Physics
Interfacial Tension
Quick fact
The interfacial tension between oil and water is about 50 mN/m, strong enough to keep salad dressing separated until you shake it vigorously.
Why this is interesting
Why does a drop of oil instantly form a perfect sphere when released underwater, while a drop of water on a greasy surface flattens into a puddle? The answer lies in a hidden tug-of-war at the boundary between two fluids.