Need to form flexible chains of nanoparticles in liquid in order to build tiny robots with flexible joints or make magnetically self-healing gels? Tnen you might want to revert to childhood and think about sandcastles. Researchers from North Carolina State University and the University of North Carolina-Chapel Hill recently showed 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.
Corresponding author Orlin Velev, INVISTA Professor of Chemical and Biomolecular Engineering at NC State, said:
“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.”
Bhuvnesh Bharti, research assistant professor of chemical and biomolecular engineering at NC State and first author of the paper, explained:
“We then add a magnetic field to arrange the nanoparticle chains and provide directionality.”
Chilling the oil can be likened to 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.
But 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.”
Bhuvnesh Bharti, Anne-Laure Fameau, Michael Rubinstein & Orlin D. Velev
Nanocapillarity-mediated magnetic assembly of nanoparticles into ultraflexible filaments and reconfigurable networks
Nature Materials (2015) doi:10.1038/nmat4364
Illustration: 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.