The science of leaving ominous messages on bathroom mirrors

Scribbling on a steamed-up mirror can be a fun way to give one's reflection a mustache after a warm shower or leave death threats in horror movies. But what's the science behind it?

It's reasonable to associate the ghostly lines on the bathroom mirror after a long shower with the greasy handprints on windows and glass doors. Sometimes they are the same. Even clean fingers have oil on them, and that oil rubs off on anything. On clear glass, it distorts light coming through it and leaves a print. On mirror glass, it leaves an inhospitable home for water, so droplets don't accumulate. If a person wipes their hand across a dry mirror, more of the oil is likely to wipe off, and that's what happens.

Long shower aficionados, though, know that images will also appear when wiping a finger along the mirror's already-wet surface. A line will form and hold, even if the shower continues.

The science of leaving ominous messages on bathroom mirrors

In this situation, oil doesn't actually accumulate on surface of the mirror. Since the mirror is already wet, the finger just glides on the water. The images aren't the result of the famous dispute between oil and water. Instead, they're due to differences in surface area.

A droplet of water has a very high surface area. The water molecules all pull together, exposing a long, rounded edge to the air. Touching these droplets, depending on the amount of force used, doesn't always wipe them away. More often, it collapses them – breaks the surface tension of the droplets and makes them form a thin film.

This film has less surface area than droplets of water. As the humidity decreases, or as the mirror heats, the water on it begins to evaporate. The more surface area is exposed, the more water will evaporate – the way water in a shallow puddle will evaporate faster than the same amount in a deep bowl.

Since water attracts water, droplets will accumulate more water around themselves, tugging molecules of steam into their bodies. A film of water will do the same. It pulls water in all along its even surface, instead of into clumps as droplets do. And so, the image will hold a lot longer than it seems like it should.

[Via Does Anything Eat Wasps]