[thedoc (OP)]

Alex Hoffman asked the Naked Scientists:
   
My 8 year old son has this question: "what is the most liquid thing in the world?" I imagine that he is talking about viscosity. What determines a fluids viscosity? Does it change with things like temperature and pressure?
What do you think?

[chiralSPO]

TL;DR: superfluids have 0 viscosity, and would probably be the best answer to your son's question. I have also provided answers from a chemist's perspective on why normal substances are more or less viscous.



As a chemist, I will say that viscosity is largely determined by interactions between molecules in the liquid. For simplicity, I will only talk about pure substances (in which all molecules are identical) rather than mixtures or solutions (combinations of multiple types of molecules).

There are 4 important characteristics that molecules have which are important for thinking about how they interact with each other:

1) size: substances composed of bigger molecules tend to be more viscous that those composed of small molecules.

2) shape: for two molecules of similar size, long skinny ones will be more viscous than nearly spherical ones. (Think of stirring a bowl of spaghetti vs stirring a bowl of macaroni.)

3) charge/charge distribution: molecules that have an overall charge will be very sticky--like static charge makes foam packing peanuts stick to a cardboard box. Even molecules that are overall neutral, but have uneven charge distribution like a more positive end and a more negative end can be very sticky (this is called polarity)

4) polarizability: Even molecules that are overall neutral and are not polar (see # 3) on their own can still become polar when they are close to charged or polar molecules.

Liquid helium is one of the least viscous liquids, and this makes sense because it is composed of the smallest possible, completely spherical, neutral, nonpolar and very difficult to polarize units. (I can't really call them molecules, because they are only single helium atoms).

There are other more exotic forms of matter, called superfluids, which have zero viscosity (liquid helium actually becomes a superfluid if it is isotopically pure and cooled between its melting and boiling points). These properties are not so much in the realm of chemistry as they are of physics, so I am not nearly as knowledgeable on the subject, but I am sure another member is.

[evan_au]

Superfluid Helium is reknowned for its ability to creep up the sides of a glass container, over the top, and escape by running down the sides. Due to its very low surface tension, the meniscus is far higher than you would find with water or other liquids.

See: https://en.wikipedia.org/wiki/Superfluid_helium-4