Scientists drew fittingly from Roman mythology when they named a unique class
of miniscule particles after the god Janus, who is usually depicted as having
two faces looking in opposite directions.
For years, scientists have been fascinated by the tantalizing possibilities of
these particles for their potential applications in electronic display devices,
sensors and many other devices. However, realizing these applications requires
precise control over the positions and orientation of the particles, something
which has until now eluded scientists.
Duke University engineers
say they can for the first time control all the degrees of the particle’s
motion, opening up broad possibilities for nanotechnology and device applications.
Their unique technology should make it more likely that Janus particles can
be used as the building blocks for a myriad of applications, including such
new technologies as electronic paper and self-propelling micromachines.
Typical Janus particles consist of miniscule spherical beads that have one hemisphere
coated with a magnetic or metallic material. External magnetic or electric fields
can then be used to control the orientation of the particles. However, this coating
interferes with optical beams, or traps, another tool scientists use to control
positioning.
The breakthrough of Duke engineers was to devise a fabrication strategy to coat
the particle with a much smaller fraction of material. This discovery allows these
particles to be compatible with optical traps and external magnetic fields, allowing
for total control over the particles’ positions and orientations.
“Past experiments have only been able to achieve four degrees of control
using a combination of magnetic and optical techniques,” said Nathan Jenness,
a graduate student who completed his studies this year from Duke’s Pratt
School of Engineering. He and co-author Randall Erb, also a graduate student,
were first authors of a paper appearing online in the journal Advanced Materials.
“We have created a novel Janus particle that can be manipulated or constrained
with six degrees of freedom.”
The researchers have dubbed the unique particles they created “dot-Janus”
particles.
Using optical traps on dot-Janus particles, researchers controlled three degrees
of movement – up and down, left and right, forward and backward, while constraining
one degree of rotation - side-to-side tilting. Using magnetic fields, they controlled
the remaining two degrees of rotation - forward and backward tilting, and left
and right turning.
“The solution was to create a particle with a small cap of cobalt that covers
about a quarter of the particle,” Erb said. He and Jenness conducted their
research in the laboratory of Benjamin Yellen, Duke assistant professor of Mechanical
Engineering and Materials Science. “This gave the particle just enough of
a magnetic handle to allow it to be manipulated by magnetism without interfering
with the optical tweezers.”
The researchers said that the fabrication of these unique dot-Janus particles
combined with the ability to control their orientation will have important ramifications
in the burgeoning field of nanoengineering.
“Being able to more completely control these particles affords us a greater
ability to measure the mechanical properties of biomolecules, including DNA,”
Yellen said. “It may also be possible to control the behavior of cells by
manipulating dot-Janus particles attached to cell surfaces. These biological applications,
as well as the ability to control the assembly of nanostructures, establish the
broad scientific value of these findings.”
The research was supported by the National Science Foundation and the Nanoscale
Interdisciplinary Research Team. Robert Clark, former Duke dean of engineering
and now in the same position at the University of Rochester, was also part of
the research team.
Posted August 11th, 2009
