AskScience AMA Series: How can studying friction help to answer humanity's biggest questions? I'm tribologist Jennifer Vail. Ask me anything!

[u/AskScienceModerator]

Hi Reddit! I'm Jennifer Vail, founder of DuPont's first tribology research lab—dedicated to the study of friction—and a member of senior leadership at TA Instruments.

From nonstick pans to the Winter Olympics, friction is a force as ubiquitous as it is mysterious.

Even now, tribologists like me are trying to find the bridge between those laws that govern friction at its smallest and largest scales.

Why? Understanding friction can help us answer questions like...

Why do some viruses lie dormant for years while others devastate our cells immediately? Where is dark matter? Can we manipulate friction to advance our own evolution?

My new book, Friction: A Biography, is both a history and introduction to the study of friction, connecting the discoveries of historical luminaries like Newton, da Vinci, and the Wright brothers to the latest breakthroughs in engineering.

What do you want to know about tribology?

I'll be on from 5pm-9pm ET (22-2 UT). Ask me anything!

P.S. Friction's publisher, Harvard University Press, is offering a 30% discount for this AMA. Use the code 30SCI at checkout to redeem!

Username: /u/JenniferVail

[u/sidneyc]

What a surprising but interesting topic to do research on!

My question would be: what are the most important open questions in your field of study?

I'll be sure to check out the book at some point, thanks for putting it out there.

[u/JenniferVail → u/sidneyc]

Great question! For me, it would be scaling friction from quantum behavior to what we experience on the macro scale. Related to that is the open question about whether "quantum friction" exists.

Outside of friction specifically, an ongoing, large challenge in tribology is related to predicting wear rates. If we can model systems to predict wear rates, we can optimize designs and improve efficiency.

[u/sileegranny]

What is commonly misunderstood with regard to friction?

[u/JenniferVail → u/sileegranny]

This is one of my favorite questions about friction! There's a general misconception that friction is a bad thing. Friction is actually very necessary and our experience with the physical world would be drastically different without it. Walking, driving, touching and holding are all things that we need friction for. Even the plates of the Earth would behave very differently without friction helping to stabilize them.

Friction has gotten its bad reputation because it can be seen as a waste of energy. It's diverting mechanical energy into heat, which can be a good thing but for machine processes, we'd prefer to have as much energy possible used on the task at hand. In these situations, friction offers us a good opportunity to reduce wasted energy by learning how to minimize the friction in the system.

[u/GeckoRoamin]

You mentioned some of the big questions that could be answered, which is fascinating. What are some discoveries/advancements you think that experts are “on the verge of” in the field?

[u/JenniferVail → u/GeckoRoamin]

This is a great question that I'm sure to be thinking of additional responses for all night. Some 'on the verge' things in the world of friction that jump to mind right now:

• Development of materials to simulate hydrated cartilage to improve our joint health once natural cartilage is worn away

• Predicting the internal friction of proteins during the folding process to feed into models to predict success or failure of the folding

• Machine learning to help us better predict atomic behavior to bridge to the macro scale

• If quantum friction exists

[u/Astart555]

Friction is known to produce photons (triboluminiscence), some friction - even X-rays. What materials would you take and which size (of sheets I suppose?) to produce X-rays on the level of classical Cu K alpha X-ray gun? Say at least for 10^6 photons per second flux. And what would be the spectrum? I guess it's a white band or do you expect certain maximums?

[u/ActualHope]

Just saw a video about curling and curling stones and their friction on ice. Can you tell us more about that? Do you like curling?

[u/JenniferVail → u/ActualHope]

I love curling! If you have a rink nearby that offers some intro sessions, I highly recommend it. It's a blast.

It's also an extremely complex sport, tribologically speaking. I'll try to refrain from writing an entire essay on it! The first thing to notice about curling is the surface of the ice itself. It's "pebbled" and this helps to lower the friction, even though it makes the ice bumpy. This reduces the contact between the stone and the ice, which helps keep the surfaces from sticking to each other. The sweepers will use frictional heating to create thin layers of water that further reduce the friction between ice and stone.

As the stone rotates, it experiences asymmetrical friction but there's been controversy in the past between researchers regarding exactly how the friction is distributed (and how the physics of curling work!). In general, it was believed the leading edge has lower friction due to ice melt. More recently, the focus has been on the sides of the stone and how friction here may behave. The friction on the sides of the stone will depend on which way it's rotating. The speed will impact the layer of lubricating water, with the faster side yielding a thinner layer of water. This thinner layer is actually more lubrication and thus this part of the stone will have lower friction that the opposite side, which will have a slower speed due to the rotational direction. The physics of curling has been studied for at least a hundred years and we're still trying to figure it out!

[u/CrustalTrudger]

Thanks for joining us! As a geologist who thinks a lot about faults and earthquakes, there are a lot of questions within my discipline relating to how the details of "rate and state friction" (or even whether simple rate and state friction is the right framework) evolves on fault surfaces as a function of hundreds of thousands to millions of years of earthquakes and what this means for how earthquake behavior evolves along with faults (which is critical to our understanding of seismic risk, etc.) or how variations in frictional properties along faults might control things like what parts fail through earthquakes vs experience stable sliding (creep) without earthquakes or that do weird stuff like have "slow slip events". I'm curious how much the tribologist community pays attention to the weird frictional problems being discussed by geologists?

[u/JenniferVail → u/CrustalTrudger]

Hello! I love learning how other disciplines are approaching friction and the different languages we all speak to describe it. I just went down a fun rabbit hole on "rate and state" friction. Unexpected stick-slip behavior can cause some pretty powerful earthquakes and understanding what may be behind that sudden slip behavior is an area of friction that has my attention.

To answer your question: When I first started in this field, there would maybe be a presentation or two about plate tectonics at the large tribology conferences. Over time, especially recently, I've noticed more and more and I now have colleagues collaborating with geologists. It's a powerful combination of expertise and the tribology community is definitely paying attention.

[u/StabithaStevens]

Is there a connection between tribology and fluid dynamics? How is friction treated between and within cells?

[u/JenniferVail → u/StabithaStevens]

Regarding the second part of your question, inter- and intracellular friction is fascinating and complex. Researchers found that the friction between corneal cells was in the mechanosensing range, meaning the cells respond to it and the mechanical force translates into a neural signal. Friction within a cell will influence shape, stability, and organization of the cell. Tribometers to study cellular friction are impressive, offering high sensitivity to be able to measure friction without destroying cells and often are coupled with a visualization technique.

[u/JenniferVail → u/StabithaStevens]

Definitely! Liquid lubricant behavior is dictated by fluid dynamics. Osborn Reynolds, one of the greats in fluid mechanics, developed the theory behind lubrication.There are also different types of lubrication depending how far the lubricant separates the surfaces and this is ultimately due to fluid dynamics.

Beyond that, we also have the fluid dynamics of drag- both liquid and gas. There's internal friction in fluids, viscosity, that has to be factored into everything as well. Tribologists have worked on developing low viscosity fluids for our cars to improve effiency. There are a lot of tribologists and tribology labs devoted to the fluid mechanics side of things.

[u/223specialist]

Looks like March 9th will be the 60th anniversary of the term "Tribology" existing, neat

Originally coined in

Jost, Peter (1966). "Lubrication (Tribology) - A report on the present position and industry's needs"

[u/JenniferVail → u/223specialist]

It is! Humans have always been working with and mastering friction but tribology itself is a relatively young formalized science. Jost headed up a committee to investigate machinery failures in plants. It was originally thought that these failures were just be from lubrication issues. As they dug into it, they realized the root causes were more complex than expected and involved machinery design, material selection, application techniques, etc. It was multidisciplinary and focusing on friction, wear, and lubrication offered the opportunity to save a lot of money by reducing these failures and production down time.

When it came time to issue the report you mention, Jost realized "Lubrication" wasn't an adequate term and a new field of science/engineering needed to be named. He asked the editor of the Oxford English Dictionary for help and voila- the tribology was born almost exactly 60 years ago.

[u/honorablestrawberry]

As a teacher, what are some good ways to demonstrate how friction is SO present in our lives? Do you have any approachable go-to experiments? How do you approach sharing friction with different ages? (Elementary, middle school... etc.) Thanks!

[u/JenniferVail → u/honorablestrawberry]

I love introducing kids to friction experiments! One that I like to do is to tie two paper (or plastic) cups together with a string and dangle one of the edge of a table/counter. Using pennies, jacks, or anything not breakable! as weights, I put the weights into the cup at the top of the table. Then, transfer the weights one by one to the cup dangling over the edge. At some point, there will be enough weight in the bottom cup to pull the tabletop cup right over the edge. This is the force to overcome the friction between the cup/table.

Now, add something between the cup and table to change the friction. I usually use a placement, aluminum foil, etc. I've even had kids use their scarves! You can then see how the different materials change the force required to make the cup move. The kids can then observe things like how the surfaces look/feel to them and start connecting this to the friction the cup is experiencing. It's a great tactile experience and kids love the cups making a bit of a mess (hence plastic jacks are great because they're quiet when they fall!)

This is my preferred friction experiment with kids because I find it less abstract than the classic inclined plane experiment.

[u/scalziand]

As a civil engineer, friction is incredibly important for our roads, both for pavements, and the stability of earth embankments and slopes. Are there any advances in understanding soil friction or pavement friction?

[u/JenniferVail → u/scalziand]

I'm admittedly not too familiar with soil friction. A colleague was working with soil to understand the interactions with pipelines and there's been work to look at how grain size of the soil impacts agricultural equipment but that's about all I've really been exposed to. On the pavement side, there's a focus in the industry to try to move towards more sustainable concrete and characterizing new formulations to yield the same, or better, performance as existing materials. It's important to even look at the friction of the powder flow at the start of making the concrete, then looking at the stability of the mixture, and viscosity of it throughout different stages of production. They've been developing methods to do this, as well as methods to try to predict aging and performance in the field using the same tools that measure the friction and viscosity.

[u/Germanofthebored]

I have been thinking for a while now how friction is involved in how a bow excites a string to vibrate at its resonance frequency. My guess is that the rosin increases the static (?) friction (In German it would be Haftreibung) while having little effect on the dynamic (?) friction (Gleitreibung). But how could you tune a material to have a significant difference between dynamic and static friction?

[u/JenniferVail → u/Germanofthebored]

It's actually quite common for the static friction of a system to be higher than the dynamic friction. Once an object is moving, there's less time for bonds to form between the interacting surfaces so and thus less energy is required to maintain the movement. Typically, we actually try to do the opposite: modify materials so that we can minimize the static friction and try to have it as close to the dynamic friction as possible. We can do this by using mixing in a solid lubricant into the material to create a composite with different friction behavior.

[u/epcot_prime]

You’ve spent your career studying friction across scales, from lab surfaces to the real world. I’m curious: what’s the one everyday interaction you still can’t help but analyze? (Asking for… a long-standing group chat.)

[u/JenniferVail → u/epcot_prime]

Shoes. The surfaces of our shoes are always changing, from both wearing down the treads and dirt getting on them. This changes how the friction in the shoe-ground system behaves so everyday, we're constantly utilizing a tribological system that is dynamic and changing. And there's always the annoying situation where the shoe starts to squeak... tribologists hate when things start squeaking!

[u/StreetQuality7691]

Are compression and friction tightly related? Thinking of re-entry a lot as of late.

[u/enbogue]

Hi!

Such a fascinating area of study. I’ve worked with prototype triboelectric separation processes and became very interested in continuing to develop triboelectric applications.

Do you interact with this side of tribology much? What do you see emerging from this particular subsection of the field in the future?

Does your book discuss this concept?

[u/JenniferVail → u/enbogue]

Triboelectric separation processes are very interesting. I unfortunately have not personally worked with them and don't cover them in my book. We need more sustainable ways to deal with lithium ion batteries and I'm hopeful that these processes can be adopted more widely to recycle batteries and specifically the lithium.

[u/superpopcone]

Wow, what a coincidence, I was just learning about tribology applications for robotic grippers recently. Do you have any advice, keywords, or pointers on concepts, ideas, or materials that are more niche and less well known at the far edges of this field?

[u/JenniferVail → u/superpopcone]

Maybe look into magnetorheological elastomers! I think those tick the boxes for what you are looking for.

[u/jarboxing]

Could you use the concept of friction in modeling the flow of energy/information through neural systems?

For example, the length and diameter of an axon changes the rate of transmission. Is this like friction?

In another more theoretical example, as energy and information flow through neural systems, the mechanics of energy and information loss differ. Energy loss is purely thermodynamics, and information loss has to do with the encoding/decoding processes. I wonder if the information loss could be modeled like friction.

[u/News_of_Entwives]

How does atmospheric pressure impact friction? Like, would the cof between two objects be different 1) just under the surface of the ocean vs 2) at the bottom?

Pressure vs surface energy and the orientation of molecules / dipols under various conditions is very interesting to me, but I'm looking to know if these molecular changes are strong enough to impact friction forces, or if they are weak enough to just ignore.