These false-color SEM images reveal microscopic flower structures created by manipulating a chemical gradient to control crystalline self-assembly.
To create the flower structures, Noorduin and his colleagues dissolve barium chloride (a salt) and sodium silicate (also known as water glass) 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 byproduct, 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.
Images courtesy of Wim L. Noorduin