No friction? No problem!

Imagine a world without that dual-edged force called friction. Friction directly allowed for man to invent fire, while continuing to be a pain in Cub Scout Pinewood Derby races. Love it or hate it, it has always played a role in constructing our world as we know it. Certainly a world without friction could only be a physicist’s dream, right?

Before we start this discussion, perhaps we should consider where friction comes from. From our introductory physics class, we know that the static friction, for example, is governed by this equation:

Fs ≤ µFn ; Fs = static friction, µ = coefficient of friction, Fn = normal force

The normal force (or contact) is the upward acting force on an object, acting perpendicular to the surface to prevent the object from sliding. But what about this coefficient of friction? Where can it come from? Naively, we consider the below possible explanations:

  • Surface impurities in the object/contact point, i.e. ‘roughness’
  • Mass of the object, as normal force generally opposes the gravitational force
  • Elevation to object, seen with the diagram below
inclined1
Force diagram of an object on an inclined plane. Normal force acts at an angle to the gravitational force, so friction will also be dependent on the slope. Source: StudyPhysics

While we can write equations to describe what the coefficient of friction can depend on, in truth we are unsure how to properly characterize this material dependent property. Do the surfaces stick to each other? Is it a microscale interaction (i.e. atomic) or does it occur due to larger surface interactions? Proposed solutions range from understanding the surface energy profile of the sliding surfaces to the surface chemical impurities (page 34). So there seems to be some combination of the smaller and larger interactions- that’s critical to understanding how to remove macroscale frictional forces.

tribology-nanocomposites
An overview of the various friction forces that act on an object, at different length scales. Fun fact- the study of surfaces in relative motion is called tribologySource: RPI

The field of research is called superlubricity, and the key is choosing the right material. Scientists have recently shown heavy interest in DLC, or diamond-like carbon, which has a very dense atomic structure like diamond but is different structurally. It’s hard like a diamond, but more importantly, is very useful for surface friction studies. Interestingly, structural changes to DLC occur at low temperatures and is highly hygroscopic, so to minimize friction, the environment is a key component to setup properly.

Thus, the study in question analyzes DLC movement over nanodiamonds grafted into graphene on a silicon dioxide base. The diamond-diamond interaction, as you can imagine, greatly reduces the friction between the DLC object and the base. But more interesting is how- the group describes the formation of flakes (or ‘scrolls’) between the contact points of the graphene and the nanodiamonds. Seemingly, the graphene wraps around the nanodiamond to increase the effective surface contact, reducing the overall contact area by ~65-70%! What promotes the wrapping? Wait for it… wait for it… van Der Waals forces! Yup, these ones.

f3-large
Pictorial representation of the graphene wrapping around the nanodiamond. Note that in a highly humid environment, the wrapping doesn’t occur. Source: Berman, Science

So as the DLC moves across the graphene surface, the nanodiamonds act as rollers to greatly reduce the surface friction that it experiences. And the best part is that ‘the friction mechanism at the mesoscale for an ensemble of graphene patches is not different from nanoscale.’ [1] However, as noted in the above image, water prevents this vDW interaction at the nanoscale and therefore increases the friction/tear between the two surfaces. So one can imagine that this technology would be present on the next Mars rover or in any controlled environment. That being said, one shouldn’t rule out the potential applications in the lubrication industry! At the end of the day, our high school chemistry is the governing principle behind promising technologies. Take a moment to thank your AP Chem teacher :).

[1] http://science.sciencemag.org/content/348/6239/1118.full

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