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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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    $\begingroup$ 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. $\endgroup$
    – user107153
    Jun 3, 2016 at 7:23
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    $\begingroup$ 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. $\endgroup$
    – Luaan
    Jun 3, 2016 at 9:37
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    $\begingroup$ Water is good at lubrication, the problem is it vaporizes like crazy. What good is a lubricant that is gone in 5 minutes? $\endgroup$
    – Davor
    Jun 3, 2016 at 11:53
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    $\begingroup$ @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. $\endgroup$
    – DK2AX
    Jun 3, 2016 at 12:07
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    $\begingroup$ 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). $\endgroup$
    – Luaan
    Jun 3, 2016 at 12:49

6 Answers 6

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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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    $\begingroup$ What about surface tension and wetting? Shouldn't they be important parameter too? $\endgroup$
    – user_na
    Jun 3, 2016 at 7:34
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    $\begingroup$ Yes, they are important, too. Wetting generally decreases with viscosity but they're not identical. $\endgroup$
    – Luboš Motl
    Jun 3, 2016 at 7:37
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    $\begingroup$ 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. $\endgroup$
    – Neil
    Jun 3, 2016 at 9:46
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    $\begingroup$ 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. $\endgroup$
    – Luboš Motl
    Jun 3, 2016 at 10:02
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    $\begingroup$ 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? $\endgroup$
    – DK2AX
    Jun 3, 2016 at 10:37
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A good lubricant tends to effectively minimize direct contact among components of any device that need it

Keeping this in mind, 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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    $\begingroup$ An excellent observation. Viscosity is not the deciding factor here. $\endgroup$
    – Neil
    Jun 3, 2016 at 9:49
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    $\begingroup$ 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. $\endgroup$
    – Luboš Motl
    Jun 3, 2016 at 10:04
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    $\begingroup$ 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. $\endgroup$
    – Drunken Code Monkey
    Jun 3, 2016 at 16:39
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    $\begingroup$ talc is another solid lubricant, graphite isn't rollers, its sheets, $\endgroup$
    – Jasen
    Jun 4, 2016 at 5:56
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    $\begingroup$ @LubošMotl Graphite is not a microscopic roller bearing it is a lubricant. There are no graphite nanoballs rolling between the surfaces. In the begining of the sliding the graphite covers both counterparts filling all voids and sliding contact between, say steel-steel/graphite changes to graphite-graphite. $\endgroup$
    – Crowley
    Jun 7, 2016 at 13:58
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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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    $\begingroup$ This is the only full answer here as yet. $\endgroup$
    – Selene Routley
    Aug 1, 2017 at 1:24
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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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    $\begingroup$ 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. $\endgroup$
    – DK2AX
    Jun 3, 2016 at 12:10
  • $\begingroup$ "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. $\endgroup$
    – Mazura
    Jun 4, 2016 at 4:29
  • $\begingroup$ "[...] 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 $\endgroup$
    – Mazura
    Jun 4, 2016 at 4:29
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    $\begingroup$ "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). $\endgroup$
    – Mazura
    Jun 4, 2016 at 5:03
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    $\begingroup$ @andynitrox Turpentine is a pretty good lubricant, in the right circumstances. Under most "human-scale" conditions, it's too volatile to be good for this. It's also a good solvent in many circumstances, and dissolving your working parts is a bad trait for a lubricant. It does, however, benefit from both a fairly low viscosity and from non-polarity, both of which are leading factors in overall lubricant performance. Ultimately, though, the question of what makes good lubricant is just too complicated an issue to be answered by one or two base material properties. $\endgroup$
    – Matthew Najmon
    Jun 6, 2016 at 20:53
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@tbf is right; lubrication, and tribology in general, is complicated. That's why there is that high effort to understand it and to design advanced materials.

There are several phenomena that cause the friction force exist and the ones you have neglected causes that oils are superior to water in most industral applications.

In dry sliding we can identify adhesion (dominant for two super-smooth glassy surfaces), asperity skipping and deformation (dominant for two rough and hard surfaces) and ploughing (dominant for sliding hard rough surface against soft one). Some add chemical bonding as separate cause, others consider it as a part of adhesion and another ones consider it as a condition.

The lubricants are chosen to lower the friction and wear and there is no universal superlubricant ideal for any application. One must consider:

  • All materials in sliding contact;
  • Range of the applied forces;
  • Temperature;
  • Sliding velocities;
  • Environment (air/liquid flow, chemical surrounding, sliding frequency, debris presence, ...)

To the question, the mineral oil is good lubricant in case of sliding two metals because it passivates the surfaces and prevents their contact (adhesion is therefore neglected), if the viscosity is low enough it also decreses the interaction between asperities of both surfaces. On the other hand, water can chemically react with the surfaces and because of its low viscosity cannot prevent asperity interaction. But it says nothing in general.

Notes:
The most common lubricant on the Earth is water - joints in bodies of all vertebrates are water-lubed.
As Abhinav noted, graphite and all solid lubricants mentioned in comments below his answer are good lubricants and you cannot define viscosity there.
Turbomolecular pumps use magnetic bearings where "lubricant" is vacuum.

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Water can not bear normal loads as well as oil.
Water is bound to escape from high pressure bearings to lower presser places in an open lubrication loop leaving bear contacts.
Water can create bubbles around cavities and corners and break the laminar flow which will compromise the separation of moving parts. Water will react chemically with surfaces.
There are lubricants mechanically designed to be near water viscosity but inert chemically and with wider temperature tolerance like brake fluids.
Many of the high speed revolving parts have been designed taking advantage of load bearing property of oil to actively and dynamically balance the system into its proper configuration under a range of different loadings or RPM, which is more practical with oil. Automatic transmission is but one case.

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