As shapes, both possess certain symmetries that are easily recognizable in
the natural world. Now, at an extremely small level, researchers from
University and the University of Massachusetts have created a unique set
of conditions in which tiny particles within a solution will consistently assemble
themselves into these and other complex shapes.
New nano-structures. Credit: Duke University
By manipulating the magnetization of a liquid solution, the researchers have
for the first time coaxed magnetic and non-magnetic materials to form intricate
nano-structures. The resulting structures can be "fixed," meaning
they can be permanently linked together. This raises the possibility of using
these structures as basic building blocks for such diverse applications as advanced
optics, cloaking devices, data storage and bioengineering.
Changing the levels of magnetization of the fluid controls how the particles
are attracted to or repelled by each other. By appropriately tuning these interactions,
the magnetic and non-magnetic particles form around each other much like a snowflake
forms around a microscopic dust particle.
"We have demonstrated that subtle changes in the magnetization of a fluid
can create an environment where a mixture of different particles will self-assemble
into complex superstructures," said Randall Erb, fourth-year graduate student.
He performed these experiments in conjunction with another graduate student
Hui Son, in the laboratory of Benjamin Yellen, assistant professor of mechanical
engineering and materials science and lead member of the research team.
The results of the Duke experiments appear in Feb. 19 issue of the journal
The nano-structures are formed inside a liquid known as a ferrofluid, which
is a solution consisting of suspensions of nanoparticles composed of iron-containing
compounds. One of the unique properties of these fluids is that they become
highly magnetized in the presence of external magnetic fields. The unique ferrofluids
used in these experiments were developed with colleagues Bappaditya Samanta
and Vincent Rotello at the University of Massachusetts.
"The key to the assembly of these nano-structures is to fine-tune the
interactions between positively and negatively magnetized particles," Erb
said. "This is achieved through varying the concentration of ferrofluid
particles in the solution. The Saturn and flower shapes are just the first published
examples of a range of potential structures that can be formed using this technique."
According to Yellen, researchers have long been able to create tiny structures
made up of a single particle type, but the demonstration of sophisticated structures
assembling in solutions containing multiple types of particles has never before
been achieved. The complexity of these nano-structures determines how they can
ultimately be used.
"It appears that a rich variety of different particle structures are possible
by changing the size, type and or degree of magnetism of the particles,"
Yellen foresees the use of these nano-structures in advanced optical devices,
such as sensors, where different nano-structures could be designed to possess
custom-made optical properties. Yellen also envisions that rings composed of
metal particles could be used for antenna designs, and perhaps as one of the
key components in the construction of materials that display artificial "optical
magnetism" and negative magnetic permeability.
In the Duke experiments, the nano-structures were created by applying a uniform
magnetic field to a liquid containing various types of magnetic and non-magnetic
colloidal particles contained between transparent glass slides to enable real-time
microscopic observations of the assembly process. Because of the unique nature
of this "bulk" assembly technique, Yellen believes that the process
could easily be scaled up to create large quantities of custom-designed nano-structures
in high-volume reaction vessels. However, the trick is to also be able to glue
the structures together, because they will fall apart when the external field
is turned off, he said.
"The magnetic forces assembling these particles are reversible,"
Yellen said. "We were able to lock these nano-structures in their intended
shapes both by using chemical glues and by simple heating."
The Duke team plans to test different combinations of particles and ferrofluids
developed by the University of Massachusetts team to create new types of nano-structures.
They also want to try to make even smaller nano-structures to find the limitations
of the assembly process, and study the interesting optical properties which
are expected from these structures.
"While we have shown that we can get small magnetic particles to form
complex and beautiful structures, we believe that based on theory and the results
of preliminary experiments, we should be able manipulate even smaller particles
by using other magnetic particles and ferrofluids," Yellen said.
The research was supported by the National Science Foundation.