Someone recently asked me - and I'm paraphrasing - "Why does my shower curtain try to hug me when I take showers?" This is actually a very difficult problem to solve, without even taking into account the friendliness of plastic sheets. It turns out that showers are frighteningly complex places...at least, from the perspective of physics. I mean, think about everything that's going on: you've got thousands of hot, tiny, fast-moving water droplets, relatively cool and hard walls, a flexible barrier on one side, gaps that let air through, and steam. Oh, and a lathered-up human being.
There are a few responses floating around, invoking various explanations. Maybe the hot water causes the air to warm up and rise, leaving a partial vacuum at your feet, drawing in air from outside the stall. Maybe the fast-moving water speeds up the air, lowering its pressure on the wet side of the curtain. Maybe the natural "stickiness" of water gently tugs on the plastic. But curtains still billow when the water's cold or when it's not moving all that fast or when it's not even near the curtain.
Shoot. What's really going on? In all honesty, we're not exactly sure!
Like I said, showers are frightening. The best guess so far is based on advanced simulations. Yes, someone once took some software originally developed to study the spray of fuel in engines and made a pretend shower. The simulation was pretty fancy, going all the way to include the change in shape of the water droplets as they moved through the air.
Here's what that fancy-pants simulation revealed: the water droplets push the air around and make a sort of mini sideways tornado ("vortex" if you like awesome words). The tornado has a low pressure zone in the middle, which sucks at the curtain. Voila! But for all its sophistication, it was still a rough simulation, all the other physics stuff, like the hot air rising and the stickiness of water, also play a role. And of course things get complicated when you put a 98-degree oblong object in the middle of it all, which is why the final answer to the question is so bedeviling.