Wet Clothes: Newsletter Challenge (04/19/05)

The clothes at the top are more exposed to direct sunlight, so they dry quicker.

ITS PRETTY SIMPLE. Your neighbors dog pee'd on the bottom so they are not dry

I've always assumed that this was just caused by gravity, maybe with some kind of "wicking" action, depending on the fabric?

capillary action

The son should take solice in the fact that he is partially right. The fabric at the top stretches more then that of the the bottom due to water and gravity. This allows more surface area and more airflow through the fabric. There is also the simple gravity effect on the water and maybe more sunlight at the top but I think it is mainly the stretching effect.

as long as the clothes are really wet, gravity is the main factor (enough water in the space between the fibres to win over capillary), so water collects in the lower part. If you then turn the shirt (wet part to top) when it is just damp, capillary wins over gravity and the upper part is the wet one. Solar activity has only an effect when the lines are close end shadowing happens.

I think the volume of water in the capillaries of the cloth is so small that the affect of gravity on it is negligable. The top part of clothes dry faster due to the simple reason that sun falls direct on it.

As written in the 4/26 issue of Specs & Techs from GlobalSpec:

Well, your son is right to some extent, but your daughter (provided she really knows the answer) is on to something. It turns out that for the most part, capillary forces will hold the water in place against gravity for damp laundry. The cloth "wicks" the moisture. (For really wet cloth gravitational forces will, of course, rule.) What happens here is that the cooling associated with evaporation as the laundry dries causes the air next to the cloth to cool as well, which causes a downward draft of air. This air continues to pick up moisture but becomes more saturated as it flows downward, which means it picks up less moisture the longer it flows. The net result is that the cloth near the top loses its water more quickly than the cloth lower down - so at some point the laundry will be dry at the top but not yet dry at the bottom. Check out E.B. Hanson, "On the Drying of Laundry", SIAM Journal of Applied Mathematics, 52 (1992):1360. Click here to read a tongue-in-cheek review. Also see "Mathematics of Laundry Unveiled", Science News 142 (1992):286.

My wife would rail my kids if they were raking (with all the dust that goes with it) while there were clean clothes on the line...