If this is really a problem for anyone, just buy a thicker, heavier curtain.
The effect in question relates to the formation of water vapour and its cushioning/insulating effects. When an iron skillet gets hot enough, you can throw a droplet or four of water at it and the droplets will “dance” around; this is because the moment the water hits the iron surface, the water that is in direct contact with the skillet evapourates, forming water vapour that is “trapped” beneath the rest of the liquid-water droplet. The liquid water can “ride” on this layer of vapour for quite some time, skidding about as the heated molecules beneath move erratically.
Before dipping his hand into molten lead I’m sure he dipped his hand into a bowl of cool (but not too cold - maybe 60 degrees Fahrenheit or so) water. The water vapourised upon contact with the molten lead, surrounding his hand with a glove of water vapour that shielded his skin from being burnt. The reason the water can’t be too cold is that, if it was, the lead might harden around his hand, forming a blisteringly hot, but solid, lead glove that would be quite painful.
This effect can also be used to hold liquid nitrogen in one’s mouth and then exhale with a big poofy cloud of vapour like a dragon … though if you’re not careful you’ll find yourself cracking/splintering your front teeth. Yikes, indeed.
I’ve heard that theory about hot water in the shower heating the air and lowering its density so that it rises and denser cool air from outside pushes in the curtain at the bottom. The corollary that cold water would chill the air so that it would sink and push the bottom of the curtain out doesn’t hold water, hot or cold. And all that crap about miniature hurricanes is a lot of computerized jibber-jabber. If the hurricane did it, then the curtain would never stop sucking in because the hurricane would continue beyond Professor Schmidt’s 30-second program run. Maybe he should get a faster computer and let it run for 5 or 10 minutes. He might also add the room size and water evaporation effects to get at the truth of the matter.
As fully explained herein, both hot and cold water INITIALLY push the curtain bottom into the shower until air densities equilibrate inside and outside. After that the curtain is then pushed back out (actually it just hangs vertically again and sticks to the tub that is now wet from condensation - you know, the surface tension effect, NOT the Coanda effect because the water isn’t flowing there).
I have long held to my own theory that watersprays of any temperature vaporize enough liquid water to its very-low-density gaseous form. Water vapor is much lighter than air because water has a molecular weight of 18 and dry air has a molecular weight of 28.85. With the same number of molecules in a mole volume (remember Avogadro?), water vapor is lighter than dry air by a factor of 18 to 28.85 – even though humid air feels heavy. Actually, this heavy feeling is a lung thing – it is just a little harder to breathe.
When room temperature air becomes saturated, at 100% relative humidity, it holds about 2.5% water vapor that has a density of 0.0468 lb/cf (pounds per cubic foot). The mixture of 70°F air saturated with water vapor has a density of about 0.0742 lb/cf. That compares to a normal air density of 0.0747 lb/cf with only 0.6% water vapor (at 68°F and 60% relative humidity). With hot water, the mixture density will be lowered to 0.0737 lb/cf from the effect of heating the air to about 85°F. The air will then hold 4.1% water vapor which helps achieve that lower density.
In either case, the denser outside air sinks down along the curtain to enter the shower which pushes the moist air out at the top until the outer room area reaches the same saturation conditions and the system is in balance. Cold water cools the shower air, thus increasing its density more quickly by both the temperature effect (colder air is heavier) and the smaller amount of water vapor that cool air can hold (only 1.75% at 60°F). Hot water must saturate and heat both shower and room air to reach equilibrium and this will take somewhat longer.
From experience in a hot shower, the curtain stops blowing into the tub in about 3 minutes in my bathroom. It never happens in the shower at the gym where the room is gargantuan in comparison to my bathroom and thus the air can’t saturate. The gym has excellent ventilation also and that keeps removing the water vapor from the room. So I always have a clammy vinyl sheet on my bare leg there. At least I used to until they switched to heavy curtains that take more pressure to move.
I have tried the cold water shower experiment a few times (from a comfortable position outside the shower) and the bottom of the curtain always moves into the shower, as it does with hot water. I have never run the water long enough with cold water to reach equilibrium density inside and outside the shower. It should take only a minute but I will leave that up to Prof. Schmidt to determine. Of course, to be more accurate, I suppose he should stand in the cold shower stream so that the body heat factor will be incorporated and he will be appropriately chastened for his error.
I’m embarrassed for Cecil. Did he forget about the basic concept of hypothesis testing? If someone came in with some proof of Astrology, he’d shoot it down. But now that a Dr. at a University came up with a computer model that shows a vortex, it appears that Cecil left down his guard and has prepared to accept it without any sort of hypothesis testing that REAL scientists demand.
Schmidt has a model which yielded a theory. I have a theory that it is thousands of little invisible gremlins that race towards the water for a drink, whenever they shower is turned on. My theory is nearly as valuable as Schmidt’s theory.