Janus particles, which take their name from a Roman god with two faces, are microscopic 'two-faced' spheres whose halves are physically or chemically different. Now, U.S. researchers have shown that some of these Janus microparticles can move like stealthy submarines when an alternating electrical field is applied to liquid surrounding them. This could lead to new kinds of self-propelling microsensors or means of targeted drug delivery. But read more...
You can see above how these particles "move perpendicular to the applied electric field direction with their polystyrene hemisphere (lighter side) forward. The liquid flow around the metal-coated hemisphere (darker side) is much stronger then around the polystyrene hemisphere, driving the particle to move by induced-charge electrophoresis (ICEP) in the direction of the dielectric hemisphere, normal to the electric field." (Credit: Orlin Velev research group, NC State University)
This research work has been led by Dr. Orlin Velev, associate professor of chemical and biomolecular engineering at NC State and several members of his research group including Sumit Gangwal, an NC State graduate student and Dr. Olivier Cayre, a post-doctoral researcher. Dr. Martin Bazant from the Massachusetts Institute of Technology, also participated to this study. In fact, Bazant has predicted today's results in a theoretical model. Here is a link to his research about Induced-charge electro-osmosis.
Even Velev was surprised by the findings. "Velev says 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 mini-submarine and the metallic 'face' in the rear, Velev added."
Velev also thinks at other usages for these Janus particles. "'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 says. Fabricating these responsive materials on the microscale and nanoscale is an exciting and rapidly developing area of science, he adds. '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, this research work has been published in a recent issue of Physical Review Letters under the title "Induced-Charge Electrophoresis of Metallodielectric Particles" (Volume 100, Number 5, Article 058302, February 8, 2008). Here is a link to the abstract which states that the phenomenon discovered by the researchers "may find applications in microactuators, microsensors, and microfluidic devices." Finally, here is a link to the full paper (PDF format, 4 pages, 447 KB).
Sources: North Carolina State University news release, February 27, 2008; and various websites
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