# Friction - its root cause?

At a fundamental level (i.e. at the atomic level), what is friction? Obviously it is not a fundamental force, so I assume it has its origins in electromagnetism. Further, it must be more than a manifestation of hydrogen bonding since friction affects all matter, yet not all matter contains hydrogen. Is it due to a van der Waals’ force? (I suppose that’s possible, but would such a force always be attractive? i.e. friction is always attractive, at least insofar as it prevents things from slipping past each other).

OK, I give up - what is going on at the atomic level when we have friction at the macro level?

(And, while we’re at it, let me ask - is viscosity essentially the same as friction?, except that it refers to a fluid whereas the term ‘friction’ is used to denote the same phenomenon occurring with solids)

Thanks!

Great question. I’ve wondered this too. Here’s my guess:

No surface is perfectly smooth. If two perfectly smooth surfaces would rub against each other, they’d just glide off, frictionlessly. But in the real world, the surfaces are not perfectly smooth, they have little bumps on them. What happens when a bump from surface A hits a bump on surface B? The harder one knocks some atoms off of the softer one, and then it can keep going. And the energy lost in pushing those atoms off is friction. When the bumps are so big and hard that they can’t be smoothened out, the surfaces don’t have enough energy to overcome the friction, so they stop moving.

Am I close?

Sorry Keeve but Wikipedia has a different take:

I agree with Keeve. I have always been taught tthat friction occurs at the macrscopic level. Surfaces are not smooth and surfaces consist of electrons so they try to repel each other. Each surface has little bumps that repel, and thus retard motion, when they encounter the bumps in the other surface.

Friction is a companion field to lubrication. Lubricants work because they form a thin film that separates the surfaces by a greater distance than the roughness of the surfaces. This means that the only retarding force is the shear within the lubricating film.

Keeve’s idea is very interesting (i.e. loss of energy due to “knocking off” of atoms).

I had looked at the Wiki article, Askance. In the part that you quote, there is actually very little said about the (putative) mechanism of friction other than, “chemical bonding between the surfaces - by the stickiness of the two surfaces”. So friction is “stickiness”. Not too helpful! At best, that Wiki statement is pretty nebulous. In fact, it really just substitutes one mystery for another. In place of “friction” we have “chemical bonding” or “stickiness”. My question remains, what is the nature of friction (or its possible equivalent, “chemical bonding”).

But don’t forget the van der Waals forces. Without the van der Waals forces, geckos would slide down off walls.

After reflecting a bit more on Keeve’s idea, it occurs to me that atoms wouldn’t need to be actually knocked off. Simply having one molecule bump into other would start both of them vibrating. Wouldn’t the vibrations carry away energy (i.e. some of the mechanical energy being put into the system is lost via vibration, hence more mechanical energy must be put in; if not, the movement induced by the mechanical energy must diminish).

Sorry, I didn’t think I had to explain chemical bonds:

Something I recall from my chemistry textbooks: With some metals (such as copper), if you polish two pieces of very high purity as smooth as you can make them, they will have an extremely high co-efficient of friction when brought together (to the point of actually bonding together), much higher than two rough or impure pieces. In this case, the friction comes much more from electromagnetic attraction than mechanical unevenness.

I’m afraid this whole idea of roughness at the atomic level is misconceived. The first thing you need to do when you’re down there is discard all sense of what you are used to. An atom is almost entirely empty space and, absent atomic forces, two could pass through each other virtually every time without even noticing each other. It is almost solely these atomic forces that generate friction and stickiness, as the gecko example shows.

Right you are.

And a followup question. Two gauge blocks can be wrung together and they will adhere. Is this van der Waals or air pressure, the air having been driven out from between the blocks?

Never mind, Wiki says it’s both. Along with a light film of oil, wringing excludes all the air and brings the surfaces to close together that molecular interaction occurs. I assum the attractive part of the interaction is van der Waals force.

And I didn’t think I’d have to restate (see my OP) that, yes, it presumably is due to some form of chemical bonding. My question is what type of bond.

As an aside, why be so snide in GQ? Do you think that I would ask a question, or request a clarification, just for fun?

I have always understood this to be one of the Great Mysteries of physics. A problem that is mind-bogglingly intuitive and simple, yet the greatest minds in history have been unable to crack it.

I’m betting that you’ll get nothing in this thread but guesses, until a real physicist shows up and explains that no one currently knows the definitive answer.

(The wiki paragraph quoted a couple of times already comes from this (dubious?) source, which also states…)

Then there’s Casimir friction, which likely has large effects on both protein dynamics and nanomachine design.

I think the interactions atom-by-atom amount to high and low spots in a force field, and atoms going over the high and low spots vibrate and transmit their vibration back into the solids, where they amount to heat. The high and low spots in a force field are the stuff of the van der Waals interactions, and perhaps other interactions too, but primarily van der Waals. On a small enough scale, and with a physics-oriented mindset, I think the distinction between different kinds of chemical bonds, and indeed between chemical bonds and other interactions, breaks down. It’s all electrostatic attraction and repulsion.

Note that extremely smooth surfaces like crystalline faces and polished silicon wafers and flame polished glass still exhibit plenty of friction.

I would presume covalent bonding (which is the sharing of pairs of electrons between atoms, typical chemical bonding). Your OP mentions “hydrogen bonding” - what did you mean by that?

The problem in offering a discrete answer to this question is that the concept of friction covers a wide variety of phenomena at various macroscopic and microscopic levels (down to the level of electrochemical bonds) that may or may not be interrelated and thus, no one explanation is going to suffice to describe all surface friction phenomena. To compound this, the term “friction” is also sometimes used to describe internal continuum mechanical losses (hysteresis) in solids and Newtonian fluids and lossy behavior in aeroelastic, turbulant conditions with non-Newtonian fluids. I consider this use of the term friction to be inappropriate, or at least highly confusing, but it is frequently use in technical jargon and therefore an accepted use of the term.

If you want a simple answer for solid surface-to-surface static friction can be broken into two essential domains; that caused by surface roughness which results in normal forces between “interlocking” surface features (think of dragging two pieces of sandpaper against one another), and the tangential forces that result from having to break attractive electrical bonds between two surfaces, or between a surface, a adhesive medium, and another surface (like two plates of glass with a thin layer of water in between them). When you get to dynamic friction things get far more complex; surfaces that had low friction statically can suddenly have high friction dynamically (particularly in the solid-fluid-solid case where the internal viscosity of the fluid suddenly dominates the effect), and conversely, rough surfaces with high static friction may have much lower dynamic friction because of the intermittant contact permitted by surface roughness. In addition, the basic character of the interface may change; for instance, in a car engine, when it’s not running the shafts may sit directly on the bearings, but when it is spinning at high speed it is supported by a thin but pressurized layer of oil which prevents any surface-to-surface contact, minimizing wear.

As a practical matter, at least on the high school physics or machine design level we represent solid surface-to-surface friction as a single coefficient for static friction and another one for dynamic friction, or perhaps a curve for dynamic friction. In reality, once you get into the nanomachine/cellular/molecular level, “friction” behavior dominates inertial mechanics and you have to cope with the actual individual mechanisms rather than just pretending that it’s all one big bag. The entire phenomena has to itself the field of study of tribology which is both absolutely fascinating and fantastically boring at the same time, depending on how deeply you delve into it and what your interest is in applied continuum mechanics.

So…it’s complicated; too complicated to sum up in a couple of paragraphs. Anything important usually is. I wish I could offer up some general reference for the layman on the topic, but I don’t have anything in my library that breaks it out at that level.

Stranger

If there’s a library handy get The Feynman Lectures on Physics and look at volume I chapter 12 pages 3 to 5. Here’s a short excerpt:

I forgot to mention that the interactions that cause the “atoms to seem to cling together” are the van der Waal / London dispersion forces.

ETA As I believe someone has already mentioned.