The difficulty in proving that either theory holds true lies in the fact that points of contact are necessarily embedded at the juncture of two materials and are therefore hard to observe. One of the original breakthrough experiments on these theories projected light through transparent materials held together to measure the growth of apparent contact points. While this lent credence to the contact quantity theory, there was not yet a way to assess the bond strengths at those individual points of contacts or to be sure that the observations were of single points of contacts or clusters of even smaller nanoscale contacts.
It was not until Carpick and Tullis met at a conference designed to bring physicists and mechanics researchers together with geologists that they realized that the tools of the former group could resolve the latter group's contact quality theory. The solution came from moving from the massive scale of earthquakes to the smallest scales imaginable.
"We want to simplify the case," Li said. "So in our experiment we look at only one point of contact: the tip of an atomic force microscope."
An atomic force microscope is an ideal tool for investigating bonding strength where two surfaces meet. Instead of using light, atomic force microscopes measure nanoscale details using an extremely sharp probe tip that is sensitive to the push and pull of individual atoms.
The researchers simulated rock-on-rock contact with silica, a major component in most geological materials. They pressed a silica tip against a silica surface for different lengths of time and then dragged it to measure the amount of friction it experienced. They repeated these experiments with surfaces made out of different materials: diamond and graphite. Critically, both diamond and graphite are chemically
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University of Pennsylvania