Foggy mirror...

misskitty

Whenever you take a shower the bathroom mirror fogs. I discovered if one rubs a thin layer of soap on the mirror, then gently rubs it in with a cloth, the mirror does not fog. I could not think of any other reason other than some chemical reason. Why does this happen?

ShawnD

Fog is actually millions of little water beads. Soap reduces surface tension between water and glass causing the water to sleet rather than bead. The water is still there; it's just more together.

Gokul43201

I think you meant to say 'adhesion', rather than 'surface tension'.

brewnog

Soap will indeed reduce the surface tension of the beads of water, thus making the wetting angle of the droplets much smaller, and tending to allow them to form a film rather than collection of droplets.

misskitty

So does the water just sheet off the water?

ShawnD

Yeah pretty much. The same thing will happen after you wash your car.
Pay attention to how water sits on the hood of your car:
-before you wash it
-after you wash it (with soap)
-after you wax it

Before you wash it, water will sort of stay on and sort of fall off the hood. Cohesion and adhesion are not too different.
After you wash with soap, water will just slide off the hood. Adhesion is much stronger than normal.
After you wax, water will form little beads. Cohesion is much stronger than normal.

That beading effect is why I will never wax my car. If your car is left out when it rains, rain water (which is far from clean) will bead all over the car. With cohesion being so strong, there is a lot of added friction between water and your car, so that water just stays there. The water eventually evaporates after the rain has stopped, and the crud that was in the rain water is now all over your car in the form of brown dots. Your car looks like crap after that, and it's incredibly difficult to clean.

Gokul43201

This is true. The soap molecules (stearates I guess) will reduce the surface tension of water. I was a bit confused by the "between water and glass" bit.

hypatia

I use rain-x no fog{for cars} for my indoor mirrors, it works for a really long time with one application with out the haze that soap can leave.

brewnog

Onion or Potato juice also does the same thing.

hypatia

But I would'nt want the smell of onion, or any food that might attract bugs.
When I dive I spit into my facemask to keep it from fogging. But I can't get myself to stand and spit on my mirror..lol