[chris (OP)]

On Earth, liquids take the shape of their container. But what about in microgravity, for instance aboard the ISS? And how would the chemical composition of the liquid affect its behaviour, comparing, say, a polar liquid, like water, with a non-polar substance, like hexane? And what about the size of the molecules? How would that affect the behaviour? Would decane behave differently to hexane?

[Geezer]

For a start, let's please stop perpetuating the myth of MICROGRAVITY! (Please see other thread.)

The ISS is subject to pretty much the same gravity as everyone on Earth. If it wasn't, it would have hurtled off into deep space a very long time ago.

The effect experienced by people and objects on the ISS is weightlessness. Every time we jump up in the air we are weightless too. The only difference is that people on the ISS are weightless for a very long time, and that can have some pretty serious consequences.

[RD]

intermolecular forces can defeat normal 1g gravity, e.g. capillary motion.

So effectively reducing gravity is not going to produce new liquid behavior, IMO.

[CliffordK]

I've seen pictures of balls of liquid floating in space.  So, I would agree there is an amount of intermolecular cohesion.

Assuming a non-full container, in most cases, the liquid would not expand to fill the container, although perhaps a pressurized soda bottle might experience greater diffusion of the CO₂ through the liquid, and in fact be more space filling.

Different liquid/container combinations have different types of menisci.  Those liquids that have a concave meniscus would likely cover the walls of the container with a central cavity (or several bubbles).  Those liquids that have a convex meniscus would likely ball up away from the container walls (the same idea as the capillary motion discussed by RD).

[damocles]

The behaviour of liquids is easiest to think of in terms of surface tension or surface free energy (same thing). There is an energy cost for a liquid to have surface area, so in the absence of a container, in a vacuum, and in absence of other forces, the equilibrium configuration of a liquid is a spherical blob! However, because surface tension forces are not particularly large, what you more typically see in practice in spacecraft shots are wobbly blobs!

With a container, a lot depends on the affinity of the liquid for the surface:
-- In a clean glass container, the water will tend to spread itself over the glass, because the glass has a higher energy cost for uncovered surface than the water does, and a very low energy cost for glass/water interface, so the glass will pull the water over its surface like a sheet.
-- In a waxed container, the water will find contact with the wax even less desirable than contact with the vacuum, and so it will tend to ball up away from the waxy surface, and may even detach and leave completely.

[SeanB]

Have a look at the series of videos done on the ISS by Don Pettit, very nice demonstrations. Available on Youtube.

[chris (OP)]

What about intermolecular forces? And how do polar versus non-polar liquids behave?

[damocles]

What about intermolecular forces? And how do polar versus non-polar liquids behave?

Intermolecular forces are key to the whole situation. A molecule in the bulk of a liquid can bask in the attractive field of typically 8-12 other molecules around it; on the surface it only has close neighbours on one side, and so there are only typically 5-8 other molecules involved. That is why there is an energy cost in producing extra surface, and is really the basis of surface tension/surface free energy.

There is no fundamental difference in this aspect between polar and non-polar liquids, except that polar liquids usually have slightly higher surface tension (slightly higher intermolecular attractive forces), and polar liquids adhere better to conducting surfaces (i.e. metals). Another way of saying this very thing is that polar liquids have low interfacial tension with metallic surfaces.