Ponytails in space would be pretty unmanageable. Or at least that is my prediction based on a paper I have just read. The paper has just come out in the prestigious physics journal Physical Review Letters, and is titled: Shape of a Ponytail and the Statistical Physics of Hair Fiber Bundles. First off, I should say that I have met all 3 authors and, at least the last time I saw them, none of them had ponytails. But one of them does work for a company that makes a awful lot of shampoo, so that may have been part of the motivation behind the study.

In the paper the authors successfully predict the shape of ponytail, i.e., how it fans out from the point where the hair is tied together, as illustrated in the image up top. Their model basically predicts that it is mostly due to competition between two opposing effects. The first is gravity, which tends to pull the hair down and so tends to compress the hair and prevent it fanning out.

The second is the forces between individual hairs in the ponytail that tend to push them part. It appears that it is these forces that causes the hair in a ponytail to fan out. So it is competition between this and gravity that sets the shape of ponytail.

The forces between the hairs basically are very similar to the forces you get with a tangle of power leads by your TV or PC. Hairs are long and thin like power cables and both are elastic in the sense that if you bend them they tend to resist with an opposing force. If you try and compress a tangled mess of power cables into a smaller volume to tidy up, you end up bending these cables which try and unbend when you let go, causing the mess of power cables to expand as soon as you let go. Similarly gravity tries to compress the hairs, but as they are all a little curvy, this bends the hairs which try and expand back – this expansion opposes gravity and sets the ponytail shape.

In the absence of gravity there is nothing to oppose the springy force between hairs and so I predict that ponytails must be a mess in zero-gravity.

One final thing I learnt is that our hair appears to go through a ‘glass transition’ when we wash it. When the hair gets wet it adsorbs water which greatly softens the hairs – which is what going through a ‘glass transition’ means in this context. It is a bit like window glass that is heated up so hot that it softens. Then when we dry it the individual hairs never dry completely straight (even if we have what we call ‘straight hair’). As the hairs are not completely straight they twist into each other. Then any attempt to compress hair tends to straighten out the hairs, which then push back.