For the past decade, scientists have been toying with the notion of encapsulating medicine in microscopic balls.
These so-called nanospheres could travel inside the body to hard-to-reach places, like the brain or the inside of a tumor. One problem researchers face is how to build these nanospheres, because you'd have to make them out of even smaller nanoparticles.
"With micro- or nanoparticles, you cannot just touch them with a finger and put them together and stick them together. You need a method to do that," says Alvaro Marin.
So Marin and his colleagues at the University of Twente in the Netherlands came up with a hands-free method to assemble nanoparticles.
They took a million or so of nanoparticles (the ones they used happened to be made of plastic) and mixed them up in a drop of water. Then they put the drop of water containing the nanoparticles on a silicon chip covered with tiny bristles.
When they did that, something curious happened: The bristles helped keep the droplet intact and retain its spherical shape. It did not collapse and wet the surface of the silicon chip.
As Marin reports in the journal PNAS, the next step was pretty easy. He waited. After a while, the water evaporated.
As the water evaporates, "the particles inside the droplet have time to rearrange and stack up in a nice, neat orderly fashion, much like oranges in a grocer's stand," says David Pine of New York University. Pine was not involved in the research.
More fun visuals from Marin's lab: Watch a drop of water freeze to form a chocolate-kiss-like shape.
In other words, just by waiting until the water evaporated, the nanoparticles organized themselves into a slightly lumpy ball that looks a bit like a soccer ball.
By choosing the right nanoparticles, Pine says you could make nanospheres that would release their contents in response to some external signal. They could become a new kind of drug delivery system.
At least that's the idea. Someday. Maybe.
Marin is now at the Institute of Fluid Mechanics and Aerodynamics at Bundeswehr Universitat in Munich. He is fascinated by fluids and how they move. He produced a video of the evaporating water droplet, as well as one about what happens when a water droplet freezes that won an award from the American Physical Society Division of Fluid Dynamics as part of its Gallery of Fluid Motion.