May 5, 2006
Nanoscale water bridges act like glue
Water has been found to form ice at room temperature if it is placed between a tiny tungsten tip and a graphite surface. Joost Frenken and colleagues at Leiden University in the Netherlands have found that the water effectively acts like a glue in this situation, even though water is normally thought of as a lubricant. The finding could be useful to researchers studying micro- and nanoelectromechanical systems, which can fail if the friction between the surfaces is too high (Phys. Rev. Lett. 96 166103).
Most surfaces are never smooth and completely flat. If two bodies are brought together, they will not touch over the entire area of apparent contact, but will rather touch over a large ensemble of tiny contacts. Under normal, humid conditions, water vapour can condense out onto these contacts, creating a tiny "capillary bridge". These bridges tend to make the surfaces stick together and make it harder for them to slide over each other.
However, researchers have not been sure if this is also true when the liquid is confined to small gaps between surfaces. To investigate this problem further, Frenken's team carried out a series of experiments on an instrument called a "tribolever", which uses tiny amounts of bending inside a miniature silicon sensor to sense forces as small as 20 picoNewton.
The experiments involved attaching a sharp piece of tungsten wire onto the sensor and carefully scanning it back and forth over the surface of clean, high-quality graphite. The researchers found that icy nanoscale water bridges - lasting for several seconds - formed between the two surfaces at room temperature. In this geometry, the water effectively acted like a glue, and not like a lubricant, joining the two surfaces together.
"Our work provides a new understanding of what happens on the nanometre scale between contacting and sliding bodies," says Frenken.
The team will now investigate using different materials for the tip and substrate and varying other parameters like temperature and tip speed. They will also study to what extent the ice contributes to friction under practical circumstances.
About the author
Belle Dumé is science writer at PhysicsWeb.