a phenomenon of adhesion that happens between two smooth, flat surfaces. The surfaces seem to be "glued" or "stuck" together, while in fact there is no glue.
It is not entirely clear why wringing happens (the physics of surfaces is still very much an evolving field), but we think that there are three forces in action:
- Air pressure from the surrounding environment; this is a factor, but it is clear that this is not enough.
- surface tension from oil/water that remains on the surfaces in microscopic amounts
- electronic attraction forces (again, probably not enough)
For wringing to occur, the average irregularity of the surface has to be on the micron (or microinch scale). Interestingly, after two surfaces have "wrung" together, the wringing layer has a measurable thickness, on the order of 25 nanometers. Things are also more complex than that, since the wringing layer thickness can change in time.
Wringing has a practical application when stacking gage blocks or testing surfaces for flatness agains either gage blocks or optical flats.
some info from http://www.starrett.com/gag-blk-wrng.html, other from www.nist.gov | 
