Crystals seem nothing like flowers. But it’s a matter of perspective, and some technical ingenuity. Just ask Wim Noorduin, who invented a way to build microcrystals that, when seen through an electron microscope, resemble blooming irises, violets, and carnations.
The hair-width nanocrystals, which graced a recent cover of Science, were grown as part of Noorduin’s study, “Rationally Designed Complex, Heirarchical Microarchitectures.” Noorduin, a materials postdoc at Harvard, was working to find ways to control microcrystal growth, and one of his experiments involved watching a salty solution absorb carbon dioxide. He knew roughly what to expect, but was stunned when the crystals sprouted florets, complete with petals. He decided to shift his study to the most beautiful specimens–and to find new ways to tweak their appearance.
“We were interested in making spontaneous, self-assembling structures,” says Noorduin. “We wanted to make various shapes in a controllable way, using only very simple physical chemical processes.”
Nanomaterials scientists have been etching detailed microstructures on different materials for some time, but crystals are a different ballgame. To grow them, you lay down the substrate and drop in the precipitate. After that, they simply grow. You cede control. But Noorduin was able to crack this problem. He discovered that by varying the acidity and concentrations of the chemicals, he could change the shapes of the resulting crystals. Changing the acidity of the solution caused the crystals to coalesce into a thin stem-like shape. As CO2 concentration increases, they grow wider and flatter, more like a violet. (The color in these images was added artificially — electron microscopes create black-and-white images.)
Perhaps the most surprising thing is that you can do all of this at home. Noorduin and his collaborators began by dissolving two easily obtained materials, barium chloride and sodium metasilicate, in water. Naturally occurring CO2 then diffuses and precipitates — that is, it attaches to free-floating barium ions and the result is insoluble. It drops to the bottom as crystal barium carbonate. Those crystals release acid, which triggers the metasilicate to form a silica polymer, and both continue to grow together. To control the CO2, “I just put something on the beaker, like a small lid, basically, but it’s still a little bit open. And then when I want to increase the concentration, I just take the lid off for a while,” he says. “You can really do this experiment in your kitchen, it’s very simple.”
Noordin hopes that this research could be used eventually to build microscopic optical and electronic components, though that’s far away for now. “It’s just intriguing to see that some kinds of biological systems, like corals and also microsized animals that are able to crystalize their skeleton, that they can do that so controllably and that they also are able to make so many different kinds of shapes, that’s really amazing,” Noorduin says. “These structures are really small. And the samples that we grow them on, I think are an inch, approximately,” he says. “So that means that there are thousands of these structures growing on one sample, and if you then take them in the electron microscope, it really looks like a complete world that you created.”
All images: courtesy of Wim NoorduinGo Back to Top. Skip To: Start of Article.