Every high school student is taught that opposite charges attract. Even in the complex world inside living cells, simple rules of thumb like this one usually continue to told, and go a long way to explaining how the machinery of life all holds together.
Recently, however, researchers have found an intriguing exception to this rule, which may have implications for the development of new materials for sophisticated sensors and optical devices, says Nily R. Dan, an assistant professor in UD's Department of Chemical Engineering, in a July 16 Science article.
"Our work addresses fundamental questions about how charges interact," Dan says of her joint work with researchers at the University of Pennsylvania. "In complex systems, such as multi-component materials or living cells, opposite charges don't necessarily attract. Understanding the conditions in which oppositely charged objects do or don't attract helps us understand basic strategies of self-assembly, which may be used to design new drug carriers or smart materials that respond to their environment."
Dan's article--coauthored by UD postdoctoral researcher Helim Aranda-Espinoza and by Yi Chen, Tom C. Lubensky, Philip Nelson, Laurence Ramos, and David A. Weitz of the University of Pennsylvania--examines the interactions between microscopic charged particles and oppositely charged synthetic membranes.
One would expect that the particles would adhere and cover the entire membrane surface. However, the experiments show that, frequently, only a small fraction of the membrane area is covered by adhering particles. Why do the remaining bare, oppositely charged surfaces repel particle adhesion? The researchers showed that in some cases the system prefers instead to separate into a highly adhesive zone and another, repulsive, zone by transmitting electrochemical jumps over long distances along the impenetrable membrane. While the system imitates living cells (the membrane) interacting with oppositely charged proteins (the particles), it may also be used as a model for the design of new, colloidally based materials.
Dan's research was supported by the National Science Foundation.
Journal
Science