Vol. 19, No. 31
May 18, 2000
In the March 24 issue of Science magazine, Orlin Velev, Abraham Lenhoff and Eric Kaler--all from UD's Center for Molecular and Engineering Thermodynamics and Department of Chemical Engineering--described how they were able to grow these colloidal crystals in water drops suspended on fluorinated oil. The droplets acted as templates for the highly ordered and smooth particles, which are a millimeter or less in size.
The method used is extremely simple, Velev said. "Everything we do takes place in a couple of beakers at room temperature. So why use complicated and expensive microfabrication technology when instead templated self-assembly can create a variety of complex self-sustained microstructures in a beaker?"
The form of these new particles can be easily and systematically changed by the researchers--from flattened or dimpled spheres to doughnut shapes.The shape is controlled by the interplay of droplet size, gravitational force and changes in interfacial tensions set by added surfactants. The particles can be coated with a metal, such as gold, or one hemisphere can be made magnetic, so that the spheres can be oriented or flipped by magnetic fields. The assembled particles can also be modified by adding other organic, inorganic and metallic particles.
"The most important part of this research is the method, which allows us to synthesize a whole new class of microstructured particles," Kaler said.
Because the "doughnuts" have uniform porosity and pack together efficiently, they might be useful as media for chromatography, which is a process for separating components in a mixture of chemicals.
The new magnetic particles also could be useful for electronic papers, which display electronic text on thin, flexible sheets filled with millions of microscopic capsules that show either dark or light images in response to electrical charges.
The researchers say that the colloidal crystal assemblies are quite robust under moderate mechanical manipulation, unlike non-crystalline structures, which easily break into powder. This mechanical strength combined with the ability to make novel shapes opens many new applications for these assemblies.
Research will continue on the process, Lenhoff said, because "by changing the compositions or by altering a parameter of the process like surfactant concentration or type, we foresee that we can create tens or even hundreds of new types of particles of different shapes and functionalities" -Cornelia Weil