# Run, don’t walk, on custard

I enjoyed a rhubarb yoghurt a couple of days ago and I was putting the washed container in the recycling today. Like many yoghurts it was thickened with starch (tapioca starch in this case).  Basically people who make yoghurts add starch and similar things to yoghurt so people think they are getting more than what they are getting, which is milk, water and a bit of fruit. They will then pay a pound for what is a mixture of cheap (starch is dirt cheap) ingredients plus some fruit. Yoghurts can be very profitable.

The yoghurt was OK, not so exciting, but you can have fun with just water plus starch

Sorry, it is in Spanish, but you get the idea. You will see people running on the water plus starch, but if you wait till around 1:40 you will see what happens when one of the guys just stands on the water plus starch. He sinks as the fluid slowly flows round him.

The same idea would work with thick custard or yoghurt. The physics behind it is straightforward. Water flows easily from the tap because the time for water to flow from the tap is something like a second, while water molecules spin and wander around in a time that is around a trillion times smaller. So if you want water molecules to move around in a second they are happy to oblige. A second is very long time for a molecule that spins round a trillion times in that length of time.

Starch molecules are maybe a 1000 times bigger than a water molecule but if you have enough of them they can entangle, a bit like a ball of wool after a kitten has played with it. The then entangled starch molecules move much slower than water molecules. Indeed they can take more than 1 second to move their own length.

Now if you try and shift starch molecules round in less than a second, unlike water they resist. You are trying to move them faster than they normally move and the resistance they put up to the force (e.g., from your weight) means the water plus starch acts like a solid. So, you can run on it.

However, if you just stand on it then gradually the starch molecule do move, the liquid flows and you sink into it. Thick yoghurt and custard are like a solid if you hit them fast enough but are like a liquid if you slowly push them, and this directly reflects how fast molecules far too small to be seen move around.