NC State researchers
develop a technique to assemble nanoparticles into filaments (left) in liquid.
The filaments can be
broken (middle) and then re-assembled (right).
Image courtesy of
Bhuvnesh Bharti.
(August 6, 2015) If you want to form very flexible chains of nanoparticles in
liquid in order to build tiny robots with flexible joints or make magnetically
self-healing gels, you need to revert to childhood and think about sandcastles.
In a paper published this week in Nature Materials,
researchers from North Carolina State University and the University of North
Carolina-Chapel Hill show that magnetic nanoparticles encased in oily liquid
shells can bind together in water, much like sand particles mixed with the
right amount of water can form sandcastles.
“Because oil and water don’t mix, the oil wets the particles
and creates capillary bridges between them so that the particles stick together
on contact,” said Orlin Velev, INVISTA Professor of Chemical and Biomolecular Engineering
at NC State and the corresponding author of the paper.
“We then add a magnetic field to arrange the nanoparticle
chains and provide directionality,” said Bhuvnesh Bharti, research assistant
professor of chemical and biomolecular engineering at NC State and first author
of the paper.
Chilling the oil is like drying the sandcastle. Reducing the
temperature from 45 degrees Celsius to 15 degrees Celsius freezes the oil and
makes the bridges fragile, leading to breaking and fragmentation of the nanoparticle
chains. Yet the broken nanoparticles chains will re-form if the temperature is
raised, the oil liquefies and an external magnetic field is applied to the
particles.
“In other words, this material is temperature responsive,
and these soft and flexible structures can be pulled apart and rearranged,”
Velev said. “And there are no other chemicals necessary.”