<p>Yes, a droplet of water would do that.</p>

<p>However, at the planetary scale, the contribution of superficial tension is negligible. At that scale, gravity dominates all other forces.</p>

<p>In fact, you could model a planet as a loose mass or rubble, kept together by gravity alone, and the model would be very close to reality.</p>

CC BY-SA 3.0
2014-12-05T19:55:18.680Z
2014-12-05T19:55:18.680Z
Florin Andrei
<p>Video: <a href="https://www.youtube.com/watch?feature=player_embedded&amp;v=9ZEdApyi9Vw" rel="nofollow">Space Station Astronauts Grow a Water Bubble in Space</a> </p>

<p>Surface tension tends to draw the water into a nice sphere.</p>

CC BY-SA 3.0
2014-12-04T13:55:29.350Z
2014-12-04T13:55:29.350Z
Wayfaring Stranger
<p>Strictly speaking (as far as I know), hydrostatic equilibrium applies whenever a fluid balances external body forces with the pressure gradient.  From <a href="https://en.wikipedia.org/wiki/Hydrostatic_equilibrium" rel="nofollow">Wikipedia</a>:</p>

<blockquote>
  <p>In continuum mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure gradient force.</p>
</blockquote>

<p>I think the concept happens to be most frequently used in areas where gravity is the external force, but it could in principle be anything else. So, though I stand to be corrected, I think a droplet isolated in space long enough could be said to be in hydrostatic equilibrium, even though the most relevant force is the surface tension, rather than gravity.</p>

CC BY-SA 3.0
2014-12-04T07:53:19.007Z
2014-12-04T07:53:19.007Z
Warrick
<p>We have found instances of asteroids that are mostly rubble piles, but spinning fast enough that they would fly apart if bound only by gravity. The most likely binding force is static charge, although some sort of glue is not out of bounds. I don't know if either of these would be called "hydrostatic", but if the question is really "Is gravity the only force that can bind a non-solid object together?" the answer is no.</p>

CC BY-SA 3.0
2015-01-05T02:59:18.560Z
2015-01-05T02:59:18.560Z
ganbustein
<p>Wouldn't a droplet of liquid (say water) floating around in space become spherical and "in hydrostatic equilibrium" due to surface tension but not gravity?</p>

CC BY-SA 3.0
2014-12-04T00:41:57.553Z
The title has nothing to do with the question.
ProfRob
CC BY-SA 3.0
2014-12-04T01:22:46.950Z
@RobJeffries I edited it a little to make it more descriptive.
HDE 226868
CC BY-SA 3.0
2014-12-04T15:01:27.713Z
It is important to state if the droplet is confined by a surrounding fluid (e.g. air) which exerts a pressure upon it. Without such a fluid the molecules of a liquid like water will quickly "boil off" i.e. fly away due to their unrestricted thermal motions. Also you need to indicate what gravitational field applies at the position in space.  Surface tension is a result of net attractive inter-molecular attractions (e.g. van der Waals forces).
steveOw
CC BY-SA 3.0
2014-12-04T15:14:41.830Z
@RobJeffries:   The gist of my question has been edited away.   As is stands now, this is a prerequisite for my original question, but I don't see why you thought the title and question didn't match....
ThePopMachine
CC BY-SA 3.0
2014-12-04T16:07:52.683Z
@PopMachine The title was a little unclear. Were you asking if the definition of a planet should include gravity and/or hydrostatic equilibrium? Or something else. At any rate, you can and should roll it back or edit it if my edit was wrong.
HDE 226868
CC BY-SA 3.0
2014-12-04T16:11:05.357Z
@HDE226868:   It's fine the way it is, because having gone and looked again, the definition of planet and dwarf planet does include the concept of self-gravity, so the answer would have been pretty dull.
ThePopMachine
CC BY-SA 3.0
2014-12-04T23:52:21.190Z
a droplet of liquid would boil and would be in no sort of equilibrium
ProfRob
CC BY-SA 3.0
2014-12-04T23:54:08.890Z
However, the answer to the current title of the question is yes. For instance rotation can form an important part of hydrostatic equilibrium for some bodies.
ProfRob
<hydrostatic-equilibrium>
Can objects enter hydrostatic equillibrium through processes other than the influence of gravity?
ThePopMachine
# Can objects enter hydrostatic equillibrium through processes other than the influence of gravity?

(Asked by: username_0 on 2014-12-04)

Wouldn't a droplet of liquid (say water) floating around in space become spherical and "in hydrostatic equilibrium" due to surface tension but not gravity?

**Tags:** hydrostatic-equilibrium
> **Comments:**
> **username_1**: The title has nothing to do with the question.  
> **username_2**: @RobJeffries I edited it a little to make it more descriptive.  
> **username_3**: It is important to state if the droplet is confined by a surrounding fluid (e.g. air) which exerts a pressure upon it. Without such a fluid the molecules of a liquid like water will quickly "boil off" i.e. fly away due to their unrestricted thermal motions. Also you need to indicate what gravitational field applies at the position in space.  Surface tension is a result of net attractive inter-molecular attractions (e.g. van der Waals forces).  
> **username_0**: @RobJeffries:   The gist of my question has been edited away.   As is stands now, this is a prerequisite for my original question, but I don't see why you thought the title and question didn't match....  
> **username_2**: @PopMachine The title was a little unclear. Were you asking if the definition of a planet should include gravity and/or hydrostatic equilibrium? Or something else. At any rate, you can and should roll it back or edit it if my edit was wrong.  
> **username_0**: @HDE226868:   It's fine the way it is, because having gone and looked again, the definition of planet and dwarf planet does include the concept of self-gravity, so the answer would have been pretty dull.  
> **username_1**: a droplet of liquid would boil and would be in no sort of equilibrium  
> **username_1**: However, the answer to the current title of the question is yes. For instance rotation can form an important part of hydrostatic equilibrium for some bodies.  

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## 4 answers

### Answer by username_4 on 2014-12-05. Score: 1

Yes, a droplet of water would do that.

However, at the planetary scale, the contribution of superficial tension is negligible. At that scale, gravity dominates all other forces.

In fact, you could model a planet as a loose mass or rubble, kept together by gravity alone, and the model would be very close to reality.


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### Answer by username_5 on 2014-12-04. Score: 1

Video: [Space Station Astronauts Grow a Water Bubble in Space](https://www.youtube.com/watch?feature=player_embedded&v=9ZEdApyi9Vw)

Surface tension tends to draw the water into a nice sphere.


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### Answer by username_6 on 2014-12-04. Score: 1

Strictly speaking (as far as I know), hydrostatic equilibrium applies whenever a fluid balances external body forces with the pressure gradient. From [Wikipedia](https://en.wikipedia.org/wiki/Hydrostatic_equilibrium):

> In continuum mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure gradient force.

I think the concept happens to be most frequently used in areas where gravity is the external force, but it could in principle be anything else. So, though I stand to be corrected, I think a droplet isolated in space long enough could be said to be in hydrostatic equilibrium, even though the most relevant force is the surface tension, rather than gravity.


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### Answer by username_7 on 2015-01-05. Score: 0

We have found instances of asteroids that are mostly rubble piles, but spinning fast enough that they would fly apart if bound only by gravity. The most likely binding force is static charge, although some sort of glue is not out of bounds. I don't know if either of these would be called "hydrostatic", but if the question is really "Is gravity the only force that can bind a non-solid object together?" the answer is no.


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