Physics of a Syringe: Surface Tension & Water Retention

In summary, a vacuum is not necessary to prevent syringe contents from spilling out. The plunger seal and friction between the plunger and barrel prevents the contents from spilling.
  • #1
Da Apprentice
59
0
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?
 
Physics news on Phys.org
  • #2
Hi Da Apprentice! :smile:

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. :wink:
 
  • #3
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.
 
  • #4
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.
 
  • #5
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.
 
  • #6
Blaze_409 said:
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.
 
  • #7
Blaze_409 said:
… 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 :wink:) have no sucky-power.

Vacuums have lots of nothing

in this case, no pressure …

so no downward force on the column …

that's all! :smile:
 
  • #8
Blaze_409 said:
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 said:
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 said:
Vacuums (like black holes :wink:) have no sucky-power.

Vacuums have lots of nothing

in this case, no pressure …

so no downward force on the column …

that's all! :smile:

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.
 
  • #9
Andy Resnick said:
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 :smile:

friction holds the plunger, but the plunger doesn't hold the contents :wink:

(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)
 
  • #10
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...
 
  • #11
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.
 

1. What is surface tension?

Surface tension is a physical property of liquids that causes the surface of the liquid to behave like a stretched elastic membrane. It is caused by the cohesive forces between the liquid molecules, which are stronger on the surface due to the lack of neighboring molecules. This creates a "skin" on the surface of the liquid that allows it to resist external forces and maintain its shape.

2. How does surface tension play a role in a syringe?

In a syringe, surface tension is responsible for the formation of a meniscus, which is the curved surface of the liquid at the top of the syringe. This meniscus is created by the surface tension trying to minimize the surface area of the liquid, pulling the liquid up the sides of the syringe.

3. How does water retention affect the function of a syringe?

Water retention, or the ability of a material to absorb and hold onto water, can affect the function of a syringe in two ways. First, if the material used to make the syringe is not water-retentive, it can lead to leakage and loss of liquid. Second, if the material is too water-retentive, it can cause the syringe to become harder to push or pull, making it difficult to accurately measure and dispense liquids.

4. Can the surface tension and water retention of a syringe be altered?

Yes, the surface tension and water retention of a syringe can be altered by changing the material it is made of. Materials with lower surface tensions, such as plastic or glass, can reduce the amount of liquid that clings to the sides of the syringe. Materials with higher water retention, such as rubber or silicone, can help to prevent leakage and make the syringe easier to use.

5. How does the physics of a syringe impact its accuracy in measuring liquids?

The physics of a syringe, specifically surface tension and water retention, can greatly impact its accuracy in measuring liquids. The surface tension of the liquid and the material of the syringe can affect the formation of the meniscus, making it difficult to accurately measure the volume of liquid in the syringe. Water retention can also cause variations in the amount of liquid that is retained in the syringe, leading to inaccurate measurements.

Similar threads

  • Other Physics Topics
Replies
32
Views
8K
  • Mechanics
Replies
26
Views
1K
Replies
26
Views
2K
  • Introductory Physics Homework Help
Replies
9
Views
1K
  • Introductory Physics Homework Help
Replies
15
Views
2K
Replies
16
Views
3K
Replies
7
Views
2K
  • Introductory Physics Homework Help
Replies
2
Views
4K
Replies
8
Views
2K
  • Mechanics
Replies
8
Views
2K
Back
Top