Wednesday, February 22, 2017

Big Consequences of Friction at the Nanoscale

How steep does an incline need to be before a box will slide on it? It's a classic question in physics classrooms, and the answer depends on two factors—the box's weight and a factor called μ (mu): the coefficient of friction. The value of μ depends on things like the box's material, the texture of the incline's surface, and whether the box is already moving or sitting still, but in some situations there's another surprising factor that can affect how easy it is for an object to start sliding along a surface—how long the object has been sitting still.

Massive earthquakes are often attributed to the buildup of stress caused by the friction between tectonic plates. While this is true, it doesn’t get to the heart of the issue. If you really want to know what causes an earthquake, you have to get up close and personal with the surfaces of the tectonic plates and find out what’s happening at the nanoscale.

A sign marking where the San Andreas Fault crosses the road.
Image Credit: Michael R Perry (CC BY 2.0)

This is tricky, but it’s important. Scientists can model friction pretty well—after all, it plays a role in everything from manufacturing to biomechanics—but the model used, known as the “rate-and-state friction model” (or “RSF” for short), is not based on a fundamental understanding of friction. Instead, the RSF model is based on experiments and observations. The model has a good track record of fitting data already measured, but the best, most trustworthy models are those based on fundamental physics that not only are a great match to experimental results and observations, but that can actually predict friction without needing any information or measurements ahead of time.

In research just published in the American Physical Society’s journal Physical Review Letters, a team of researchers led by scientists at the University of Pennsylvania provide new insight into friction at the nanoscale. Their work will help establish a better physical basis for models of the static friction that develops between the surfaces of rocks, like what we see prior to earthquakes, and of other materials.

Imagine two jagged edges trying to slide past one another, as in the case of tectonic plates. At some point, the kinetic friction (the friction between two moving surfaces) becomes so strong that they stop moving. In rocks and many other materials, this leads to something called frictional aging. Frictional aging means that the static friction (the force required to get two stationary surfaces moving) changes with time: it increases dramatically at first, and then continues to grow more slowly.

This frictional aging has been documented, but its cause is still up for debate. It really comes down to three possibilities—the boundary between the surfaces changes shape so there is more contact area between them, the contact area stays the same but new chemical bonds form between the two surfaces, or some combination of these two processes.

Recent research supports the idea that, at least at the nanometer scale, new chemical bonds form between surfaces of silica, a main ingredient in seismic rocks, during the aging process. They seem to form more quickly at first, causing static friction to increase rapidly and then level off with time. The results also suggest that load, which you can think of as the pressure pushing the two surfaces together, plays some role in aging.

To investigate the role of load on aging, Ph.D. student Kaiwen Tian and his advisor, Robert Carpick from the University of Pennsylvania, conducted a series of experiments that involved measuring the increase in static friction between a silica tip and a silica surface over different contact times, with different amounts of pressure. A clear relationship emerged: as the load increased, the static friction increased proportionally to that load for each contact time.

To understand why this might be the case, the team collaborated with Izabela Szlufarska from the University of Wisconsin–Madison, who performed simulations and calculations of the effect. Together, they expanded on previous work to develop a physics-based model for frictional aging that mathematically relates aging to time and load. Predictions from this model match experimental results well, and the model really digs into what is happening on the nanoscale.

As previous work suggests, the model says that new chemical bonds between the two surfaces lead to a rapid increase in static friction, but this slows down and eventually levels off over time. However, the number of bonds depends on the load. The higher the load, the greater the contact area between the two surfaces and, therefore, the more places there are for bonds to form. The researchers also considered whether the load helped individual bonds form more easily, but they found this effect to be small.

What does this mean for earthquakes? Well, that depends. The model shows that load and time effect is important for aging, but only if the materials can form these chemical bonds. This was the case in the silica-silica experiment, but it’s not clear yet whether this is the case for rocks—and that’s something that has proved very hard to measure. The researchers are attempting to check that next, and to see if the surfaces actually do change their shape with time, in addition to forming bonds, under different conditions. In the quest for a better understanding of earthquakes, let’s hope that mystery is solved sooner rather than later.

Kendra Redmond

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