Physics of a Syringe: Surface Tension & Water Retention

[Da Apprentice]

I was wondering, why is it that when water is "sucked up" by a syringe that this water stays inside the syringe and doesn't leak out?... Is this because of the surface tension of the water?

 

[tiny-tim]

Hi Da Apprentice!

No, it's because of the air pressure pushing upwards …

total force upward = pressure times area, and that's more than enough to balance the weight, if the tube isn't too long.

 

[KingNothing]

Actually, I believe it is more complicated than that. Water tends to stick to the walls of it's container - this is why you see the edges in a glass of water climbing up the sides a bit. And because of surface tension, it doesn't like to form really 'small' volumes - this is why if you spill water, it tends to come out in drops and puddles rather than spreading totally evenly.

Put those together, and it would take a lot of extra energy to force the water down the thin needle.

 

[Andy Resnick]

Nobody has mentioned the friction between the syringe plunger and the syringe walls. Disposable syringes consist of a rubber tipped plunger within a plastic barrel, and the friction is quite considerable, preventing unwanted movement of the plunger. Precision syringes consisting of ground-glass plungers and barrels can often discharge the contents on their own as the resistance to movement is quite small.

 

[Blaze_409]

I think the explanations here are flawed.

A syringe in effect is a vacuum. In the closed position put your finger over the tip and try to pull the handle on the syringe.(note even if you pull it up, when you let it go it comes back down).

And we know that air vacuum's(physics not the household applaince) don't exist except in space. space is a vacuum

The idea is that a medium,(water/air etc.) that you suck up won't spill back out because of the suction created by the vacuum in the syringe.

 

[KingNothing]

I think the explanations here are flawed.

The idea is that a medium,(water/air etc.) that you suck up won't spill back out because of the suction created by the vacuum in the syringe.

This is what tiny-tim was saying above, just in different words. The air pressure from outside the syringe is too great compared to the pressure inside the syringe for it too fall out.

 

[tiny-tim]

… The idea is that a medium,(water/air etc.) that you suck up won't spill back out because of the suction created by the vacuum in the syringe.

Vacuums (like black holes ) have no sucky-power.

Vacuums have lots of nothing …

in this case, no pressure …

so no downward force on the column …

that's all! ​

 

[Andy Resnick]

I think the explanations here are flawed.

A syringe in effect is a vacuum. In the closed position put your finger over the tip and try to pull the handle on the syringe.(note even if you pull it up, when you let it go it comes back down).

And we know that air vacuum's(physics not the household applaince) don't exist except in space. space is a vacuum

The idea is that a medium,(water/air etc.) that you suck up won't spill back out because of the suction created by the vacuum in the syringe.

This is what tiny-tim was saying above, just in different words. The air pressure from outside the syringe is too great compared to the pressure inside the syringe for it too fall out.

Vacuums (like black holes ) have no sucky-power.

Vacuums have lots of nothing …

in this case, no pressure …

so no downward force on the column …

that's all! ​

I don't understand the logic. A full syringe held vertically by the barrel will not (generally) discharge the contents. If the plunger is free to move, the plunger will fall due to gravity and the contents will be discharged. The reason the plunger stays is due to friction between the plunger and barrel.

Try it and see- fill the syringe with air and observe what happens. Fill a precision syringe with air and see what happens.

 

[tiny-tim]

The reason the plunger stays is due to friction between the plunger and barrel.

Yes, obviously the contents won't stay unless the plunger stays …

friction holds the plunger, but the plunger doesn't hold the contents ​

(but on reflection, i think i was too quick to agree to the vacuum suggestion …

i think the contents usually are in direct contact with the plunger, but that the reaction force between them is reduced to area times atmospheric pressure minus weight)

 

[pallidin]

IHMO, Andy has is right.
The friction of the plunger seal inhibits plunger movement in the primary AND reverse direction. The reverse direction "holding" force is greater than the atmospheric or internal pressure to expel syringe contents.
Case closed.
Next patient...

 

[Skrambles]

It's a combination of the surface tension AND the pressure difference. Try filling a syringe from the rear with no stopper and nothing covering the needle. Very little of the water will drip out from the needle. If you were to put something with less surface tension into the syringe, mineral oil for example, then you would have to clean up the mess you just made. I know this because I had to clean up just such a mess.