Particles' Surprising Moves

Microscopic Janus particles -- spheres whose halves are physically or chemically different -- have been found to propel themselves in unexpected ways when an electric field is applied.

Researchers at North Carolina State University (NC State) in Raleigh said they have demonstrated for the first time that Janus particles -- so-called "two-faced" particles named for the Roman god with two faces -- will move like stealthy submarines when an alternating electrical field is applied to liquid surrounding the particles.

North Carolina State University researchers observed "two-faced" micrometer-sized particles that propelled themselves perpendicular to the direction of an electrical field, not in the direction of the field, as would be expected. (Image courtesy Orlin Velev, NC State University)

The discovery advances knowledge about how potential "smart" materials such as tiny engines or sensors can move around and respond to changes in their environment. Janus particles could be used as microscopic mixers, molecular "shuttles," self-propelling microsensors or as a way of targeting drug delivery, the researchers said. A paper describing the research was published in the Feb. 8 edition of Physical Review Letters.

The researchers -- Orlin Velev, PhD, associate professor of chemical and biomolecular engineering at NC State and lead author of the paper; Sumit Gangwal, an NC State graduate student; Olivier Cayre, PhD, a post-doctoral researcher in Velev's lab; and Martin Bazant, PhD, from Massachusetts Institute of Technology -- created tiny particles with gold on one side and plastic on the other and applied low frequency alternating current to the water containing the particles. The electric field was of voltage and frequency similar to that found in a typical home wall outlet.

Velev said the micrometer-sized particles convert the electrical field into liquid motion around them and then unexpectedly propel themselves perpendicular to the direction of the powered electrodes, not in the direction of the electrical field, as would be expected. The particles always travel in the same orientation: with the plastic "face" as the front of the microscopic submarine and the metallic "face" in the rear, he said.

The phenomenon, called "induced-charge electrophoresis," which had been predicted in a theoretical model by the MIT collaborator, had not been demonstrated previously.

Velev said these materials have the potential to perform a variety of applications.

"You can imagine other types of Janus particles comprising a 'smart gel' that responds to a change in its environment and then releases drugs, for example," Velev said. Fabricating these responsive materials on the microscale and nanoscale is an exciting and rapidly developing area of science, he added. "We are able to create tiny Janus particles of the same size and shape and are beginning to learn how to give them functionality," Velev said. "The next step is to create more complex particles that are able to perform more specialized functions in addition to propelling themselves around."

For more information, visit: www.ncsu.edu