In the epic battle of soap bubble versus drop of water, the surprising winner is the bubble. This is because of the bubble's ability to "self-heal" as the drop first splashes down through it, and then back up through it again. We'll give you some of the physics behind why the bubble can heal itself.

Seeing the soap bubble pierced, not just once but twice is cool enough, but then the colored drop of water washes down the side of the bubble like the bubble is a solid object. It's so cool. How does something so fragile self-heal? To understand, we look at the Marangoni effect.

The effect is no more complicated than the tendency of molecules in a fluid to pull away from liquids with low surface tension, and towards liquids with high surface tension. It's responsible for, among other things, the "tears of wine" that you see trickling down in a wine glass.

Soap lowers the surface tension in water. This is part of why bubbles can form. If you somehow arranged water molecules in the shape of a soap bubble, but didn't add any soap, the water molecules pull at each other strongly enough that the bubble would pull itself apart.

In a soap bubble, the soap-to-water ratio isn't homogeneous. Some places have a little more soap than others. Those places with mostly water, have a higher surface tension. The molecules pull together there, and as they pull together, they bring more soap molecules into the mix. Meanwhile, places with too much soap stretch out until the soap molecules are so thinly dispersed that it starts pulling together again. Simple as it is, a soap bubble is a self-healing, or at least self-stabilizing, structure. When we see the droplet go through the bubble, it's dumping a lot of water into one part of the surface of the bubble, which causes the bubble to pull together around that part. The soap rushes in, keeps the bubble together.

[Via Science Daily]