Why is Water Left in a Cup?

Please, I would like to know why water never fully drains from a cup after drinking??Why is viscosity illustrated with spherical objects only, doesn't viscosity exist in objects of other shapes??Thank you very much.. Please reply..
- Tamar (age 17)
Yenagoa, Bayelsa State, Nigeria

The water sticks to the cup. That's largely because most cup materials (glass, ceramic) have some electrically charged spots on their surface. Water molecules tend to stick to other electrical charges the same way they stick to each other. The positively charged hydrogen atoms in H₂O can stick near a negative charge, and the negatively charged oxygen can stick near a positive charge.

Some materials, like teflon, stick very little to water. I bet if you had a teflon cup you could drain the last bit of water from it.

I'm not sure why viscosity was illustrated with a spherical object. Often the simplest examples of viscosity are illustrated with flat plates and a viscous fluid between them. Viscosity can be found in all sorts of different shapes of fluids. Perhaps what you saw was a calculation of the friction coefficient of a rigid sphere in a viscous fluid. The calculation is more complicated for more complicated shapes.

Mike W.

(published on 05/17/2015)