Take a really dry dish cloth and try to wipe up some liquid you spilled on the kitchen counter. I will take up only so much of the liquid.

Then try it with a damp cloth (or a wring out a wet one). It will take up much more of the liquid.

It seems counter-intuitive. Why does a damp cloth absorb more liquid?

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    $\begingroup$ I think the effect is actually that the dry cloth can take as much as the damp one or more, it's just that the damp cloth takes the liquid in much quicker. I believe it will have to do with the wet fibers in the cloth "attracting" the liquid much better. Hopefully there is someone better acquainted with mesoscopic physics to give a fuller answer. $\endgroup$ – Void Oct 14 '14 at 12:53

The sort of dishcloths generically known as J Cloths are made from a material called Viscose rayon:

J Cloth

This material is derived from cellulose and like cellulose it interacts with water. Water breaks hydrogen bonds formed within the fibres. This makes the fibres softer, and the exposed hydroxyl groups make the surface more hydrophilic. It's the latter process that makes a damp cloth more able to soak up water than a dry cloth.

The absorption of water on a fabric procedes by wicking. This requires a low contact angle and no hydrophobic areas on the fibres where the meniscus can get pinned. Incidentally, pinning is the basis of many superhydrophobes - even a relatively hydrophilic surface can be made hydrophobic by giving it the correct microstructure.

Anyhow, the contact angle of dry Viscose rayon is around 30-40º, which is fairly low but still high enough to prevent wicking and cause pinning. That's why a dry cloth is slow at absorbing water. It will absorb the water eventually, but the timescale may be many seconds or even minutes. After the Viscose has interacted with water and formed free hydroxyl groups at the surface the contact angle falls to effectively zero. This makes wicking, and therefore water absorption, much faster.

The dry cloth finds itself in a catch 22 situation. It has to interact with water to become hydrophilic, but until it becomes hydrophilic the water can't spread into the fabric for the interaction to occur. As anyone used to doing the washing up can tell you, the solution is to force the water to wet the fabric by wetting then squeezing it.

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    $\begingroup$ Terrific answer. You're showing some of your chemist roots... $\endgroup$ – Floris Oct 14 '14 at 17:13
  • $\begingroup$ @Floris: indeed. Physics is more fun though - would you rather answer questions on black holes or doing the washing up? :-) $\endgroup$ – John Rennie Oct 14 '14 at 17:53
  • $\begingroup$ John - I think I'm better at doing the dishes... :-) $\endgroup$ – Floris Oct 14 '14 at 18:03
  • $\begingroup$ @JohnRennie Just wow! We don't use J cloths but thicker ones, I just looked up the materials: 60% Viscose, 20% PES, 20% PP. I guess the same applies for them due to the Viscose. $\endgroup$ – awendt Oct 15 '14 at 6:18
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    $\begingroup$ @awendt: PES is probably polyester, and PP is polypropylene. These are both quite hydrophic and do not interact with water, so the behaviour is being dominated by the viscose component. $\endgroup$ – John Rennie Oct 15 '14 at 7:41

I suspect it's a surface tension question.

When a dish cloth is slightly damp, then water is already between the fibers. Putting a drop of water in touch with that fiber (and water), the water will be drawn from the drop into the space between the fibers.

By contrast, if the cloth is really dry, then when the drop is touching the fiber, it first needs to "grow" before it can discover the space in between the fibers where the energy would be more favorable - and in order to grow, you need to increase the energy contained in the surface.

Systems like to go "straight downhill". Surface tension provides an energy barrier. When you wring the cloth, you force the water into the fibers - and once it's there, you provide a lower barrier for more water.

It's a bit handwaving, but I suspect it's not far off. Looking forward to comments / corrections / suggestions from the community.


protected by Qmechanic Oct 15 '14 at 11:15

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