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Static frictional force depends on the coefficient of static friction; kinetic frictional force depends on the coefficient of kinetic friction. To minimize friction, one of two things could happen: Normal force could be minimized, or the coefficient of static/kinetic friction could be minimized. In a lot of real-world scenarios, however, it is hard to minimize the normal force between two objects, and so minimizing the coefficient of static/kinetic friction is important.

What techniques exist to minimize the coefficient of static/kinetic friction?

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    $\begingroup$ This is a very general question. Not sure it's not too broad for this forum. In general, smoothing the surface, changing sliding to rolling, adding space between the surfaces, and adding lubrication (oil, graphite, teflon, ball bearings, air cushion...) are the most common techniques. $\endgroup$
    – Floris
    Commented Sep 30, 2014 at 15:24
  • $\begingroup$ Are you trying to reinvent the wheel? $\endgroup$
    – b_jonas
    Commented Sep 30, 2014 at 17:34

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In general, smoothing the surface, changing the interaction from sliding to rolling, adding (air) space between the surfaces, and adding lubrication (oil, graphite, teflon, ball bearings, air cushion...) are the most common techniques.

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    $\begingroup$ +1, plus these techniques are based on some properties of materials (plus some mechanics) like surface tension, viscosity etc, which make the intermediate material to be able to move more smoothly on itself rather than on other materials $\endgroup$
    – Nikos M.
    Commented Sep 30, 2014 at 16:02
  • $\begingroup$ @NikosM. exactly. These lubricants typically become a "space between the surfaces" by virtue of their properties. $\endgroup$
    – Floris
    Commented Sep 30, 2014 at 16:16
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Friction is an adhesive process. When you touch two surfaces together the atoms at the two surfaces come into contact and form interatomic bonds. In the extreme case of very clean and smooth surfaces you can get cold welding.

So reducing static friction is basically a matter of stopping the surfaces from adhering to each other. For example applying a thin (i.e. molecular dimensions) layer of oil or grease will make the contact an oil-oil contact instead of iron-iron (or whatever the materials are made of). Even an adsorbed monolayer such as a surfactant will reduce adhesion, and many lubricating oils contain surfactants such as molybdenum dithiocarbamates and zinc dialkyldithiophosphate for this reason.

I take slight issue with Floris, because it's usually only possible to form the sort of oil or air films that he mentions when the surfaces are in motion i.e. the friction is dynamic. When relative motion is stopped the separating films will normally drain away and the surfaces will come into contact. Under these circumstances you need some material of the sort I describe above that will reduce the adhesive forces between the surfaces.

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Friction coefficient for ice and ice is minimum. The very high pressure on the ice due to the very narrow blades of the skates causes the ice immediately below the blades to melt. The thin layer of water (melted ice) between the blades and the ice surface reduces the friction between the blades and the ice surface and so the ice skater can glide easily on the ice.$_1$

If the temperature of the two materials are moderated properly, the similar technique can be used to reduce friction coefficient for other suitable materials. Coatings on the piles are also used to reduce the friction.

Credits: $_1$ Mini Science Encyclopedia (5th Edition) - Ang Woon Chuan

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