Stringy and bobbly spit - why spit beads

03 October 2010

A very simple experiment, with spit

Ingredients

A hand A little spit - preferably your own

Instructions

Get a small drop of spit between your thumb and forefinger.

Slowly pull your finger and thumb apart forming a lovely string of spit about 10-15mm long.

Watch the string for 10-15 seconds and see if anything interesting happens.

If you want another go, and it doesn't work, try some fresh spit.

Result

You should find that the string of spit, slowly turns into a string of droplets, held together by a thin line of spit.

Spit Beading

Explanation

Spit is mostly water, but it doesn't quite behave like water, it is a little bit more viscous than water and it is much better at lubricating than water. This is because it is full of long thin protein molecules, which tangle with one another making the liquid slightly thicker, which helps it lubricate.

When you stretch out the liquid you will also tend to stretch out the molecules a bit.

Spit between two fingers Stretching the spit
The spit is mostly made up of water with some long chain protein molecules dissolved in it. When you stretch the spit the protein molecules tend to be stretched out.

The rest of the spit is water, and water has a property called surface tension, there is a constant force attempting to minimise the surface area of the water.

Not very curved liquid Surface tension on curved surface
Surface tension will almost cancel itself in a only slightly curved piece of water If the surface is more curved, much more of the force is acting inwards, so the pressure is higher.

If the surface is curved this acts to increase the pressure in the water, the more curved the water is the higher the pressure, This means that if part of the string of spit is slightly narrower than another, its pressure will be slightly higher, so spit will move from the narrower section to the wider one. This means that the spit slowly forms the beads.

Spit moving Bead of spit
Because the pressure is slightly higher in the narrower parts of the string than the wider ones, so fluid moves towards the wider parts. Eventually almost all the water moves into the bead, only being supported by a string of protein between them.

The protein polymers are stretched in between the droplets and tangle up with one and other, effectively creating a string like structure between the beads.

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