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Author: Peter Taylor

Solid, Liquid, Mash: Why deciding what's a liquid isn't that simple

Updated Monday, 14th April 2014
When a traveller was stopped from taking mash on to a plane, the TSA may have been acknowledging the difficulty of  defining exactly what a liquid is. Peter Taylor explains.

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Mashed potato When I was at school life was much simpler and I thought I knew everything that was important to know. As I have got older I seem to understand less and less and the world seems a much more complex place with a myriad of contradictions.

For example, back then matter had three states; solid liquid and gas – although I am so old that now a fourth state, a plasma, has been added. A liquid has a fixed volume but no specific shape, it flows to take up the shape of the container.

This all seems simple enough until I was reminded of a recent debate around mashed potatoes. When travelling by air "Liquids, aerosols and gels over 3.4 ounces cannot be brought through security", but what about mashed potatoes?

It seems such a debate occurred at Chicago's O'Hare Airport and as you would expect the security guards confiscated the mashed potatoes, but I'm not sure this resolved the issue for all time that mashed potatoes are a liquid. They have a particular volume but do not flow easily to fill up the container but if left long enough I suppose they would and this provides the reason why things are never simple. Liquids flow but the rate at which they flow depends upon the viscosity of the liquid.

Water is a classic liquid we can pour it into a glass and it will fill the bottom of the glass, but if I put a dollop of strawberry Jam in the glass it doesn't immediately flow to take up the shape of the container, however,  if I leave it long enough it does. The classic example of this is the tar drop experiment that has been running in Brisbane since 1927. The highly viscous pitch has produced eight drops in the last eighty or so years and another is about to fall. (You can watch for the drop online.)

To get a clearer understanding you need to look at what is going on at the atomic level. In a solid the atoms or molecules are held in specific ordered positions with forces holding them in place that ensure they don't easily move. In a liquid the atoms or molecules are disordered but still attracted to each other so they take up a specific volume but they can flow over each other, the forces of attraction can be more easily broken and reformed. In viscous liquids these forces are stronger and so the rate at which they break and reform is slower.

Again this all makes sense until you consider liquid crystals. We have recently been making some new liquid crystals and at certain temperatures these materials have melted from their crystalline state but still have a degree of order so they do not behave as simple liquids. Another complexity that enhances our lives through display screens.

When I was at University I was told that glass was a liquid in that it was not an ordered solid and the molecular structure of the glass should enable it to flow if you left it long enough but even that has since been strongly questioned.

But this is why I love science and doing research. Things are never what you expect them to be and if they were research would be boring. I love it when I get a result that doesn't fit because it means there is some new phenomena to investigate. So I am glad I know less and less because it means there is more and more to discover.


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