Focus: Taming Evaporation
If you let a glass of salt water stand overnight, it can leave beautiful crystals behind. The lab equivalent of this experiment is a common way to study the crystallization process, but evaporation as a way to concentrate material is hard to control, which makes these studies difficult. In the 3 March PRL, researchers describe a new device that can increase a solution’s concentration in a precisely managed way. The device could be useful in laboratory studies of crystallization and other condensation phenomena in fluids, and it may also lead to improved samples for studies of protein structure and better manufacturing methods for high-tech materials with microscale structures.
Molecules dissolved in water will crystallize out if their concentration is increased to the point of saturation. Suspensions of liquid drops or tiny particles, such as plastic beads, will in a similar way form into solid clumps. Researchers would like to better understand the microscale interactions between molecules and particles that cause them to crystallize or dissolve. Ultimately, some would like to exploit a better knowledge of crystallization to create microscale structures for high-tech materials.
One way to study such crystallization is to observe the surface of a drying water droplet. As the water evaporates, the solution becomes more concentrated, until crystals begin to form. But evaporation at a surface layer induces small scale turbulent flows that disrupt the process, says Armand Ajdari of the School of Industrial Physics and Chemistry in Paris.
To concentrate solutions in a controlled way, Ajdari and his colleagues created a pair of parallel channels in a slab of plastic. One channel, a dead-end “finger” some tens of microns on a side and a few millimeters long, connects to a reservoir containing the solution to be studied. The other channel is open at both ends, and air is pumped through it. Separating the two is a 10-micron-thick membrane.
In a process called pervaporation, water in the finger permeates through the membrane and evaporates into the adjacent airstream. Solution at the end of the finger becomes highly concentrated and may crystallize, while solution close to the reservoir is less so. And the system generates a whole range of concentrations in between, all of which can be photographed through transparent walls.
The team tested their system with a potassium chloride solution. As the concentration in the finger slowly rose over a period of some hours, a “wave” of crystallization moved from the end of the finger back toward the reservoir, at a rate they could control with the airflow rate. Because water loss by pervaporation is so gentle a process, says Ajdari, crystallization occurs in an exceedingly predictable way. He and his colleagues showed that the crystallization closely followed a simple theory based on the known characteristics of potassium chloride.
With the potassium chloride “calibration” in hand, the device should accurately reveal properties of less well-understood solutions and suspensions, the researchers say. They also believe their device could help biologists fine-tune the growth of high-quality protein crystals, which are difficult to grow but essential for determining the complex atomic structures of proteins. The team has also experimented with colloids–liquids containing many tiny particles such as plastic beads–to observe how a solid structure of particles emerges as the liquid evaporates, a possible model for crystallization of molecules. They may also be useful, Ajdari says, as templates for the creation of microstructured materials such as photonic crystals, in which pores on the scale of light wavelengths lead to unusual optical properties.
Greg Randall of the Massachusetts Institute of Technology notes that the device allows condensation, crystallization, and other fundamental processes to be studied “in a very controlled manner, on a very small scale.”
David Lindley is a freelance writer in Alexandria, Virginia, and author of Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science (Doubleday, 2007).