2. Friction at the nanoscale
In general, friction between two macroscale surfaces is proportional to normal force. However, the frictional characteristics of nanoscale surfaces cannot be fully described by the framework of Amontons' laws of friction: at the nanoscale, friction becomes far more complicated because different processes contribute to energy losses during sliding and thus lead to friction.
For one, the concept of surface roughness plays a crucial role. On a molecular scale, almost all surfaces are rough: under a microscope, even a surface that looks perfectly smooth to the naked eye appears like a mountainous landscape. Friction only arises at the locations where the highest hilltops on the two sliding surfaces touch one another.
Other possible mechanisms of frictional energy dissipation at nanoscale levels are wear related, molecular deformation, thermal effect, electronic effects, chemical bonding, phonon effects, ambient chemistry effects (you can find a detailed discussion of these mechanisms in ACS Nano, "Approaches for Achieving Superlubricity in Two-Dimensional Materials").
2.1 Intermolecular forces
The amount of friction generated between two surfaces depends on the factors listed above, including the minute intermolecular forces acting between them.
The forces of attraction between atoms or molecules, which the Dutch physicist Johannes Diderik van der Waals discovered in the 1870s, are electrostatic forces that exert a major impact at the nanoscale and affect the interaction of molecules. They are the dominant forces between uncharged nanostructures and they ultimately decide what functionality nanostructures can provide.
Without these – very, very weak – intermolecular forces, life as we know it would be impossible. They are responsible for a number of properties of molecular compounds, including crystal structures, condensing, melting and boiling points, surface tension, and densities. Intermolecular forces form molecules like enzymes, proteins, and DNA into the shapes required for biological activity.
As objects get smaller, these forces become more important. Henri can immediately think of a great example of this when he grinds the beans for his morning espresso: whole beans don't stick to the side of the grinder, but a fine grind will stick to everything (thanks to electrostatic charge accumulation).
One reason for this is the surface-to-volume ratio. As bodies become smaller, the surface forces (which act due to the contact they make) become larger relative to the body forces (like gravity or electromagnetism) and increasingly determine the body's behavior.
Van der Waals forces can also be repulsive – although number of systems in which repulsive van der Waals forces could occur is limited – for instance when two surfaces approach each other in liquid: the same force which causes attraction in air (and which is responsible for stiction, i.e. static friction, and adhesion) can be made repulsive by choosing the right combination of surface materials and intervening liquid. This force has the characteristic that it increases very rapidly with very small changes in separation when the surfaces are close to each other.
Research has shown (Langmuir, "Superlubricity Using Repulsive van der Waals Forces") that if repulsive van der Waals forces exist between two surfaces prior to their contact then friction is essentially precluded and supersliding is achieved. The steeply repulsive van der Waals force acts to prevent the surfaces achieving contact and they tend to slip past one other, rather like two magnets when you try to push them together.
This opens the possibility that, in certain material systems, the controlled use of repulsive van der Waals forces could be a way to reduce, if not eliminate, friction (see: Slip sliding away in the nanoworld).
2.2 Friction in mechanical micro- and nano-systems
Whenever two surfaces come into contact, there is friction. On a larger (macro) scale – relevant for machines – engineers use gears, bearings, and all kinds of lubricants to reduce friction.
While these means can reduce friction in the macroscopic world, the origins of friction in extremely small devices such as microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS) require other solutions – because at this scale normal lubricants don't work.
Despite the unprecedented accuracy by which nano-devices are nowadays fabricated, their enormous surface-to-volume ratio leads to severe friction and wear issues, which dramatically reduce their practical benefits and lifetime.
Apart from MEMS and NEMS, friction is the main factor limiting the efficiency of nanofluidic devices (where fluids are confined to the nanoscale). For most liquids, the solid surface of surrounding channel walls poses friction resistance to the flowing liquid, causing a – sometimes complete – loss of velocity at the liquid/solid interface.
While this effect is not particularly worrying for macroscale fluid transport, it becomes a major issue for nanofluidics. For regular surfaces the energy required for nanofluidic applications is enormous because of the tremendous hydraulic resistance inside nanoscale conduits.
Simply put, the smaller the channel, the more pressure you need to drive the liquid. Imaging drinking a beverage with a straw: the thinner the straw is, the more difficult this becomes.
Graphitic nanoconduits, for example carbon nanotubes (CNTs) and graphene nanochannels and their membrane forms, may provide a solution. It has been discovered that superfast water transport occurs in these structures (Nature, "Nanoscale hydrodynamics: enhanced flow in carbon nanotubes" and Nature Materials, "Fast diffusion of water nanodroplets on graphene"), and that has inspired great interest for numerous nanofluidic applications.
An understanding of nanoscale friction at the interface between a liquid and a solid is crucial for the development so-called lab-on-chip devices where miniscule amounts of a fluid are transported through tiny channels, and of efficient membranes for water desalination and power harvesting.
So, basically what matters most for these tiny mechanical devices is what sticks to what. If you have too much friction, then you get parts of a device sticking together that shouldn't and the whole thing clearly is not going to work.
Understanding the loss of energy that results from friction (which is called dissipation) at the interfaces of sliding nanoscale surfaces is a prime challenge for the development of nanoscale functional devices. Decreasing nanoscale friction in NEMS or MEMS systems has huge implications for controlling mechanical energy losses and wear of these devices.
In the 1980s, physicist Serge Aubry postulated that, if the lattice spacing between particles in one surface were to differ slightly from the lattice spacing in the other, friction between the two surfaces should disappear entirely.
In 2018, researchers were able to confirm these predictions through numerical simulations and experiments (see: Physical Review X, "Experimental Observation of the Aubry Transition in Two-Dimensional Colloidal Monolayers"). They demonstrated that it is possible to entirely suppress static friction between two surfaces. This means that even a minuscule force suffices to set objects in motion.
They also showed that it is possible for static friction to be generated as desired if the contact pressure between the two surfaces is increased.
This way of varying static friction creates new opportunities for moving objects easily across surfaces and to lock them into place safely. Especially in micromechanical parts, where only small forces are at play, a vanishing static friction can lead to hugely improved levels of efficiency.