Learn a trick that makes fabrics temporarily waterproof. Short answer: start wearing them really tight. Eighties leggings tight.

Pour water on a loose handkerchief and it will soak right in. That's not the hanky's fault. It was designed to soak things up, and must be honestly grateful that this time around the liquid it is soaking up is just water.

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Stretch the handkerchief, good and tight, over the top of a glass of water, invert the glass, and no water will get through. This handy demonstration proves it. It's obviously the same piece of cloth, so there doesn't seem to be a reason why water can push through it in one position and be corralled by it in another.

But what a difference a flat surface makes. In water, it allows surface tension to come into play. Surface tension is the result of cohesion; the fact that water molecules are attracted to each other. They pull together everywhere, but especially along the surface of the water. This cohesion allows the water to form a kind of skin.

Examples of cohesion and surface tension are everywhere. Everyone has seen droplets of water splattered on floors or tables. The droplets form little balls, usually squished out of shape on the side that's resting on the surface. It's enough of an everyday experience that no one thinks about why, without surface tension, it shouldn't happen. Gravity is pulling down on the water. It should be flattened across the surface of the table like road kill from a steamroller. Instead, part of the drop of water is pushing up, defying gravity. This is because the surface tension forms a membrane that keeps the water together. So instead of looking like a squashed opossum, it forms something like a little water balloon.

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Water balloons can burst, and that's where the other part of the hanky's feat of strength comes in. One water droplet will stand up. Add more and more water to it and it deflates and turns into a film. The surface tension can stand up to a certain amount of internal pressure from the water, and after that the cohesive forces can't contain anymore. The skin breaks and the water splooshes out. (That's a technical term. Feynman used it.)

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The sploosh is easy to come by if half a glass of water is applied to the surface membrane. The cotton chops up that surface into tiny parts. The tiny threads that make up the fabric make a lot of tiny little surfaces. In order for the water to fall, the water pushing down, and the air pushing up, has to apply enough force to break the membrane. It's easy to apply that force across the entire surface of the glass, the way it would be easy to break a water balloon stretched across an entire cauldron. It gets a lot harder to apply that force to each tiny surface that peeks through the cotton threads.

And so the fabric that absorbs all that lovely blue liquid in personal undergarment commercials stays leak free if it's pulled tight enough. This is a trick that will work with other fabrics, such as ordinary clothes, as well. The problem is, they do have to be pulled tight and flat enough so that none of the water can break rank and flow. Ladies, you will have to tuck all your pointy bits away. Gentlemen, you too.

Via Physics.org and Kitchen Science Experiments.

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