Why Does the Shower Curtain Attack You?

This is part six of Lunch Break Physics, the series that has so far argued a suction cup isn’t sucking, a fridge doesn’t make cold, the sky isn’t blue because it reflects the sea, ice floats because water freezes into a roomy cage, and planes fly by throwing air downward. Every episode so far has ended with a clean answer. This one doesn’t, and I find that oddly wonderful for something as everyday as a shower curtain. Standard physics, sources at the bottom, and corrections are very welcome in the comments.

The usual explanations

Ask around and you’ll hear two confident explanations. One is static electricity: the curtain builds up a charge and clings to your wet leg. The other is that hot air rises, climbs out over the rail, and drags the curtain up and in on its way.

Neither one explains the billow, and you can check both at home. Static cling needs the curtain to touch something. But the curtain billows in before it reaches you, and it relaxes when you turn the water off. (Static may still add to the final stick once the curtain does touch your leg.) As for heat, a cold shower pulls the curtain in too [1], so warm air can’t be the whole story. It may add a little in a hot shower, but it isn’t needed.

The demo

Turn the water up and the spray drags air down with it. The curtain leans in, and it falls back straight when you switch the water off. The pressure number in the readout is illustrative, not a measurement.

The demo stops at the lower pressure inside. It doesn’t try to show what causes it.

What the explanations agree on

When the shower runs, the air on the shower side of the curtain ends up at slightly lower pressure than the still air of the bathroom. Air always pushes from high pressure toward low, so the bathroom leans on the outside of the curtain harder than the shower air leans back from the inside, and the curtain moves in.

Back in part one a suction cup stuck to the wall for the same reason. Higher pressure outside, lower pressure in the sealed pocket, and the atmosphere pressing the cup flat. The twist is that I made the suction cup’s low pressure myself, on purpose, by squeezing the air out and sealing the gap, so the cause was never in doubt. In the shower the running water makes it for us, and how it manages that is where the agreement ends.

Where it stops being settled

There are at least three serious candidates for what lowers the pressure, and they genuinely disagree.

The Bernoulli explanation is the popular one. The spray drags air along with it, the idea goes, the fast air on the inside face of the curtain sits at lower pressure, and the curtain gets pushed toward it. It sounds tidy, and it’s the version you’ll find most often. Faster air doesn’t automatically mean lower pressure, though. Bernoulli’s rule links the two only for steady, frictionless flow of a single fluid, along one streamline. A shower is a churning, turbulent mix of air and droplets. Schmidt, who simulated the whole thing, found that Bernoulli wasn’t what moved the curtain [1].

Then there’s the buoyancy or chimney explanation: the warm shower heats the air, the warm air is less dense and rises out over the rail, and cooler bathroom air pushes in low to replace it. It can’t be the whole story, because cold showers pull the curtain in as well [1][2].

In 2001 David Schmidt, a mechanical engineer at the University of Massachusetts Amherst, ran a proper computer simulation of a shower on his home PC, in the evenings over two weeks. He split a model bathroom into fifty thousand cells and solved the equations of fluid motion for thirty seconds of shower time [1]. He found that the spray drives a horizontal vortex, a slow sideways whirlpool of air with a low-pressure core, and the bathroom air pushes the curtain toward that core.

A whirl of air has a low-pressure core for the same reason a tornado or a dust devil does. Air circling an axis is being forced off a straight line, and turning a corner needs a force pulling inward, toward the centre. In a steady swirl, what supplies that inward push is mostly the air pressure itself, higher out at the rim and lower at the axis, so a real low-pressure hole runs down the core of the spin [4]. Schmidt’s shower vortex is a sideways version of that. The spray keeps driving it, so it doesn’t die out in his simulation [1], and the curtain happens to hang right alongside its low-pressure centre. He described it as a bit like a sideways dust devil, found with fluid-simulation software normally aimed at jet engines [3], and the work earned him a 2001 Ig Nobel Prize [2].

Schmidt’s vortex is the most detailed candidate, but it’s still one simulation of one model bathroom. Wikipedia files the shower-curtain effect among the unsolved problems in physics, listing the Bernoulli, buoyancy, vortex, and even boundary-layer ideas without a definite conclusion [2]. That’s a lack of agreement, not an official verdict.

A little bit of math

You can see why the effect is so feeble with one line. The force on the curtain is the pressure difference times the area it acts on:

Force  =  (pressure difference)  ×  (curtain area)

None of the sources here gives a measured value, so treat this as a back-of-envelope guess. Say the difference is 5 Pa, against roughly 101,000 Pa of atmosphere at sea level. A curtain is about 0.7 metres wide and 1.8 metres tall, so roughly 1.3 square metres. That gives 5 × 1.3 ≈ 6 newtons, about the weight of a couple of apples, if you pretend the difference is the same over the whole sheet. (A real one varies from place to place.) Schmidt calls the force weak, enough only for light, thin curtains [1]. A heavy vinyl curtain feels the same small push, but more weight and stiffness resist it, so it barely moves.

The same trick, elsewhere

The same pattern, a moving or swirling fluid setting up a pressure difference and something nearby getting pushed toward the low side, shows up elsewhere too. These are clean members of the family, not the disputed part.

A tornado or dust devil has the same low-pressure core as Schmidt’s vortex, standing upright and on a much bigger scale. The air circling the axis needs an inward push, so the pressure at the centre drops [4]. A perfume atomiser, or an old scent bottle, does it in miniature. When you squeeze the bulb, a jet of air blows across the top of a little tube dipped in the scent. The pressure above the tube drops, and the liquid gets pushed up into the jet.

False friends

What they have in common is that the shower’s own air doesn’t make the pressure difference.

Static cling is the myth we started with. Static electricity is real and it genuinely can make a dry curtain stick to a dry leg, but it runs on electric charge. It doesn’t need the shower running either.

The suction cup from part one is the family resemblance. It ends with the same pressure difference, which is why it fools people, but the cause is completely different: the suction cup holds a sealed, static low-pressure pocket that you made yourself by pushing the air out, while the shower’s low pressure is made continuously by moving water and moving air.

Sometimes the curtain clings to a tiled wall it happens to touch. That’s surface tension in a thin water film gluing two wet surfaces together. The curved water edges create a small pressure difference inside the film itself.

And if the bathroom door is open and the house has a breeze, bulk airflow can shove the curtain around. The sheet still feels that as air pressure, but it’s the house moving the air.

Things you can check

Lunch Break Physics runs Tuesdays at noon. Last week: how planes generate lift. Next Tuesday: how noise-cancelling headphones work, where adding sound makes things quieter. Got an everyday-physics puzzle you’d like poked at? The comments are open.

Sources

• [1] Why Does the Shower Curtain Move Toward the Water? (David Schmidt, Scientific American, 2001). The researcher’s own account. He argues that the Bernoulli and buoyancy explanations both fail (buoyancy predicts a cold shower would be safe, and it isn’t), and his 50,000-cell CFD simulation instead finds the spray drives a low-pressure horizontal vortex, with the higher-pressure bathroom air then pushing the curtain in. The force is weak, so only light curtains are affected and weighting the hem fixes it.
• [2] Shower-curtain effect (Wikipedia). Catalogues the effect under “unsolved problems in physics” and lays out the competing hypotheses (buoyancy/stack effect, Bernoulli, Schmidt’s horizontal vortex, and the Coandă boundary-layer effect) with no settled conclusion. Notes that Schmidt received the 2001 Ig Nobel Prize in Physics for the partial solution.
• [3] Run rings around a curtain (The Guardian, 2001). Reports Schmidt’s use of computational fluid-dynamics software normally used to model jet engines to find the vortex spinning at the bottom of the shower as the cause of the curtain’s behaviour.
• [4] Tornado: physical characteristics (Encyclopædia Britannica). A vortex core is “a roughly cylindrical area of lower atmospheric pressure” bounded by the fastest circulating winds. The air turning around the axis requires an inward force, supplied by a pressure that is lower at the centre than at the rim, so a sustained vortex carries a low-pressure core.