Materials researchers at Iowa State University, working in part under a grant from the National Science Foundation, have demonstrated a novel coating that makes surfaces "smart"—meaning the surfaces can be switched back and forth between glassy-slick and rubbery on a scale of nanometers, the size of just a few molecules.
Possible applications include the directed assembly of inorganic nanoparticles, proteins, and nanotubes, and the ultra-precise control of liquids flowing through microfluidic devices that are finding their way into biomedical research and clinical diagnostics.
The new coating is a single layer of Y-shaped "brush" molecules, according to principal investigators Vladimir V. Tsukruk and Eugene R. Zubarev, lead authors on a report of the work in the September 16 issue of the journal Langmuir.
Each molecule attaches to the surface at the base of the Y, which forms a kind of handle for the brush, and extends two long arms outward to form the bristles. The coating can be switched because one arm is a polymer that is hydrophilic, or attracted to water, while the other is a polymer that is hydrophobic, or repelled by water.
Thus, say the researchers, when the coated surface is exposed to water, the molecules collapse into a series of mounds about 8 nanometers wide, with the hydrophilic arms on top shielding the hydrophobic arms inside. Conversely, when the surface is treated with an organic solvent such as toluene, the surface spontaneously reorganizes itself into mounds that have the hydrophobic arms on top.
Not surprisingly, the two states are very different when it comes to properties such as stickiness and the ability to become "wet."
In future work, the Iowa State team hopes to coax the mounds into an ordered pattern, instead of the current random scatter, which may allow the researchers to make surfaces that are lubricating in one direction and sticky in others.