Jan 24, 2019 | By Cameron

Materials with dichroic properties go back at least as far back as the 4th century when the Lycurgus Cup was (likely accidentally) made, a glass vessel that appears opaque green when lit from the front and translucent red when lit from behind. The effect is caused by elongated gold and silver nanoparticles that reflect some wavelengths of light while transmitting others, though the maker of the Lycurgus Cup probably didn’t know that and was merely adding the metallic powders to make the glass extra shiny. But Vittorio Saggiomo and colleagues from Wageningen University in the Netherlands did understand the cause of the optical effect when they developed a method of 3D printing with gold nanoparticles to create objects with dichroic properties.

The researchers started with creating a dichroic solution using a modified version of the Turkevich method that reduces gold ions to gold nanoparticles using a citrate. The solution undergoes multiple temporary color changes, initially from yellow to blue, and then after a couple minutes, it shifts to a dark black before finally settling on the dichroic opaque brown/transparent purple. The paper states:

“The time-dependent study shows the formation of small gold nuclei that in time cluster together forming nanowire-like structures concomitant to the first-color change. The second change of color, from ink-black to purple, is accompanied by an enhancement of the scattering, giving the purple solution a brown reflection. While boiling, the gold nanowires fragment, creating nanoparticles with a large head and a slim and long tail, comparable to a tadpole. Over time the tail starts to shrink.”

They then infused the dichroic gold nanoparticle solution into polyvinyl alcohol (PVA), a plastic commonly used for producing supports on 3D prints as it dissolves in water. That plastic was extruded into filament that can be used on any standard FDM 3D printer (in this case an Ultimaker 2). Parts 3D printed in the material look brown with light shining on the front, but put a light behind it and it goes to a transparent purple.

The effect imbued by the Turkevich method is a well-studied phenomenon. It’s one of those lovely occurrences that happens where physics and chemistry meet. Joining the light-bending effect with 3D printing will certainly bring some new attention to the old trick, especially since only a tiny amount of gold nanoparticles is necessary to achieve the result. Besides finding appeal among artists, dichroic plastics could be used to 3D print optics and sensors. They wouldn’t be the first 3D printed lenses.



Posted in 3D Printing Application



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Larry wrote at 1/24/2019 6:55:45 PM:

This is super interesting, but have you heard of a guy named Bill Masters? I recently read an article on OZY about how Masters developed the first 3D printing patent - then lost it. Anyone know anything about this? Here it is for reference: https://www.ozy.com/flashback/and-he-could-have-been-the-father-of-3d-printing/81198#.WeXu6REalHI.twitter

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