# Why does water with no gravity acting upon it form a sphere?

• vorcil
In summary, the surface tension of water causes its molecules to be attracted to each other, forming a sphere in the absence of gravity. This is due to the minimization of surface area and potential energy. The first law of thermodynamics applies in a vacuum, and the difference in tension between liquids and solids is due to the lattice structure of solids. Cohesion forces are strong enough to maintain a spherical shape even without gravity, as seen in droplets on the space station, and it just takes much longer for amorphous solids to flow into a sphere.

#### vorcil

Why does water with no gravity acting upon it form a sphere?

i know it has something to do with the fluid tension but otherwise i have no idea why, in space, water forms a sphere?

Surface tension.

In other words, the molecules are attracted to each other, so they naturally form the shape which minimises the number of them that are not surrounded by others.

cesiumfrog said:
Surface tension.

In other words, the molecules are attracted to each other, so they naturally form the shape which minimises the number of them that are not surrounded by others.
Nice explanation

And of course, a sphere has the smallest surface area of all shapes with a given volume.

Like what cesiumfrog says. Volumetric tension (aka 'surface tension'). Every molecule pulls on every molecule that's its immediate neighbor, until it gets close enough, and then it stops pulling. However this could describe a solid too. The atoms in iron will all attract their neighboring atoms, but an iron rod won't flow into a ball. The property of internal tension isn't quite sufficient to tell us why a liquid will deform into a ball and a solid will not.

You're question is a good one. And the tension answer is the one I've always been satisfied with--until now.

Maybe someone with some better skills on the nature of materials can tell us why the tension in a liquid is different than the tension in a solid.

Why do the molecules in a liquid slide past each other while still maintaining cohesion, while in a solid they remain fixed?

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Yeah I'm still not satisfied by surface tension?

for example if you made a huge unshaped blob of water in space, it will become a very good sphere,

where does the energy that moves the water molecules come from?
does the water lose energy(possibly thermal) while it reforms into a sphere?

vorcil said:
where does the energy that moves the water molecules come from?
does the water lose energy(possibly thermal) while it reforms into a sphere?

The system looses potential energy and transforms it into thermal energy. The sphere is the minimal potential energy configuration of the molecules. Potential energy is defined by the relative position of molecules to each other, and the forces between them (electromagnetic + gravitation).

does water freeze in space?

Does the first law of thermodynamics apply in space, or in a vaccum?
i.e will heat be lost without a medium or matter for that heat to pass to?

The first law of thermodynamics is conservation of energy. It applies everywhere.

dx said:
The first law of thermodynamics is conservation of energy. It applies everywhere.

what's that law that states that a desire for equlibrium must be achieved between two areas of different temperature then:?

and does it apply to matter in a vacuum and weightless environment?

Phrak said:
Like what cesiumfrog says. Volumetric tension (aka 'surface tension'). Every molecule pulls on every molecule that's its immediate neighbor, until it gets close enough, and then it stops pulling. However this could describe a solid too. The atoms in iron will all attract their neighboring atoms, but an iron rod won't flow into a ball. The property of internal tension isn't quite sufficient to tell us why a liquid will deform into a ball and a solid will not.

You're question is a good one. And the tension answer is the one I've always been satisfied with--until now.

Maybe someone with some better skills on the nature of materials can tell us why the tension in a liquid is different than the tension in a solid.

Why do the molecules in a liquid slide past each other while still maintaining cohesion, while in a solid they remain fixed?

Iron also has a surface tension. The Helmholtz free energy of some amount of water or iron is proportional to the volume (the bulk contribution) and the area separating the compound from the atmosphere.

The difference between iron and water is caused by the fact that the water molecles are not bound to a lattice. Then the shape of the surface can change. Therefore it will become a sphere as that minimizes the free energy.

In case of solid iron rod, the minimum free energy configuration cannot be reached as the rod is frozen into a local free energy minimum.

Phrak said:
<snip>

Maybe someone with some better skills on the nature of materials can tell us why the tension in a liquid is different than the tension in a solid.

Why do the molecules in a liquid slide past each other while still maintaining cohesion, while in a solid they remain fixed?

Fluids flow under stress, solids do not.

Count Iblis said:
Iron also has a surface tension. The Helmholtz free energy of some amount of water or iron is proportional to the volume (the bulk contribution) and the area separating the compound from the atmosphere.

The difference between iron and water is caused by the fact that the water molecles are not bound to a lattice. Then the shape of the surface can change. Therefore it will become a sphere as that minimizes the free energy.

In case of solid iron rod, the minimum free energy configuration cannot be reached as the rod is frozen into a local free energy minimum.

This leaves leaves out amorphous solids--at least I think so.

If you ever watch space station videos showing water or orange juice large droplets being released (and then swallowed), the large droplets quickly become spherical. It seems difficult to believe it's just a surface related effect.

Jeff Reid said:
If you ever watch space station videos showing water or orange juice large droplets being released (and then swallowed), the large droplets quickly become spherical. It seems difficult to believe it's just a surface related effect.
Cohesion forces are stronger than you think. Mercury forms balls & blobs even against gravity. But without gravity, cohesion forces have little to work against, so even water can do it.

Phrak said:
This leaves leaves out amorphous solids--at least I think so.
No. It just takes longer -- a lot, lot longer -- for glasses to flow (and form into a sphere).

In researching this answer, I was going to bring up panes of glass in medieval cathedrals that has over the years have become a bit thicker at the bottom of the pane than at the top. Alas, this is but a scientific myth. Medieval craftsman could not blow perfectly flat glass. The thickness had a gradient across a pane of glass. Craftsmen intentionally placed panes of glass with the thickest side along the bottom. Glass can flow at room temperature, but it takes a lot longer than 500 years for the flow to become observable. The relaxation time for glass at ambient temperature is on the order of 1032 years.

Zanotto, "Do Cathedral Glasses Flow?" Am. J. Phys., 66:392-396 (1998)
http://alevi.usc.edu/Html/publications/Zanotto1998cathederalGlass.pdf

D H said:
The relaxation time for glass at ambient temperature is on the order of 1032 years.
Seems to me you'd get some flow out of solid granite in that much time.

vorcil said:
Why does water with no gravity acting upon it form a sphere?

i know it has something to do with the fluid tension but otherwise i have no idea why, in space, water forms a sphere?

yup...it is more or less surface tension...also, because of the molecular interactions (along with the gravity of each individual particle), you could assume the forces of attraction to be between each individual molecule. If you model it as such, you could also assume that the sphere would be the ideally modeled shape (the pressure is equal on all sides, so with a spherical shape, it is in equilibrium).

D H said:
The relaxation time for glass at ambient temperature is on the order of 1032 years.

I guess I won't wait around for the utube video.

zoobyshoe said:
Seems to me you'd get some flow out of solid granite in that much time.

Wait, so basically everything is a liquid? except that the time it takes for it to flow it just enormus?

vorcil said:
Wait, so basically everything is a liquid? except that the time it takes for it to flow it just enormus?
I was being facetious: what's the point of classifying glass as a liquid if it takes, like, 10 billion years to exhibit any flow? I can imagine any solid would demonstrate some deformation by gravity in that much time.

Like http://en.wikiquote.org/wiki/Potter_Stewart" [Broken], we know a solid when we see it; however, there is a gradual transition between what we'd call a liquid and what we'd call a solid.

Compare with the electrical insulator vs. conductor dichotomy, for example, where we set an arbitrary band gap magnitude (e.g., 2 eV) to distinguish between the two extremes. Copper goes on one side, nylon on the other. It's only with those intermediate cases, semiconductors, that we return to examine the definition.

Water is a liquid, granite and window glass are solids. I'd classify http://en.wikipedia.org/wiki/Pitch_(resin)" [Broken], perhaps an intermediate case, as a solid for typical human time scales and at room temperature. According to Wikipedia, pitch is a liquid; by the criterion of the Wikipedia editors (extremely slow flow at room temperature), though, even lead would be classified as a liquid.

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There is a first order phase transition that separates a solid from a liquid.

Would the hallmarks of a phase transition be kinetically observable for something like pitch, though? For example, could you cool or heat it, observe a phase transition, and thus settle the matter of its classification at 25°C?

Mapes said:
Like http://en.wikiquote.org/wiki/Potter_Stewart" [Broken], we know a solid when we see it; however, there is a gradual transition between what we'd call a liquid and what we'd call a solid.

If I can add to this, some materials undergo a more definite transition between states at a given pressure. So the distinction here is how 'fuzzy' the line is between states. It's not an entirely anthorpomorphic point of view, as a meaure of platicity vs. temperature may not be rescalable.

For other materials, this isn't even true; the type of bonding that occurs between atoms changes with temperature.

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OK, it looks like my comment in #23 is being misinterpreted, or maybe I should have been more precise. I wasn't claiming that the transition between the liquid and solid states of a single given material is gradual. I was saying that if material A has a viscosity of 1 cP and material B has an effective viscosity of 1050 cP, then the first is clearly a liquid and the second would be considered a solid; however, the dividing line between the classifications would seem to be arbitrary. My example was pitch, which is sometimes classified as a liquid but rebounds nearly elastically for sufficiently small stresses.

Again, I was not arguing that phase transitions are gradual in general; I was suggesting that a continuum exists between the classifications of solid and liquid for various materials taken at a single temperature.

Part of the confusion here is the difference between crystalline solids versus amorphous solids. Liquids that solidify in the form of a crystalline solid undergo a first order phase transition. Liquids that solidify in the form of an amorphous solid undergo a second order phase transition.

Mapes said:
Like http://en.wikiquote.org/wiki/Potter_Stewart" [Broken], we know a solid when we see it; however, there is a gradual transition between what we'd call a liquid and what we'd call a solid.

Compare with the electrical insulator vs. conductor dichotomy, for example, where we set an arbitrary band gap magnitude (e.g., 2 eV) to distinguish between the two extremes. Copper goes on one side, nylon on the other. It's only with those intermediate cases, semiconductors, that we return to examine the definition.

Water is a liquid, granite and window glass are solids.

I like this answer. A couple years ago Gokul90210 had me convinced glass was a liquid. Now I see he was just trying to pass pornography off as "art".

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zoobyshoe said:
I like this answer. A couple years ago Gokul90210 had me convinced glass was a liquid. Now I see he was just trying to pass pornography off as "art".

:rofl::rofl: That's got to be the quote of the year.

surface tension
So what happens to a blob of water hovering in the space station if someone inserts a drop of liquid soap into that blob of water (soap normally reduces surface tension of water)?

glass - liquid - solid
Glass undergoes a smooth transition between solid and liquid (amorphous solid), I don't know if pressure can change this behavior. On the other extreme, at atmospheric pressure, carbon dioxide transitions from solid to vapor (sublimation).

The simple answer is: Sphere is the lowest energy state.

Mapes said:
Would the hallmarks of a phase transition be kinetically observable for something like pitch, though? For example, could you cool or heat it, observe a phase transition, and thus settle the matter of its classification at 25°C?

If it is solid at some temperature T and there would in principle be an observable difference between the solid at temperature T and a hypothetical supercooled liquid at temperature T, then you could detect the difference without actually creating that supercooled liquid. This is because the properties of the supercooled liquid at T are analytical continuations of the properties in the liquid phase above the melting temperature and these will then differ from the measured properties of the solid around temperature T.

hi dear

Each molecule pulls on each molecule of the next instant, that is pretty close and then stop firing. But it can also describe a solid. Attract the iron atoms surrounding the atom, but not a rod of iron, with a ball. The properties of internal tension is not sufficient to explain the reason for the formation of a liquid in a bottle and a sound not.The loses potential energy and converts it into heat. The area is the minimum of potential energy of molecules.

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hileri cruze said:
hi dear