Oct 5, 2005
Giving nanoscale friction the slip
Researchers at the Georgia Institute of Technology, US, have shown for the first time that capillary condensation of water is thermally activated. The findings could have implications for nanoscale friction effects, food processing and dip-pen nanolithography.
"Since formation of the capillaries affects friction and adhesion between particles, if we understand this relationship, we can understand how small particles and nano-surfaces glue together," said Elisa Riedo of Georgia Tech.
Riedo and colleagues studied the phenomenon by drawing a spherical atomic-force microscope (AFM) tip across a glass slide. The tip radius was 25 nm and the root mean square roughness of the glass surface was about 1 nm. The researchers kept the relative humidity at 37% but varied the temperature of the chamber and the speed at which they moved the AFM tip.
Measuring the resistance to movement of the tip indicated the frictional force. At low speeds, the frictional force decreased as the speed increased. At higher speeds the frictional force was almost constant.
"When you move very slowly there is time for a capillary to form at each tiny bump or asperity in the surface," said Riedo. "But when you move faster you have fewer capillaries. If you go fast enough the capillaries do not have time to form."
The speed at which the frictional force stabilized - i.e. the speed at which capillaries no longer formed - depended on temperature. The team used this data to calculate capillary nucleation times at different temperatures. They discovered that the nucleation time decreased from 4.2 ms at 299 K to 0.7 ms at 332 K.
"The more energetic the water molecules are, the more likely it is that they will form capillaries," said Robert Szoszkiewicz of Georgia Tech. "We found that nucleation times grow exponentially with the inverse of temperature."
The research indicates it may be possible to reduce adhesion between surfaces by lowering the temperature or by moving the surfaces before capillaries have time to form.
"To form water bridges, molecules need to overcome an energy barrier," said Riedo. "The thermal energy can provide the energy they need, however it takes time for these bridges to form. The longer the surfaces are together, the stronger the contact will be because more bridges can form."
The researchers believe that the discovery may also have implications for dip-pen nanolithography. "You might use the temperature dependence to increase the velocity of the ink flow, decrease it, or make the flow improbable," said Riedo. "There are a lot of implications for the technology. Each of the materials involved will have its own properties regarding velocity and how rapidly it forms capillary bridges."
The researchers reported their work in Physical Review Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.