You’re lucky you asked that question today, because today is bargain day. I’m going to give you four answers for the price of one. It’s not because I’m feeling generous, but because I can’t make up my own mind about which one to believe.
To my knowledge, the National Science Foundation has not yet funded a comprehensive university research project designed to solve this perplexing problem, so scientists have been left to debate their theories over coffee and beer. Here are four contending solutions to the great shower curtain mystery. Ya pays yer money and ya takes yer choice.
(1) Hot air rising. The story goes that the air inside the shower is heated by the water and, as everybody knows, hot air rises. (But if you don’t know why hot air rises.) If the hot air in the shower is rising, then cold air has to rush in to replace it at the bottom, and in the process it blows the curtain inward. This is a nice, simple, appealing and wrong explanation. Just try it with cold water instead of hot, and you’ll see the curtain move inward just as much. (You don’t actually have to be in the cold shower; you can do the experiment while standing outside.)
(2) Electrostatic charge. When water streams out of a narrow opening such as the hole in a showerhead, it can pick up a static electric charge. It’s not too different from scuffing your feet on a carpet, whereupon some electrons are scraped off your shoes onto the carpet and you develop a positive charge. Electrons can also be scraped off, or onto, the water by the showerhead, depending on what it’s made of.
But if the water’s molecules were to pick up, say, a negative charge on their way out of the showerhead by picking up some negative electrons, those extra electrons would repel some electrons from the surface of the shower curtain, because similar charges repel each other. That would leave the curtain’s surface with a deficiency of negative charge, and its inherent positive charges would dominate. The negative water and the positive curtain would then attract each other, as is the wont of opposite charges, and the curtain would move toward the water.
This isn’t quite as far-fetched as it may sound. (Wait’ll you see some of the other explanations.) Induced electrostatic charge, which is what the phenomenon is called, does happen and is well-known. Have you ever opened a carton packed with those styrene foam packing peanuts, especially when some of them are broken into small fragments? Just try to keep those maddening motes from jumping around or sticking to your hands as you try to brush them away. It’s because of induced electrostatic charges.
On a dry winter day, put a few small fragments of styrene foam packing peanuts on a table. (If you haven’t received a package lately, you can use torn-up scraps of lightweight paper.) Now walk across a nearby carpet while scuffing your feet to acquire a body charge. Go quickly to the table and try to touch the plastic peanuts. Even before you touch them, they will jump up to meet your hand. Your body’s static charge induced an opposite charge in the plastic and the resulting attraction of opposite charges was enough to make them leap up toward you.
Whether or not induced electrostatic attraction is strong enough to move a shower curtain, however, is up for grabs.
(3) Bernoulli’s Principle. The water is carrying along some entrained air, making an air current near the inside surface of the curtain. According to Mr. Bernoulli, the faster a gas moves across a surface, the lower its pressure against that surface. Since there is no speeding airstream on the outside of the curtain, the air pressure on the inside is lower and the curtain moves inward.
(4) The Coanda Effect. Fluids have a tendency to stick closely to a curved surface over which they are flowing. This phenomenon is known as the Coanda Effect in honor of Henri Coanda (1886–1972), a Romanian aeronautical engineer who first called attention to it.
Hold a drinking glass horizontally in the stream of water from a slowly running faucet, so that the stream falls onto one side of the glass. Notice that when the water gets to the bottom it doesn’t fall straight down. It sticks to the glass and follows its curved surface beyond the bottom before falling off.
In the shower, if the curtain is already curved inward somewhat, perhaps from one of the other effects, the water flowing over its surface may pull it farther inward because of Coanda stickiness.
Figuring out exactly why a flowing fluid sticks to a curved surface took Coanda and other aerodynamic engineers more than twenty years. Here’s the ultimate explanation.
The molecules of a fluid exhibit some stickiness toward one another; what some molecules are doing affects their neighbors because they are sort of tied to one another. (Techspeak: Fluids have a certain viscosity.) If one layer of a flowing fluid’s molecules should have some adherence to a surface over which it is flowing, the rest of the molecules will be dragged down to the surface along with them to some extent, and the fluid as a whole will tend to stick more than we might expect.
In the case of the water on the glass, the first layer of water molecules wants to stick because water wets glass. (It doesn’t wet wax, for example.) The second layer wants to stick to the first layer, so it is also weakly attached to the glass. The third layer sticks to the glass through the first two layers and so on, with each successive layer sticking less strongly than the preceding layer. Many other layers are dragged along for as long as the stickiness exceeds the pull of gravity, and then the water finally falls off the glass, having gone farther around the curve than we would have expected.
The attraction that air molecules have for one another is a lot smaller than in the case of water (Techspeak: The viscosity of air is a lot less than that of water), so it will stick to the shower curtain’s surface a lot less, but the effect is still there. Both the water and its entrained air probably contribute to the attraction of the shower curtain.
That is, if you believe that the Coanda Effect is the true cause of your flapping, slapping curtain. Me, I favor the electrostatic explanation.