When I am taking a shower, the shower curtain slowly moves towards my legs.
Also, it seems that the hotter the water, the faster it gets to my skin.
Why is that?
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67$\begingroup$ Ig Nobel prize was awarded on research of topic ;) You can start at en.wikipedia.org/wiki/Shower-curtain_effect and follow links from there. $\endgroup$– Artur BiesiadowskiCommented Jun 28, 2017 at 10:14
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6$\begingroup$ I have found if instead of using the curtain to completely seal the bath you are able to leave a gap at the end furthest away from the shower (where you're unlikely to spray water anyway) this can significantly reduce or even stop this annoyance. $\endgroup$– Matthew1471Commented Jun 28, 2017 at 13:49
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2$\begingroup$ @Matthew1471 I also had always had a gap on both ends, and never noticed this phenomenon until I used a different shower. It's very minimal in my shower now because I do leave a gap on the end. $\endgroup$– JMacCommented Jun 28, 2017 at 14:32
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1$\begingroup$ Related: lifehacks.stackexchange.com/q/15407/20114 $\endgroup$– DonQuiKongCommented Jun 28, 2017 at 19:11
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2$\begingroup$ Considering that there are so many hypotheses (see WP article linked above and the answers) and a number of other factors (shower geometry, curtain geometry, person geometry, ambient temperature, humidity and pressure, spray characteristics, materials, water chemistry ...), I would assume that all mechanisms (convection, vortex, Bernoulli, electrostatic etc.) have some sort of effect, whose strength depends on all the other parameters. $\endgroup$– RaketenolliCommented Jun 29, 2017 at 7:21
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4 Answers
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It is due to the convection in the shower, as mentioned in Steeven's comment.
The water is significantly above room temperature. Obviously it is going to heat the air (and you can feel the warm air/vapour yourself).
The air in the shower is both warming up, and becoming more humid. Both of those factors lower the density of the air, making it raise up.
When the air flows up, it creates a low pressure zone in the shower. Once the pressure gets low enough compared to the room outside, the pressure difference pushes the curtain into the shower until enough air can get in to account for the air lost to convection.
Warmer water will cause greater convective flows, so the effect should be more noticeable.
Disclaimer: Note that the heat and humidity are not the only cause of convective flows; and convective airflow is likely only part of the situation. As some comments (and the wikipedia page on this subject) mentioned, the flow of the water downwards may actually generate an air vortex in the shower. This vortex creates a low pressure zone, which drives the convective process regardless of water temperature.
There are also other potential reasons, and without experiments and potentially (multi-physics) simulations, it's very unclear what the primary cause of this phenomenon is.
I still feel this answer properly addresses the second question about why warmer water makes the process more noticeable.
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$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– rob ♦Commented Jun 29, 2017 at 17:15
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EDIT on reflection, my answer, as noted in the comments below, is not correct (to be specific, it's dead in the water), it simply would not produce the mass of less dense air required. END EDIT
The Bernoulli Principle is probably the main cause. This states that an increase in velocity, of the air associated with the falling water, produces a reduction in air pressure around your body, and then ambient air pressure in the bathroom pushes the curtain towards you.
Another possible reason is the Coanda Effect, which you can observe when you hold a spoon near a stream of water from a tap and as the water moves over the curved back of the spoon, the spoon will move inwards toward jet center.
I have to say I include this Coanda Effect for interest only, I believe the first reason to be far more significant.
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2$\begingroup$ If the increase in velocity due to falling water was the significant factor, it should work irrespective of the water's temperature. Also, even if I haven't crunched the numbers, I think that this effect is many orders of magnitude too small to have a real effect. I think that Steeven's explanation is the right one (see comments). $\endgroup$– valerioCommented Jun 28, 2017 at 9:13
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$\begingroup$ @valerio92 thanks, that's a fair point. Although, I would (half heartedly :) argue that the increase in temperature effect is something I would have to check myself. $\endgroup$– user154420Commented Jun 28, 2017 at 9:58
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$\begingroup$ I can experimentally confirm Countto10 theory about the air velocity due to the failing water, regardless of the water temperature (at least into the range [10°C;40°C]). $\endgroup$– YSCCommented Jun 28, 2017 at 13:46
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1$\begingroup$ @valerio92 Where is “Steeven’s explanation”? $\endgroup$ Commented Jun 29, 2017 at 13:31
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1$\begingroup$ @EmilJeřábek It has been deleted. It was similar to JMac's one, though. $\endgroup$– valerioCommented Jun 29, 2017 at 13:35
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There are numerous theories and this has been a subject of eternal debate. However, given that we now have mountains of evidence, I believe at this point we can state conclusively that the mechanism is as follows:
The water falls in the front of the shower. The falling water carries air downward with it. The air that moves downward at the very front of the shower moves upward further back. This pattern of downward moving air in the front and upward moving air in the back closes to form a stable, horizontal vortex. The vortex has a low-pressure zone in the middle, which produces a constant, low-pressure against part of the curtain. The higher air pressure on the other side of the air curtain therefore pushes it inward.
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26$\begingroup$ If there are mountains of evidence, as you say, would it be too much trouble to provide some citations? $\endgroup$ Commented Jun 28, 2017 at 21:19
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3$\begingroup$ No need, the mountains are YUGE. $\endgroup$ Commented Jun 30, 2017 at 16:20
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$\begingroup$ @JimmyJames If mountains of evidence won't satisfy you, what would it help for me to provide a few citations? $\endgroup$ Commented Jun 30, 2017 at 16:42
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$\begingroup$ It's hard to move mountains [citation needed] $\endgroup$– Nick TCommented Jun 30, 2017 at 19:28
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$\begingroup$ This doesn't answer the question - it misses the water temperature factor, so -1. $\endgroup$– TimCommented Jul 1, 2017 at 7:46
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I have personally never been convinced of any explanation that involves a difference in air-pressure causing this effect.
When I first used a shower that suffered from this problem - about 25 years ago - I tried various things to understand it. One of those things was to make sure I left a large gap at one end of the curtain to allow the air to freely flow around the end of the curtain. This should allow cold air to enter the shower and warm air to leave with little obstruction. It made virtually no difference.
I fail to see how the pressure difference between the outside and inside can be so great that an opening of perhaps half a square metre cannot allow equalisation to take place.
The temperature of the room compared to the water also seemed to make little difference.
Unfortunately, I have never reached a conclusion, but my pet theory is that it may be electrostatic in nature. Indeed, the very small difference you do observe when creating an opening in the curtain is far closer amount you would expect from decreasing the surface area of the curtain than the very much larger change in the pressure difference that would result from the opening.
EDIT: Have just remembered the rest of my reasons for leaning toward electrostatic forces and away from pressure differences.
I noticed that the effect was much smaller before I got into the shower - long after the water got to full temperature. It worsened after I entered. If the effect is caused by some interaction between the water and air, surely it would DECREASE as enter the shower, not INCREASE...? @DavidSchwarz says the effect is caused by a "vortex" but when I am in the shower, the falling water column is hitting my HEAD and is therefore about 5 or 6 times shorter than when it is unoccupied.
Also, I noticed that my wife (with a significantly smaller body than mine) never complained about the problem at all. Observation showed is was much reduced for her than for me.
If the cause was electrostatic, then my larger body would increase the "charge" - AND I would be closer to the curtain to make the effect much larger. As observed.
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$\begingroup$ See my answer. This is now well-understood. The air pressure cannot equalize because the vortex is stable. $\endgroup$ Commented Jun 28, 2017 at 17:15
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$\begingroup$ @DavidSchwartz Can you cite the evidence please? I'm willing to be convinced but I'd like to know why such a small pressure difference resists equalisation so effectively. $\endgroup$– LeftyCommented Jun 28, 2017 at 17:29
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$\begingroup$ The pressure difference resists equalization because the vortex is stable. So long as the vortex remains, so will the pressure difference. It's somewhat like the eye of a horizontal hurricane. $\endgroup$ Commented Jun 28, 2017 at 17:41
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$\begingroup$ That's a nice theory - but you mention evidence...? $\endgroup$– LeftyCommented Jun 28, 2017 at 18:20
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3$\begingroup$ Everything would be some much more settled if it weren't for those damned deniers and their so called "scientific method". $\endgroup$– LeftyCommented Jun 29, 2017 at 22:06