Lengthwise or widthwise. Which towel will dry faster?

So I was putting towels on drying rack and I had extra space. Should I hang towels lengthwise to speed up drying?

If we have a standard, rectangular towel. Say like this:

Which will dry sooner?

Lengthwise:

Or widthwise:

I am leaning towards lengthwise, since the water will have more surface to dry away from, but with widthwise maybe having higher vertical difference will cause water to accumulate at the bottom of the towel and dry sooner.

• use a peg so you won't have to fold it at all – Christoph Oct 29 '16 at 13:45
• What makes you think there is an unambiguous answer to this question? You have rate limited diffusion between the two half-sheets, convection-diffusion if there is sufficient ambient convection, multi-hop migration, unspecified external heating (HVAC or sunlight) to offset passive evaporative cooling, unspecified spacing to the nearest wall (typically 1-3 inches in an American bathroom) which may create a coupled convection-diffusion problem, unspecified room volume (small volume, closed door, no forced convection: the towel never dries), et c... – Eric Towers Oct 29 '16 at 18:14
• If you don't have pegs and you want to maximise the surface area not covered, hang it with the line going diagonally through two corners. – anaximander Oct 29 '16 at 19:07
• Is there any wind? And if so, what direction? If there is wind, you should choose the orientation such that the wind is along the direction of the shortest towel length, so that you maximize transport. – Charles Oct 30 '16 at 1:43
• @KeithMcClary Experiments, documented as answers, are strongly encouraged. – rob Oct 30 '16 at 19:32

Hanging it along the diagonal will maximize the surface area, but while the corners will dry faster, the thickest area with two layers will not.
A simple way to speed up the process is to flip the towel over after some time to expose the wet insides. Or using clothespins to avoid the slow drying double layer alltogether

• Excellent work. The downside is this will use the maximum line space too. – Criggie Oct 29 '16 at 22:39
• Is my math right? The area of this is ${11 \over 8}a^2$ ?? – Brock Adams Oct 30 '16 at 4:18
• How does it maximize the surface are? The surface area of towel should be constant. – Dost Arora Oct 30 '16 at 10:49
• It's about maximizing the single-layer area. – FooBar Oct 30 '16 at 11:08

In both cases the contact area of the towel with air is a² (I'm considering that the back of towel is in contact with the other back when folded). Therefore, one can expect that the exchange of water molecules with the air happens at the same rate.

However, there is one more phenomenon that should be taken into consideration which is the liquid water moving towards the bottom edges and dropping into the floor.

In the latter case, the resistance of the water is lower (because the distance to the edge is lower) for the case when it's folded lengthwise and hence the water will reach the bottom edges faster, thus drying faster.

• I agree regarding the rate of water having to be maximised, but I feel this is very dependent on both the material and the fibre alignment, which would need to be checked before deciding on the direction of hanging. Regards – user108787 Oct 29 '16 at 15:15
• @CountTo10 I agree that it can depend on the fibers pattern, but I don't see how the material can be a factor here, since I don't expect to anisotropic properties to play a role at this scale. – cinico Oct 29 '16 at 18:06
• So...what if it was hung on its diagonal/hypotenuse? Wouldn't that maximize the resistance of water? – stackErr Oct 29 '16 at 19:13

If we really want to maximise the drying rate, we need to get the water out far more than we need to worry what angle it should hang at. This is particularly important if the towel is doubled over, as large towels invariably are.

If it is hung outside, wind direction is important, especially if we have the ability to keep the towel aligned correctly to the breeze for as long as possible.

Like most physics problems, it's much more subtle than it first appears.

It depends on how much water the towel contains.

1. Determine how much water is squeezed out of the towel, if it is still say 50 percent wet, and you don't have the means or strength to reduce this percentage, then hang it to maximise water seepage first. This is vital to the operation.

2. If you are thinking ahead, you will have drained as much water possible out first, getting it down to 20 percent is good, then hang it in a position to maximise breeze assisted / radiator drying.

So... when do you hang it widthwise, and when lengthwise

It's the amount of water you get out of it before hanging that is important,

You say what factors are important, but you don't apply them to the situation and arrive at a conclusion. Which direction maximizes seepage? Which direction maximizes breeze-assisted drying?

Sammy raises an important point, we also need to look at how the towel fibres are aligned for maximum drainage rate.

I would also conclude that, as Sammy has kindly pointed out, I should confirm that once the mechanical release of water has been achieved, in my opinion it does not make a significant difference which orientation we choose to facilitate breeze or radiator drying.

The only exception I would make to that is in the case of heavy towels with a definite pattern or fibre structure which may help as channels to remove as much water as possible.

• Your answer is informative, but I still think it misses the original question. You've listed numerous factors which optimize the time needed for the towel to dry, but the question simply asks wheather it's better to hang it lengthwise or widthwise. When citing my previous comment, you said that it is the amount of water got out before hanging that is important. Does that mean that, in the end, the orientation of the towel is irrelevant/provides just a negligible difference? – Soba noodles Oct 29 '16 at 15:33
• @Sobanoodles in my opinion, the orientation of the towel is important as regards looking at it to determine which way the fibres will act as walls to get the water out quickly (that's why I got the downvote, I would guess, because the fibres in say cotton are invisible, so it's an ideal case, not realistic in practice), but after you get the maximum amount of water mechanically removed, I cannot see that it makes a damn bit of difference which way you turn the towel, I really can't. – user108787 Oct 29 '16 at 15:43
• So, a more concise conclusion is: If it's very wet, we can't tell which way to hang it without a microscope, and if it's mildly wet, it doesn't really matter. Is that what you're saying? – Soba noodles Oct 29 '16 at 15:50
• Same here :) You've earned my upvote, thank you for this enlightening thread. – Soba noodles Oct 29 '16 at 15:55
• @TheGreatDuck than the answer by DenDenDo, in my opinion, is the best approach , (which I upvoted yesterday), as it maximises the surface area. – user108787 Oct 31 '16 at 18:48

I'm going to assume:

• I have 2 towels equally wet
• Water only moves within the towel and does not drip off (or equal amounts drip off for both towel) (I want to confine the problem to a purely evaporation standpoint)
• No wind
• Equal and constant temperature environment for both towels

I hang them as such:

I'll assert that towel A dries faster:

• Imagine the edge case of a very very long and narrow towel (make it infinitesimally small width or string-like):

• In orientation A, there is no room for water to redistribute, all surfaces participate in evaporation.

• In orientation B, the water redistributes within the material (due to gravity); the bottom has more water, and at the very very very top there is no water and it does not participate in evaporation. You now have a very tiny (nonzero) decrease in area that participates in evaporation.

So, I assert that A will dry faster than B. Lengthwise dries faster than widthwise.

This answer doesn't account for other possible orientations like hanging on the diagonal since it appears the question asked specifically about lengthwise versus widthwise.

• I don't think the gravity arguments holds water (sorry). Towels by design have strong capillary action, that's how they dry you. That's often strong enough to overcome gravity - and when it's not, you see the dripping. Hence, by your second assumption (not dripping) you already are working under the assumption that gravity is overcome. – MSalters Oct 31 '16 at 9:03

protected by Qmechanic♦Oct 30 '16 at 18:59

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