"Spring is like a perhaps hand,” wrote the American poet E. E. Cummings: “carefully / moving a perhaps / fraction of flower here placing / an inch of air there... / without breaking anything.”
With the hand of nature trained on a beaker of chemical fluid, the most delicate flower structures have been formed in a science laboratory – and not at the scale of centimetres or even millimetres, but microns.
These minuscule sculptures – curved and delicate and the diameter of a human hair – don't look like the cubic or jagged forms normally associated with crystals, but that's exactly what they are. Rather, fields of carnations and marigolds seem to bloom from the surface of a submerged glass slide, assembling themselves one molecule at a time.
By manipulating chemical gradients in a beaker of fluid, Wim Noorduin, a postdoctoral fellow at the Harvard School of Engineering and AppliedSciences (SEAS), has found that he can control the growth behaviour of these crystals to create precisely tailored structures.
"For at least 200 years, people have been intrigued by how complex shapes could have evolved in nature,” says Noorduin, lead author of a paper about these nanocrystals that appeared on the cover of Science last year.
“This work helps to demonstrate what’s possible just through environmental, chemical changes.”
To create the flower structures, Noorduin and his colleagues dissolve barium chloride (a salt) and sodium silicate (also known as waterglass) into a beaker of water.
Carbon dioxide from air naturally dissolves in the water, setting off a reaction which precipitates barium carbonate crystals. As a by-product, it also lowers the pH of the solution immediately surrounding the crystals, which then triggers a reaction with the dissolved waterglass.
This second reaction adds a layer of silica to the growing structures, uses up the acid from the solution and allows the formation of barium carbonate crystals to continue.
"You can really collaborate with the self-assembly process," says Noorduin. "The precipitation happens spontaneously, but if you want to change something then you can just manipulate the conditions of the reaction and sculpt the forms while they're growing."
Increasing the concentration of carbon dioxide, for instance, helps to create 'broad-leafed' structures. Reversing the pH gradient at the right moment can create curved, ruffled structures.
What Noorduin sees through the scanning electron microscope is actually in greyscale.
Colour has been added to the nanostructures in the images here to reinforce the notion that they could be resplendent flowers.
Watch the video below in which Noorduin shows how tweaking variables like the pH, temperature and exposure to air affect the size and shape of the crystals:
Noorduin and his colleagues have grown the crystals on glass slides and
metal blades and they've even grown a field of flowers in front of
Abraham Lincoln's seat on a one-cent coin.
"When you look through the electron microscope, it really feels a bit like you’re diving in the ocean, seeing huge fields of coral and sponges," says Noorduin. "Sometimes I forget to take images because it's so nice to explore!"
Of course, this research goes above and beyond making teeny weeny flowers – Havard is using the findings as a modelling system to help develop our understanding of the emergence of form, curvature and complex hierarchical architectures.
For more information, please visit: http://www.seas.harvard.edu/news/.