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Aug 30, 2018 | By Thomas

MIT researchers have found a way to print colloids such as polymer nanoparticles in highly ordered arrangements, similar to the atomic structures in crystals. Through this direct-write colloidal assembly process, a process combining the bottom-up principle of colloidal self-assembly with the versatility of direct-write 3D printing, the researchers can build centimeter-scale, free-standing crystals, each made from billions of individual colloids. This technique could be used to scale-up self-assembled materials for use as optical sensors, color displays, and light-guided electronics.

3D-printed colloidal crystals viewed under a light microscope. Image: Felice Frankel

"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."

Using this method, researchers have 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 3D printing technique as a new way to build self-asssembled materials that leverage the novel properties of nanocrystals at larger scales.

"If you could 3D 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.

Colloids are any large molecules or small particles, typically measuring between 1 nm and 1 μm in diameter, that are suspended in a liquid or gas. Fog is one common example of a colloid, as it is made up of soot and other ultrafine particles dispersed in air. Milk is an emulsified colloid of liquid butterfat globules dispersed within a water-based solution. 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 exhibit unique optical, chemical, and mechanical properties.

Up until now, researchers 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 3D shape, and be applied to an incredible variety of materials," says A. John Hart, associate professor of mechanical engineering and the senior author of the paper.

The researchers use a custom-built 3D-printing apparatus for the process, which consists of a glass syringe and needle, mounted above two heated aluminum plates. The needle passes through a hole in the top plate and dispenses a colloid solution onto a substrate attached to the bottom plate.

The team evenly heats both aluminum plates so that as the needle dispenses the colloid solution, the liquid slowly evaporates, leaving only the particles. The bottom plate can be rotated and moved up and down to manipulate the shape of the overall structure.

MIT Graduate student Justin Beroz said, "As the colloid solution is pushed through the needle, the liquid acts as a bridge, or mold, for the particles in the solution. The particles "rain down" through the liquid, forming a structure in the shape of the liquid stream. After the liquid evaporates, surface tension between the particles holds them in place, in an ordered configuration."

As a first demonstration of their colloid printing technique, the team worked with solutions of polystyrene particles in water, and created centimeter-high towers and helices. Each of these structures contains 3 billion particles. In subsequent trials, by changing the sizes of polystyrene particles, they were able to print towers that reflected specific colors.

"By changing the size of these particles, you drastically change the color of the structure," Beroz said. "It's due to the way the particles are assembled, in this periodic, ordered way, and the interference of light as it interacts with particles at this scale. We're essentially 3-D-printing crystals."

The team also experimented with more exotic colloidal particles, namely silica and gold nanoparticles, which can exhibit unique optical and electronic properties. They printed millimeter-tall towers made from 200-nanometer diameter silica nanoparticles, and 80-nanometer gold nanoparticles, each of which reflected light in different ways.

"There are a lot of things you can do with different kinds of particles ranging from conductive metal particles to semiconducting quantum dots, which we are looking into," Tan says. "Combining them into different crystal structures and forming them into different geometries for novel device architectures, I think that would be very effective in fields including sensing, energy storage, and photonics."

 

 

Posted in 3D Printing Application

Source: MIT

 

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