Why does a penny hold more water drops than drops of oil?

Answer 1

Small drops of liquids tend to be spherical because of their surface

tension. Surface tension is the result of intermolecular attracting

forces. When you put a small drop of a liquid on a solid surface, the

inermolecular forces of the solid surface will tend to attract the liquid

molecules. Depending on the magnitude of the different forces, the solid

surface will present certain degree of 'wettability'. If you compare how

a drop of water sits on a clean penny and on a penny that you rubbed

with your fingers, you'll notice that the drops contact angle differ.

The drop on the clean penny will cover more contact area (more wettable

surface) than the drop on the 'oily' penny (less wettable surface). Oil

reppels water because the intermolecular forces between the water and

oil molecules are very weak compared to the intermolecular forces between

similar molecules.

Now, water intermolecular forces are paricularly strong, and so the drop

of water on the penny will resist more than the oil molecules to be spreaded

on the surface of the penny. Oil drops tend to wet more the surface of the

penny and will tend to occupy more area. And so you'l get fewer oil drops

on the penny's surface.

Answer 2

Water has the ability to form hydrogen bonds with its self, which allows the individual water molecules to "hug" each other. This prevents the big blob of water on top of the penny from falling off the edge. This is the same reason (kind of) why water "climbs" up a paper towel when you place it in a pool of water. As for oil, the attractive forces between each oil molecule is not as great because there are not strong hydrogen bonds. So, the oil will fall off the penny because the "hugs" between each molecule of oil is relatively weak.

Answer 3

Soap lowers the surface tension of the water, allowing for a smaller drop before the coin runs over.