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10

How come mineral oil is a better lubricant than water, even though water has a lower viscosity?

Sketch of the problem

When two surfaces slide over each other with a gap filled with a fluid, the different layers of the fluid are dragged at different speeds. The very top layer touching the top metal surface will have the same speed as the surface itself, while the bottommost layer is stationary. The speed in the layers between is distributed linearly and there exist friction forces between those layers that slow the movement. Those frictional forces should be reduces however, if a fluid with a lower viscosity is chosen.

How come this is not so?

Does it have to do with water's polarity, so that it sticks to surfaces in a different way than oil?

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8  
I think lubrication is an extremely complicated business. However you don't in fact want the viscosity to be too low: a critical purpose of lubricants is to prevent metal-to-metal contact which is generally rapidly fatal to machinery, and in order to do this it needs to be fairly viscous. – tfb yesterday
10  
Water isn't always a worse lubricant. Ever seen those "Wet floor" signs? Floors can get extremely slippery by introducing a tiny amount of water, much more so than with oil. Ice sheets or ships are another great example - take a water-borne ship, put it in oil and it will experience a lot more friction than in water. – Luaan yesterday
6  
Water is good at lubrication, the problem is it vaporizes like crazy. What good is a lubricant that is gone in 5 minutes? – Davor yesterday
2  
@Luaan, clearly water is not always a worse lubricant than other substances, but it seems to me that under "household" conditions (such as rubbing hands or sliding a shoe on pavement), oil does a better job. The case of a ship swimming in oil vs water is only dependent on the viscosity of the fluid, it is just a regular laminar drag problem, and I am not sure if it is applicable for the original question. – andynitrox yesterday
3  
Our skin developed with one goal being "don't slip on watery surfaces". So you're looking at material specifically "designed" to prevent slippage, just like the rubber soles of your shoes. Take an old-school shoe with no rubber, and you'll see that it's extremely slippery on wet surfaces - medieval europeans went barefoot most of the time, especially in winter. Oil isn't encountered commonly in nature, so there wasn't much pressure to making skin resistant to slipping on an oily surface - in fact, the most common oil source is our own skin, "designed" for lubrication (among other things). – Luaan yesterday
up vote 69 down vote accepted

Your derivation is composed of correct statements and indeed, if something is known to act as a lubricant, we want the viscosity to be as low as possible because the friction will be reduced in this way. For example, honey is a bad lubricant because it's too viscous.

However, your derivation isn't the whole story. The second condition is that the two surfaces must stay apart. If you use a lubricant with too low a viscosity, the surfaces will come in contact and the original friction will reappear.

So the optimum lubricant is the least viscous liquid that is viscous enough to keep the surfaces apart. Which of them is the optimal one depends on the detailed surfaces and other conditions. For example, there exist situations in which water is a better lubricant than oil – for example when ice slides on ice. Some of the ice melts and the water is why the ice slides so nicely.

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4  
What about surface tension and wetting? Shouldn't they be important parameter too? – user_na yesterday
6  
Yes, they are important, too. Wetting generally decreases with viscosity but they're not identical. – Luboš Motl yesterday
5  
Grease is very viscous but it makes a good lubricant. I think that the property that makes a liquid a good lubricant has nothing to do with its viscosity. Claiming that honey is a bad lubricant because it's too viscous is just as incorrect as claiming that oil is a good lubricant because it isn't viscous. – Neil yesterday
4  
Grease surely isn't a good lubricant for engines etc. It's sticky and one can feel it. The OP is absolutely right that at the end, the friction force is proportional to the viscosity. – Luboš Motl yesterday
3  
Which physical property of a fluid (or a powder, for that matter) controls how "easy" it is for the plates to touch? Is that also the viscosity? Or maybe also the surface tension, as @user_na mentioned, since it would oppose a droplet to be squashed, when force is exerted on the top plate to reduce the gap between the plates. Can mercury for example be used as a lubricant in at least some cases? Is there also any work on the relation between those values? – andynitrox yesterday
up vote 16 down vote

From my point of view, viscosity is not the only factor involved. Grind a graphite pencil lead, and it makes a mighty fine lubricant. It might be that in the case of water placed between two surfaces, a water droplet which was supposed to act as an intervening layer, gets displaced easily, resulting in untimely contact between the otherwise lubricated parts, resulting in wear and tear, while oil components tend to stay in place as the intervening medium and act as lubricant. Graphite obviously being a fine powder does not behave as water.

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1  
An excellent observation. Viscosity is not the deciding factor here. – Neil yesterday
7  
Grinded graphite isn't a liquid (which is why the viscosity is ill-defined) and I think it shouldn't even be called a lubricant. It's a microscopic roller bearing. It has really nothing to do with the question. – Luboš Motl yesterday
8  
Graphite is not the only solid lubricant. There are solid and semi-solid lubricants made from silicon, ceramics, molybdenum disulfide, boron nitride, and polytetrafluorethylene. The common factor in these lubricants is their molecular structure and weak bonding between molecules. – Drunken Code Monkey yesterday
    
@DrunkenCodeMonkey obviously the graphite part was not needed in my answer, but still, appreciate the info. – Abhinav yesterday
3  
talc is another solid lubricant, graphite isn't rollers, its sheets, – Jasen yesterday
up vote 9 down vote

The parallel plate situation that you describe is not the typical condition encountered in practical lubrication operations. In addition to facilitating the surfaces sliding over one another, the lubricated bearing must also support a normal load. To do this, the gap between the surfaces varies with location along the bearing. For example, in a journal bearing, the shaft will not be concentric with the bearing sleeve, and, in a slider bearing, the moving surface is at a a small angle to the stationary surface. These features of the geometry allow pressure to build up in the gap between the surfaces as a result of a combination of drag flow and pressure flow. This causes an upward normal load on the sliding member. The higher the viscosity of the lubricant, the greater the pressure buildup and the greater the normal load that the bearing can support. That's why we use lubricants with higher viscosity than water.

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up vote 8 down vote

Why Oil is Slippery

Explaining why oil is slippery requires a look at its chemical properties. First, oil is non-polar, which means it does not have a positive or negative charge. Some molecules, like water, have a “charge distribution,” which means the molecule acts almost like a battery, part of it has a positive charge and part of it has a negative charge. The result, because positive is attracted to negative and vice versa, is that water and other “polar” molecules stick to each other. Oil doesn’t have this problem, so one oil molecule can slide past another more easily than one water molecule can slide past another.

Adding to the slipperiness of oil is its tendency to form distinct layers through forces called Van der Waals forces, or more specifically London Dispersion forces (a type of Van der Waals force). These forces, which are the weakest known in science, can help old things together, which would increase friction. However, oils have the unique property of forming forces only within layers because the molecules are essentially planar. Planar just means that molecules are flat as the diagram below emphasizes and only take up space in two dimensions rather than three. Without projections to attach to, forces can only be distributed within the plane and so there are no forces to bond one layer to the next. Thus, two layers of oil don’t bond to one another to any great degree. ...

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1  
That would be in like with graphite being a good lubricant, since it also has weakly-interacting plane structures that are able to slide past each other. But if polarity was the one of the leading factors, turpentine for example would be an exceptionally good lubricant. – andynitrox yesterday
    
"Carbon-graphite is a self-polishing and dimensionally stable material. Shafts polished to a fine surface finish will polish the carbon-graphite material to the same fine finish, so a thin hydrodynamic film is sufficient to provide lubrication. – Mazura yesterday
    
"[...] Plastic or polymer bearing materials often fail in submerged applications because of their tendencies to swell, soften or deteriorate. Metallic bearings are often unsatisfactory because the hydrodynamic film provided by low-viscosity liquids is not thick enough to overcome the strong atomic attraction between metal bearings and the metal shaft." –flowcontrolnetwork.com – Mazura yesterday
    
@andynitrox - Turpentine as a lubricant would mean it's submerged (you can't reliably lucubrate with a volatile solvent otherwise) and the bearing having been designed for it. – Mazura yesterday
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"London dispersion forces are particularly useful for the function of adhesive devices, because they don't require either surface to have any permanent polarity." I think it largely has to do with the adhesion of oil being greater than that of water, in conjunction with being the correct viscosity for the application (as the leading answer suggests, if your lubricant can become displaced it won't work). – Mazura yesterday

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