MIT engineers have united the principles of self-assembly and 3-D printing using a new technique, which they highlight today in the journal Advanced Materials.
By their direct-write colloidal assembly process, the researchers can build centimeter-high crystals, each made from billions of individual colloids, defined as particles that are between 1 nanometer and 1 micrometer across.
“If you blew up each particle to the size of a soccer ball, it would be like stacking a whole lot of soccer balls to make something as tall as a skyscraper,” says study co-author Alvin Tan, a graduate student in MIT’s Department of Materials Science and Engineering. “That’s what we’re doing at the nanoscale.”
The researchers found a way to print colloids such as polymer nanoparticles in highly ordered arrangements, similar to the atomic structures in crystals. They printed various structures, such as tiny towers and helices, that interact with light in specific ways depending on the size of the individual particles within each structure.
The team sees the 3-D printing technique as a new way to build self-assembled materials that leverage the novel properties of nanocrystals, at larger scales, such as optical sensors, color displays, and light-guided electronics.
“If you could 3-D print a circuit that manipulates photons instead of electrons, that could pave the way for future applications in light-based computing, that manipulate light instead of electricity so that devices can be faster and more energy efficient,” Tan says.
Tan’s co-authors are graduate student Justin Beroz, assistant professor of mechanical engineering Mathias Kolle, an associate professor of mechanical engineering A. John Hart.
Out of the fog
Colloids are any large molecules or small particles, typically measuring between 1 nanometer and 1 micrometer in diameter, that are suspended in a liquid or gas. Common examples of colloids are fog, which is made up of soot and other ultrafine particles dispersed in air, and whipped cream, which is a suspension of air bubbles in heavy cream. The particles in these everyday colloids are completely random in their size and the ways in which they are dispersed through the solution.
If uniformly sized colloidal particles are driven together via evaporation of their liquid solvent, causing them to assemble into ordered crystals, it is possible to create structures that, as a whole, exhibit unique optical, chemical, and mechanical properties. These crystals can exhibit properties similar to interesting structures in nature, such as the iridescent cells in butterfly wings, and the microscopic, skeletal fibers in sea sponges.
So far, scientists have developed techniques to evaporate and assemble colloidal particles into thin films to form displays that filter light and create colors based on the size and arrangement of the individual particles. But until now, such colloidal assemblies have been limited to thin films and other planar structures.
“For the first time, we’ve shown that it’s possible to build macroscale self-assembled colloidal materials, and we expect this technique can build any 3-D shape, and be applied to an incredible variety of materials,” says Hart, the senior author of the paper.
Read more: Researchers 3-D print colloidal crystals
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