Re: Why does a water film increase friction between some surfaces ?

Hello Nicolas,

Thanks for a good observation and an interesting question!

The short answer is that water will not increase friction, or act as an "antilubricant". But, there is no doubt as you describe, that a wet, flat bottom glass will require more force to push across a smooth surface than a dry glass. The cause of the increased force in not friction, but atmospheric pressure!

When you slide the glass across the smooth marble surface, you will do this by pushing the glass at a point somewhere above the marble surface. This push will tend to tip the glass ever-so-slightly due to the tipping force (moment) that you are exerting on the glass. This tipping up of the base will increase the volume of space contained between the base of the glass and the surface. With a dry glass, air can immediately rush in to fill this increasing volume. This is (only) because the surfaces are imperfect enough to give a path for the air to flow in.

Now, what happens with the wet glass? The liquid flows under the base of the glass and oozes out surrounding the perimeter of the base and forms a very effective seal between the glass and the surface. Now, as the volume tries to increase from the tipping force, air cannot move in to fill this space, and a low pressure, or partial vacuum forms under the glass. This vacuum tends to "stick" the glass to the surface. And, this process builds on itself- as the glass "sticks", you must exert more force to push it, which increases the tipping force which increase the vacuum!

As an interesting sidelight- if you have near perfect surfaces in contact, the "sticking" effect will take place, even when dry. Have you ever seen two clean glass panes stuck together? Because of their very smooth, flat surfaces, it can be difficult to separate them. This is the same phenomenon, the separating force tries to increase the volume between the panes, and the resulting low pressure holds them together.

As far as the "singing glass", I am pretty sure that the water is producing the same sealing/low pressure effect that, but just on a microscopic scale. There are tiny areas between your finger and the glass where the low pressure momentarily "sticks" your skin to the glass to form the stick-slip motion, and thus the beautiful music!

Best Regards,

Jay Shapiro