Laser pulses reveal contact mechanics

Marie Freebody, marie.freebody@photonics.com

The next time you are sitting at your desk full of papers, pens, files and perhaps a coffee mug, take a closer look at the points where these objects meet the desk. What do you see? If you could magnify the nanoscale interface of where the object meets the desk, you would actually see a world of mountain ridges and deep valleys.

A 3-D rendering of a thermal map of the sphere pressed into the surface with a force of 3.1 N highlights that the cooler contact areas in the center (red) occur where the heat loss from the surface is greater.

These microscopic features and, more importantly, how these features mesh together are what help tires grip a road or a gecko’s feet cling to a rock. Until now, however, there has been no way to study this complex landscape in detail while the objects are still in contact. The best researchers could do was to press one object into the other, remove it and then image the indentation it leaves using an atomic force microscope.

Now, professor Oliver Wright and his team at Hokkaido University have seen the microscopic terrain of a live contact. Their technique, reported in the December 2009 issue of Physical Review B, combines ultrafast laser technology with classical contact mechanics to image the interface between a ceramic ball and a flat surface of chromium.

A close-up of the 6-mm-diameter ceramic ball attached to the mechanical arm.

“The properties of mechanical contacts between solids depend on how their microscopically rough surfaces mesh on the nanoscale, and a physical understanding of the contact area is vital in engineering and biology,” Wright said. “We present a new way of probing contacting interfaces using ultrahigh-frequency sound and heat waves.”

In the setup, a mechanical arm presses a 6-mm-diameter ceramic ball down onto a 110-nm-thick film of chromium coating a sapphire surface. Ultrashort laser pulses (less than 1 ps in duration) are fired at the sapphire surface, which heats the chromium, causing it to expand and send a short ripple of high-frequency sound and thermal energy toward the ball.

Thermal images of the contact between the sphere and the flat surface reveal that, as the sphere is pressed with greater force (increasing from upper left to lower right), the surface contact area increases. Images courtesy of Oliver Wright of Hokkaido University.

After the sound waves bounce off the ball, they are detected by a second, probe laser. The intensity of the reflection is a measure of the strength of the acoustic echo. The Hokkaido team found that the echoes from the contact region arrive earlier, enabling imaging of the nanometer-scale deformation of the metal film.

By scanning the probe light pulses, a two-dimensional map of the thermal contact can be built up. This thermal map highlights areas of dark and light, which correspond to, respectively, where the ball and film were in contact, and where there were air gaps.

The mechanical arm presses the ceramic ball down onto the chromium surface. The red line is the excitation beam or pump laser, and the blue line is the detection beam or probe.

While spatial resolutions in mechanical contacts on the order of 0.1 mm have been obtained using megahertz-frequency sound waves, the nanoscale has never before been explored in this way.

“We implement an in situ acoustic and thermal profiling technique at frequencies of about 100 gigahertz and 1 megahertz, respectively,” Wright said. “This is more than one thousand times higher than previously used in contact mechanics.”

This work has applications in engineering for the study of friction, lubrication, and electrical or heat conduction, as well as in biology for the study of adhesion or joints.