Homemade Fog Machine

Just thought I would share one of my favorite new Physics demos with the blog. We're currently covering fluids and thermodynamics and I thought this was a really great and cheap way to demonstrate work, internal energy, and phase changes. All you need is one of those 12 oz disposable clear plastic water bottles that you have in your recycling bin (any kind).

Peel off the label so you can see through the bottle. Put 1/2 inch of water inside the bottle and put the lid on. Twist the bottle until it forms a little "bow tie" shape. It is important to do this in one fluid motion. This will make some alarming noises and feel a little scary - probably a good idea to point the lid away from people just in case.

Let go. The bottle should "bounce" back to its normal shape. Inside, you should see whitish fog form. If you take the lid off, you can make fog rings by squeezing the bottle. This will only work once per bottle.

What's happening? When you twist the bottle, you are compressing the gas inside it. This increases the internal energy of the gas, warming it up and also causing more water to enter the vapor phase. When you let go, the gas rapidly expands to fill the new larger volume of the water bottle. This quickly cools the gas and causes the water vapor to transform back into water, or fog.

Let me know if you give it a try!