Help with liquid transfer through tube

1. Jul 15, 2012

shnabz

Hello,

Have you ever drank through a straw? Notice how there is always drink left at the end that you can't drink? I believe the volume of this liquid is the same as the volume of the bore of the straw. Now, my question is;
Is there a way of pumping a small amount of liquid (2-5ml) over a 2ft distance without losing <90% of the liquid?

2. Jul 15, 2012

HallsofIvy

I don't know what you mean by "volume of the bore of the straw". The "bore of the straw" is its cross sectional area and does not have volume. If you mean the volume of the entire straw, that is clearly not true.

3. Jul 15, 2012

CWatters

I don't believe it's that great.

As you suck on the straw you create reduced pressure at the top end. Air pressure them pushes liquid up the straw to try and correct this. All is well until the level in the cup falls to the point where air starts to enter the bottom of the straw. Then air starts to make it's way up the straw and as it does so some liquid near the bottom end falls out... but it's not equal to the whole volume of the straw. I believe it depends on the viscosity and diameter of the straw.

4. Jul 20, 2012

shnabz

Okay so i used the wrong word, what i meant was, the radius of the cross section of the straw (provided it is the same throughout) multiplied by ∏ multiplied by length of straw. (Displacement)

So, lets say the straw has no flex and is very rigid, sucking on the top would remove the air, creating a negative pressure? The air is displaced by the liquid, the liquid is sucked out the other end, when there is no more liquid in the container, the liquid will be displaced by air, as soon as the air enters the straw, the pressure will be no more, and the volume of air that enters will be the same volume of liquid that falls back out the bottom into the container, therfore you will never be able to get anymore liquid out of the top.

Well...thats what i think anyway, thats why i'm here. Anyhow, can anyone answer the question? Maybe some kind of device that diverts air at the mouth end, so only a continuous flow of liquid gets through?

5. Jul 20, 2012

CWatters

as soon as the air enters the straw, the pressure will be no more

That's not correct the pressure difference still exists. A bubble of air could continue to push liquid up the straw. The problem is that for large diameter straw it's easier for the bubble to "push past" the liquid rather than pushing uniformly on the end of the liquid. For small diameter straws or fine capilary tubes the air bubble can quite happily push all the liquid along until the straw is totally empty.

There is probably some horribly complicated relationship between the viscosity of the liquid and the diameter of the straw you need to get it to work.

6. Jul 20, 2012

Staff Emeritus
That's clearer. However, it's not true. It's possible - easy even - to leave less than that volume behind. So we can't find an explanation for a fact that isn't a fact.

7. Jul 21, 2012

shnabz

I didn't ask for an explanation, just a simple answer to a simple question.

Thankyou CWaters, this is what i was looking for, or failing to understand.

8. Jul 21, 2012

Staff Emeritus
But the simple question is "why is there always liquid equal to the volume of the straw left behind", and that is not true. So, like "why is the sky yellow" it does not have an answer.

9. Jul 21, 2012

Danger

It does in Detroit... :uhh:

10. Jul 21, 2012

rorix_bw

Not true. Explain.

11. Jul 21, 2012

CWatters

Well lets see...

For a straw to work the liquid must be able to get in at the bottom. If the bottom of the straw was perfectly flat and in contact with the bottom of the cup no liquid could get in. Typically the bottom of the straw isn't in perfect contact, its at an angle. So chances are air will start to get in before the cup is totally empty.

With low viscosity liquids it's possible that air can "push past the liquid" so that bubbles of air travelling up the straw don't prevent liqud running back down the other way.

As to predicting at what point this occurs or what volume of liquid can't be sucked up (without moving the cup/straw about) that's anyones guess.

12. Jul 22, 2012

shnabz

I don't recall asking that^ question at all.

The question is:
Is there a way of pumping a small amount of liquid (2-5ml) over a 2ft distance without losing <90% of the liquid?

13. Jul 22, 2012

shnabz

@ rotrix_bw
I'm asking you to explain, because i didn't know, however CWatters has explained

14. Jul 22, 2012

shnabz

I think I could put the liquid in a container and turn it upside-down to prevent air getting in prematurely.

I wonder if there is a special straw/tube/hose that is soo thin that it prevents air from "pushing past".

15. Jul 22, 2012

rorix_bw

I understand why things might not get sucked in because the liquid is no longer in contact with the straw but i don't understand what you mean by air puishing past, ad stuff in the straw running back down ... dont you just suck harder?