May 17, 2013
Tiny Test Tube Flowers Demonstrate Wonders Of Crystal Formation
April Flowers for redOrbit.com - Your Universe Online
With spring come thoughts of green growing plants and beautiful flowers — Mother Nature showing off her finest. Wim L. Noorduin, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS), has been showing off his finest as well, growing delicate flower structures on the scale of microns in a beaker full of chemical fluid.Noorduin´s miniscule sculptures are curved and delicate, not resembling the jagged forms one normally associates with crystals, even though that is exactly what they are. Miniature fields of carnations and marigolds seem to bloom from the surface of a submerged glass slide. The tiny flowers assemble themselves one molecule at a time.
Noorduin creates these delicately tailored structures by controlling the growth behavior of the crystals by simply manipulating chemical gradients in the beaker. The results of this amazing study appear on the cover of a recent issue of Science.
"For at least 200 years, people have been intrigued by how complex shapes could have evolved in nature. This work helps to demonstrate what´s possible just through environmental, chemical changes," says Noorduin.
The crystal´s precipitation, or transition from liquid to solid form, depends on a reaction of compounds that are diffusing through a liquid solution. Certain chemical gradients attract or repel the growing crystals as the pH of the reaction shifts. Whether the structure resembles broad, radiating leaves, a thin stem, or a rosette of petals is dictated by the conditions of the reaction.
Growth in nature is often influenced by chemical gradients. Delicately curved marine shells form from calcium carbonate under water, for example, and gradients of signaling molecules in a human embryo help create the plan for the baby´s body. Bacteria living in colonies can sense and react to plumes of chemicals released from one another, according to research by Harvard biologist Howard Berg, causing them to grow as a colony into intricate geometric patterns.
To replicate this process in the laboratory, Noorduin had to identify a suitable chemical reaction. He then tested, again and again, how variables like the pH, temperature and exposure to air might affect the nanoscale structures.
The principal investigator on this project is Joanna Aizenberg, an “¯expert in biologically inspired materials science, biomineralization and self-assembly.
"Our approach is to study biological systems, to think what they can do that we can´t, and then to use these approaches to optimize existing technologies or create new ones," says Aizenberg. "Our vision really is to build as organisms do."
Her latest research has included such things as the invention of an extremely slippery material inspired by pitcher plants and the discovery of how bacteria use their flagella to cling to the surfaces of medical implants.
Noorduin and his teammates dissolved the salt barium chloride and sodium silicate (also known as ℠waterglass´ or ℠liquid glass´) into a beaker of water to create the flower structures. Carbon dioxide from the air naturally dissolves in the water. This sets off a chemical reaction which precipitates barium carbonate crystals and, in turn, lowers the pH of the solution in the immediate vicinity of the crystals. This triggers a reaction with the dissolved waterglass, which adds a layer of silica to the growing structures. This secondary reaction also 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."
If, for instance, the concentration of carbon dioxide is increased, more “broad-leafed” structures are created. On the other hand, reversing the pH gradient at the right moment can cause curved, ruffled structures to grow.
The team has grown their flowers on a variety of surfaces, from glass slides and metal blades to the spot in front of President Lincoln´s seat on a penny.
"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," describes Noorduin. "Sometimes I forget to take images because it's so nice to explore."