It’s a moonless night. The wind howls outside. A door opens slowly, as if pushed by an invisible hand.
That sound — a horror movie cliché — is the result of friction. A stealthier entrance calls for oiling the door’s hinges.
Friction is everywhere — from a violinist bowing a string to children skidding down a slide. In the right situation, the ubiquitous force can have big effects: Interleave the pages of two phone books, and the friction between the pages will hold the books together so tightly that they become strong enough to suspend a car above the ground.
But scientists can’t fully explain, at the scale of atoms and molecules, why one pair of materials sticks while another moves with ease. The extreme slipperiness of ice, for example, has been a puzzle for more than 160 years. The multitude of water molecules on an icy surface creates a sheen that can send a car spinning or a penguin tobogganing. But getting a handle on the details of how this slippery surface arises from the water molecules is surprisingly tricky.
Despite its everyday nature, “we still don’t really understand a lot of things about friction,” says mechanical engineer Ali Erdemir of Argonne National Laboratory in Lemont, Ill. On its most basic level, friction results from the interactions between atoms in two materials that are butted up against one another. But, Erdemir says, “there is a disconnect” between the large-scale processes of friction that we can see, feel or hear and the smaller, atomic properties of materials that produce those well-known behaviors.
Now, by scrutinizing atoms’ wily ways, scientists are devising new techniques to cut down on friction, going beyond known slippery surfaces like ice, Teflon and the banana peel of countless comedy gags. Some scientists have found ways to bring friction down to near-zero levels, a property known as superlubricity. Others are studying quantum effects that reduce friction.
Atomic acrobatics might help turn friction up and down at will, a useful ability since there are times when friction, a force working against the motion of a sliding or rolling object, is helpful. The frictional force of tires on asphalt, for example, lets a car turn without spinning out. But friction also saps the car’s speed, so that more energy is needed to keep the vehicle moving.
Some materials slide easily over one another, while others require extra oomph to move. That movability is described by a number called the coefficient of friction. The more slippery the pair, the lower the coefficient. The numbers below are estimates; exact values depend on conditions.
|Sliding materials||Coefficient of friction|
|Index finger on sandpaper||1|
|Tires on dry pavement||1|
|Tires on wet pavement||0.6|
|Steel on steel||0.6|
|Tires on icy pavement||0.2|
|Banana peel on linoleum||0.07|
|Steel on Teflon||0.04|
|Steel on ice||0.01|
Sources: D.R. Lide/CRC Handbook of Chemistry and Physics 2005; S. Müller et al/J. Dynamic Syst., Meas. and Control 2003; M. Scherge et al/Lubricants 2018; A.V. Savescu et al/J. Appl. Biomech. 2008; K. Mabuchi et al/Tribology Online2012
Gaining the ability to wrangle friction could have real-world consequences. It’s estimated that a third of the energy that goes into powering fossil fuel–guzzling cars is lost to friction, converted into other forms of energy like heat and sound. The same hindrance affects just about every other machine imaginable, so that an estimated one-fifth of the world’s annual energy consumption goes to fighting friction. Reducing those losses would mean “huge savings,” Erdemir says.
A real drag
Humans have been fiddling with friction for ages. Ancient Egyptians appeared to know that pouring a little bit of water on sand made sliding heavy stones across the sand easier — a necessity for building the pyramids, researchers reported in 2014 in Physical Review Letters.
Leonardo da Vinci took an interest in friction and systematically analyzed the force. More recently, scientists have invented new materials with important frictional properties, such as Teflon, created in 1938, which lets eggs slip easily from a frying pan onto a plate.
When surfaces rub together, the atoms in the two materials jostle, sending tiny vibrational waves called phonons rippling through the materials. Meanwhile, chemical bonds between the surfaces form and break as one material slides along the other. Atoms can get wrenched entirely out of place, scraping off material. This process, known as wear, explains why the tread in your sneakers rubs off over time, leaving you with soles too slick to grip the pavement.
Friction can set off sound waves that we can hear, like the scratch of rough sandpaper, the squeak of a sticky bike chain or yes, horror fans, a creaky door. Friction sometimes causes a buildup of electric charge, making static electricity that can produce quite a zap, as anyone who’s taken off a sweater and then touched a metal doorknob knows.
Different types of motion have different amounts of friction. A stationary object requires more force to overcome friction than one that’s already moving. And rolling objects have less friction than sliding ones — locking a stroller’s wheels makes it stay put unless you push with enough force to drag the stationary wheels across the floor.
Friction’s strength is defined by a number known as the coefficient of friction, which describes how much force must be exerted to move an object relative to its weight for a given pair of materials (SN: 7/16/11, p. 14). A coefficient of friction of 0 means smooth sailing, or no friction at all. Depending on the conditions, a steel skate sliding over ice can have a coefficient of 0.01, while steel on steel is more than 10 times greater, around 0.6. Banana peels’ reputation for slipperiness is well-deserved: On a linoleum floor, the slick skins have a coefficient of friction of about 0.07 (SN Online: 9/19/14). Tires on a dry road can have a coefficient as large as 1, a value that drops to around 0.6 when the road is wet (SN: 11/13/04, p. 308).
Scientists are now harnessing superlubricity, which makes materials more slick than ice and banana peels. Material pairs with a coefficient below 0.01 are considered superlubric. One method of achieving superlubricity relies on carefully selecting the structure and orientation of the rubbing materials. The aim is to dramatically reduce friction of a type known as stick-slip motion, in which sliding surfaces switch between moving and stuck states. This type of friction is common — it’s what accounts for that eerie creaking door sound, says physicist Oded Hod of Tel Aviv University.
If you could shrink down to the size of an atom, you’d see that the surface of a sheet of smooth, crystalline material is a series of hills and valleys in a regular pattern, the structure of atoms arranged in a grid. When surfaces slide by one another, the atoms in one layer don’t want their electrons to overlap with those in the other layer. “The electrons say, ‘Hey, stay away from my territory,’” says theoretical condensed matter physicist Erio Tosatti of the International School for Advanced Studies, or SISSA, in Trieste, Italy. That means the materials can get temporarily locked into place; when the atoms are arranged as they prefer, they don’t want to move from their comfortable spots. This dispute leads to intermittent sliding and stopping instead of smooth movement.
The hills and valleys created by the atoms are reminiscent of an egg carton, with a regular series of dips where each egg sits. Imagine trying to slide two empty egg cartons past each other, one on top of the other. Once the cartons reach the spot where their cups and ridges line up perfectly, they’ll get stuck. With a push, they’ll slide until the cups interlock again, and so on, sticking and slipping repeatedly. In arrays of atoms, that stick-slip process results in energy being converted, not into motion, but into other, unhelpful forms, like sound or heat.
Slide two identical egg cartons (left) over one another and the ridges and valleys get stuck together, making them harder to push. But for egg cartons of different orientations (middle) or sizes (right), the peaks won’t interlock and will slide more easily. When this principle is applied at the scale of atoms, this effect can drastically reduce friction.
Now picture rotating the cartons so that the cups and ridges no longer line up. One carton will glide along the top of the other, making for smoother motion. This idea works for atoms, too, and it’s called structural superlubricity. Two materials that stick mightily when aligned can slide with nearly frictionless ease when they rub at an angle. Likewise, consider two cartons that are made to fit different types of eggs — like chicken eggs and duck eggs. The cups in the two cartons will be spaced at different distances, since the larger duck eggs need more space. That means that the cups won’t line up exactly, and they won’t lock together, no matter how they are oriented. The same goes for two materials with differently spaced atoms.
Predicted in the 1980s and 1990s, structural superlubricity was first conclusively spotted and reported in 2004, when researchers showed that the ease with which a graphite flake slides over another graphite surface depended strongly on its orientation: At certain rotation angles, friction dropped to next to nothing, the team noted in Physical Review Letters.
More recently, structural superlubricity showed up in graphene — a sheet of graphite a single atom thick. A ribbon of graphene slides with ease across a gold surface, scientists reported in 2016 in Science. The ribbons can be hundreds of nanometers long, made up of thousands of atoms, but “they move with forces which are sometimes smaller than [those needed] to move a single atom,” says study coauthor Ernst Meyer, a physicist at the University of Basel in Switzerland. “This is really quite amazing, if you tune everything quite the right way.”
But structural superlubricity tends to require pristine conditions; dirt or blemishes on the materials will muck it up. So the effect is usually demonstrated only in a vacuum, with carefully controlled conditions and specially prepared surfaces. For those reasons, structural super-lubricity was initially confined to objects that are best measured in nanometers, or billionths of a meter, a scale on which such imperfections can be avoided. But recently, scientists have enhanced their superlubricity superpowers.
Researchers from China and Israel found superlubricity with surfaces a million times larger in area — micrometer scales. When graphite slid over a compound of boron and nitrogen, the combination boasted an ultralow coefficient of friction, less than 0.00014, the group reported in July 2018 in Nature Materials. Atoms within the compound, known as hexagonal boron nitride, are arranged into hexagons, the same shape as carbon atoms in graphite. But the hexagons in the two materials are different sizes, like egg cartons made for duck eggs compared with chicken eggs. And the friction remained low even when the tests weren’t performed under vacuum conditions, says Tel Aviv’s Hod, a coauthor of the study with Tsinghua University’s Ming Ma, Quanshui Zheng and others.
The next goal, Hod says, is to bring structural superlubricity to millimeter scales, to objects we can see and hold, even if tiny. Small moving parts of that size are common, and such reductions in friction could be useful in a variety of devices, from tiny computer components to miniature engines. “It can be in the medical industry, data storage, watches, satellites, you name it,” he says. Scaling up to that next size will be a challenge, but Hod is working on ideas to get there.
Putting it together
Throwing nanoparticles into the mix is one way of leaping to larger scales. Argonne materials scientist Anirudha Sumant and colleagues have created lubricants based on pieces of graphene that roll up around minuscule balls of diamond, forming scrolls.
The addition of the graphene nanoscrolls makes the difference, forming an army of small, slippery surfaces that work collectively to keep things moving smoothly. With that combination, the coefficient of friction dropped precipitously, to 0.004, Sumant, Erdemir and colleagues reported in 2015 in Science. Sumant says he’s working on similar lubricants for industry that could help devices like wind turbines move freely and more efficiently.
Some scientists are using their atomic-scale studies to control friction, tuning it up and down as needed. This power over friction, making a surface slippery or rough as desired, could be useful as more than a laboratory trick. Imagine car tires that are grippy when you brake or accelerate, but smooth when you’re cruising along.
Physicist Jacqueline Krim and colleagues studied the friction between a thin layer of oxygen molecules and a nickel surface that the oxygen molecules were stuck to. By vibrating the nickel surface and measuring how easily the oxygen slid over it, the researchers measured the friction. The experiment is a bit like pulling a tablecloth (the nickel) out from under some dishes (the oxygen). If friction is too high, you smash some plates.
Then, using magnetic fields, the researchers reoriented the egg-shaped oxygen molecules to stand on their ends. That reorientation decreased the friction by half, the team reported in December 2018 in Condensed Matter.
Scientists also hope to explore even more exotic effects to bolster slipperiness. Theoretical calculations say that quantum mechanical weirdness can reduce friction, Tosatti and colleagues reported in April 2018 in the Proceedings of the National Academy of Sciences. A single electrically charged atom, an ion, dragged across the surface of a material can tunnel through an otherwise impassable barrier, a process known as quantum tunneling.
An atom sliding over a bumpy surface loses energy to friction as it traverses the ridges (left). But thanks to quantum mechanics, an atom might be able to tunnel through barriers and cut down on friction (right).
Source: T. Zanca et al/PNAS 2018
Imagine a single particle traversing an egg carton. Normally, the particle would have to climb up and down each cup, requiring enough energy to navigate that landscape. But quantum mechanics indicates that a particle can occasionally skip the up and down and pass straight through one cup to another, as if burrowing through the carton. That ability reduces friction.
Although this quantum lubricity, as it is called, has yet to be harnessed in the lab, Tosatti and colleagues predict that it should be possible to demonstrate via an established technique. Scientists have used lasers to create a simulated surface that mimics the dips and bumps of a material. Dragging cold ions across the mock material can re-create friction, and possibly its quantum effects.
In the quest to tweak materials to adjust friction, scientists have been making steady progress, but it’s not so easy to draw direct connections between the physics of big and small. In her lab at North Carolina State University in Raleigh, Krim says she can adjust atomic properties of materials and study what happens. But in general, the two worlds are separated by a tough-to-penetrate forest. “There are some foot trails that have connected up,” she says. But “there is still some bushwhacking” to do.
That trailblazing could be worthwhile. Harnessing the power of materials made nearly friction free based on the atoms within would be a game changer, Erdemir says. With that capability, he says, “we can solve the world’s friction problems.”
To crack the enduring problem of why ice is slippery, a group of researchers connected the jiggling dance of water molecules to that slip-sliding samba of a pedestrian encountering an icy sidewalk.
Scientific folklore has held that the pressure from a shoe sole melts the ice and produces a lubricating layer of water, reducing the friction and sending the person wearing the shoe on a ride. Not so, says Mischa Bonn, a physical chemist at the Max Planck Institute for Polymer Research in Mainz, Germany: At temperatures well below ice’s melting point, “even an elephant … stacked on one high heel still would not exert enough pressure to melt ice substantially.” The idea that heat from the friction of a sliding object melts the ice doesn’t hold up either. The heat produced is too feeble, experiments have shown. “You need a lot of heat to melt ice,” Bonn says.
Another theory turns out to be closer to the truth. A film of mobile water molecules covers the surface of ice, reducing the friction, Bonn and colleagues determined based on simulations and experiments reported May 2018 in the Journal of Physical Chemistry Letters.
Perhaps, the researchers suggest, those loose molecules even roll around. Stepping on a slippery patch is like trying to dance on marbles; there’s no getting a foothold.
The explanation also accounts for another feature of frozen water: When ice gets too cold, it loses its slipperiness. That happens, Bonn and colleagues say, because the loose molecules get pinned down to the surface. That means there’s an optimal temperature for ice skating that’s not too cold and not too warm. By the team’s calculations, this Goldilocks temperature should be –7° Celsius (19° Fahrenheit). That’s about the same temperature that ice skating rinks try to maintain for the fastest skating speeds.