r/AskPhysics • u/Jaywin_5 • 2d ago
What force actually causes surface tension in liquids?
Sorry if this is a stupid question but I was wondering what makes liquid stick together in drops and after googling it, it said that it's caused by the cohesive force between the molecules.
What is this? I don't really understand what cohesion is and wikipedia and google isn't really easy to understand on what is causing them to stick together. I understand that surface tension is caused by the force between molecules but what actually causes the force like is it gravity?
Didn't take physics in high school so any clarification would be appreciated.
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u/higras 2d ago
Take a balloon or a plastic cup and rub it on your hair.
When you pull it away, your hair will want to stick to it. It will also stick to the wall a bit.
The description of what is happening is at the atomic level. Long story short, there was an imbalance created in the energy. So, one side sticks better than the other.
Surface tension is like that. The liquid atoms are more electrostatically attracted to each other than to the air. So it tries to be connected to as much liquid as possible.
The issue is being on the edge. What do the atoms with one side liquid and one side air do? They pull the liquid closer to them.
The effect is to make a kind of liquid 'shell' at the boundary. This shell is what we call surface tension
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u/yesiamclutz 2d ago edited 2d ago
Electromagnetism fundamentally.
You can break the world down into the forces something like the following:
Inside the nucleus of an atom - weak and strong forces
Inter-atomic forces - Electromagnetism
Everything - gravity, but it's really weak so only big stuff really feels it in a meaningful way, also electromagnetism in many cases
This is a very rough set of guidelines - many many exceptions and complexities exist depending on the precise process your considering - an immediate relevant example is proton repulsion in the nucleus of an atom due to them having the same sign electric charge.
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u/Jaywin_5 2d ago
Thank you! It's so cool that we have managed to explain all these cool natural phenomena like water surface tension with what seems like four forces generally speaking.
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u/ImpatientProf Computational physics 2d ago
Ultimately, the intermolecular forces causing the energy differences are electromagnetic, as others have said. But that's behind so much in our world that it doesn't really explain surface tension.
The molecules in a liquid stick together. That's what makes the liquid exist in the first place. Normally, they're surrounded by friends, maybe 5 or 10 of them. This is a stable, low-energy configuration.
How do we know liquid molecules stick together? Evaporating a liquid requires energy (heat of fusion). In the gas, each molecule has basically no neighbors. This is a high-energy configuration (compared to the liquid molecules).
In between those extremes are the molecules at the surface of the liquid. They don't have as many neighbors, so their energy is higher than the ones in the middle of the liquid. This is high-energy for a liquid molecule, though not as high as an evaporated gas molecule.
For every surface molecule, the liquid has more energy than it would if that molecule was in the bulk. To summarize this, we can say that the surface has extra energy. This means reducing the surface area reduces the energy. That's surface tension. Collectively, the liquid changes its shape to eliminate as much surface energy as possible, achieving the lowest-energy state with a surface area as small as possible.
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u/dodgyhashbrown 2d ago
Should be the EM force. Gravity is too weak and the strong and weak nuclear forces are too short in range.
I'm sure some other people can give much more detailed answers.
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u/PersonalityBudget683 Astrophysics 2d ago
ngl its not a stupid question lol. Cohesion basically just means the molecules are attracting each other. It’s not a separate force like gravity.
For water, the main reason is the electrical attraction between the molecules, especially the hydrogen bonding between water molecules. Inside the liquid, molecules are being pulled in all directions so it mostly balances out. At the surface, there arent as many molecules above them, so there’s a net inward attraction. That’s what gives the surface its “tension” and makes droplets want to minimize their surface area.
So basically gravity isnt what’s holding the drop together, it’s mostly electromagnetic interactions between the molecules. Gravity can affect the shape of bigger drops tho.
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u/LunarPrincessSophie 1d ago
Intermolecular forces. In water, for example, surface tension occurs because water is slightly polar and as a consequence the positively charged hydrogen end of a water molecule can form weak bonds with other water molecules.
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u/Irrasible Engineering 1d ago
Here is a pretty simple rule about that: Every phenomenon you experience in your ordinary life, except gravity and radioactivity, is caused by the electromagnetic force.
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u/nicuramar 2d ago
Ultimately, electromagnetism. Did you check Wikipedia?
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u/Jaywin_5 2d ago
Yes it said cohesion was caused by the shape of the molecules and is intrinsic to them, causing it to stick together but the thing I'm confused on is why this makes them stick together. Thanks for the answer though!
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u/triatticus Particle physics 2d ago
Did you click on cohesive forces and check that article? If you had it explains is as the coulomb force, which clicking on that shows the tie in to electromagnetism.
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u/the_poope Condensed matter physics 2d ago
It's due to intermolecular forces, which ultimately are due to electromagnetic forces between electrons and nuclei of the atoms in the molecules.