in a microfluidic environment1. This demonstration raises the possibility of making magnetic nanomotors for microfluidic devices that would be useful in biomedical and electronics applications.
To induce directional movement of nanostructures in fluidic environments, the researchers deposited silica nanohelices on colloidal polystyrene beads and coated them with the magnetic element cobalt. They then dispersed the nanohelices in deionized water in a microfluidic device and applied magnetic fields with independent amplitudes, frequencies and phases in three orthogonal directions.
The scientists found that the magnetic fields caused the nanohelices to undergo a rocking motion in the plane perpendicular to the bottom surface. This rocking motion increased to full strokes in the forward and backward directions. Structural asymmetry arising from the beads congregating at one end of the nanohelices generated unequal drag forces, which displaced the beads.
The researchers observed that bead-less nanohelices moved six times slower than beaded ones.
“By decoupling the sources of energy from the directions, we moved the nanohelices with movements closely resembling the way magnetotactic bacteria propel themselves in natural environments,” says lead researcher Ambarish Ghosh.
