How to Reduce Friction Force: Lubricants, Oils, and Volatile Substances Explained
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Volatile substances reduce friction. During experiments with a steel plate and a wooden block (more on this: Sliding friction and rolling friction), a curious event took place: someone once brought a small bottle of valerian tincture into the laboratory and opened it — the smell of valerian quickly spread through the whole room, and suddenly the friction force between the plate and the block dropped.
The episode sparked plenty of jokes, because valerian is well known as a calming remedy taken by nervous people, and here it had turned out to be a "cure for friction" as well.
A volatile substance acting as a "cure for friction".
How do volatile substances reduce friction?
Volatile substances have been shown to reduce the force of friction. Valerian tincture was not the only compound capable of this effect. The same behaviour was observed whenever a saucer of acetic essence was carried into the laboratory or a bottle of perfume was uncorked.
In all likelihood, volatile substances such as the ether in which valerian drops are prepared, along with vinegar and similar liquids, settle on the metal surface as extremely thin films and separate the rubbing surfaces from one another. It is possible they fill the tiniest pits and irregularities of the metal, smoothing them out and thereby lowering friction; it is also possible that these particles simply roll between the contacting surfaces like little balls.
Friction between slightly soiled smooth surfaces is often lower than between perfectly clean ones.
Why does oil lower sliding friction so sharply?
Sliding friction decreases especially sharply when the rubbing surfaces are lubricated with oil. A viscous oil fills every irregularity and coats the metal surface with a continuous film. In that case there is no longer metal moving against metal, but one oil film — the one clinging to the rubbing surface — moving over another film that adheres to the opposite surface.
The force of sliding friction is dramatically reduced as a result.
Which lubricating substances can be used?
Oil is not the only lubricant used to reduce friction; other liquids sometimes serve the same purpose, including water, mercury, soap solutions and the like. Solid substances ground into an extremely fine powder can form what is known as a dry lubricant.
For this purpose graphite dust is frequently applied.
