Water wire

Understanding the often anomalous behavior of water could hang by a thread between a sharp silicon point and a mica surface, according to Korean scientists. The thread in question is a strand of water just a few molecules thick that could provide nanoscale clues about water's intriguing properties.

This almost one-dimensional column of water resembles certain structures common in biology but that are rarely seen in the laboratory. Indeed, this liquid "nanowire" may soon reveal important properties of water at the molecular scale.

According to Wonho Jhe and his colleagues at Seoul National University in Korea, water is the dread enemy of atomic force microscopy. A typical AFM runs a pointy metal tip over a surface to map the lumps and ruts at the atomic scale. A single drop of water quickly coats the hovering tip however, wrecking any chance of atomic observations as it makes the tip stick to the surface unnecessarily.

Wonho Jhe

Jhe was hoping to improve the sensitivity of AFM by scanning closer to the surface, but they were struck by the odd behavior of their AFM tip and decided to take a closer look, so to speak. "We saw a very small kind of step-wise variation in the pull," explains Jhe. It might have been simple noise but the researchers' curiosity was piqued. Thinking that the effect might be nothing more than interference from water, they systematically tested the AFM tip at constant humidity. They found that the force gradient, which should rise continuously, instead increased in steps. The reading stayed constant for a while, then jumped by 0.45 Newtons per meter, stayed steady again, then jumped twice more before falling to zero. The effect disappeared when the humidity dropped below 2%, so the culprit was definitely water, but this was water behaving badly.

The researchers explain that the droplet of water pulling on the AFM tip was miniscule, about 10^-23 liters, or ten yoctoliters. The tip, they reasoned, was stretching this droplet into a one-dimensional thread, just a few molecules thick and tens of molecules long. The group is not sure why the water pulls on the AFM with discrete, stepped levels in the force gradient, but it might result from individual water molecules alternately sticking, then sliding, then sticking again to the surface as the water column is stretched.

Previously, researchers have produced one-dimensional strands of water, but only in the confines of carbon nanotubes, although much thicker water columns have also been made with other techniques. Jhe's new watery configuration is just 2.6 nanometers thick and forms in air at room temperature, making it a much more accessible "system" to produce than would be expected, provided one has access to an AFM.

Jhe's group is currently stretching, dragging, and playing with their one-dimensional water column to find its limits and to try and understand the structure. Their ongoing work might help science understand the behavior of water. Biological cells use one-dimensional columns of water to transport ions through their membranes, so Jhe's findings may shed light on cell processes.

Phys. Rev. Lett., 2005, 95, 187801; http://link.aps.org/abstract/PRL/v95/e187801