Repellent when wet
6 May 2005
Kenneth J. Wynne, PhD, a professor in the VCU School of Engineering’s Department of Chemical Engineering, and Umit Makal, a graduate student at VCU, created the polymer that is hydrophilic, or water loving, when dry, and hydrophobic, or water resistant, when wet .
Makal drew an analogy of the research to a drop of water in a Teflon-coated pan. “Water in such a pan just rolls off,” he said. “On our surface, when the pan is dry, water just loves the surface … it tries to stick to it.”
“This discovery runs counter to intuition,” Wynne said. “Water-induced hydrophobic surfaces may lead to applications for many things, including the testing of bodily fluids, switching devices, drag-reducing coatings and many others.
“Sometimes an engineer wants to guide the flow, or turn off tiny streams of fluid, such as blood, in a test tube, and this kind of material could be useful in creating channels for that purpose.”
Wynne and Makal made the discovery while working to create antimicrobial coatings by incorporating a molecule called hydantoin into fluorine-containing polymer chains. Makal was testing the behaviour of water on one of these coatings and observed that the water drops were spreading, wetting the surface.
“After we took the drop off and put it back again, it started hating the water,” Makal said. “The surface became water repellent where the original drop of water had been.”
Wynne and Makal concluded that the change was caused by a rearrangement of the polymer side chain, which exposed the hydrophobic, fluorine-containing groups to the surface and made them repel water.
“The process can be reversed by drying the surface,” Wynne said.