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#1
11-21-2002, 11:17 PM
 Jinx Member Join Date: Dec 1999 Location: Lost In Space Posts: 7,797
No Flow in the Soda Straw, But Why?

You know the old trick...put your finger over the end of a straw, and the fluid (water, soda pop, etc.) within can't flow out freely. But, why not? It cannot be air pressure pushing the fluid out when the air pressure is the same at either end of the straw! Why isn't gravity alone strong enough to cause the fluid to flow out of the straw?

It makes no sense. The column of fluid in the straw gives it a "static head" which should work in its favor, the change in elevation is down, which works in its favor, and the friction losses (against the walls of the straw) are negligible - or else it'd never flow out. So, what gives?

I've studied fluids, but where is it written that flow depends on displacement of air? (Esp. when the air is behind the column, not ahead to block it from flowing downwards, freely.)

I must have been out sick that day in science class... I'm missing something basic... - Jinx
#2
11-21-2002, 11:23 PM
 mangeorge Guest Join Date: Mar 1999 Location: Berkeley, CA Posts: 14,089
Hint: If the straw is big enough, the water will flow.
Peace,
mangeorge
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#3
11-21-2002, 11:27 PM
 Dr_Paprika Guest Join Date: Oct 2000 Location: South of Toronto, Canada Posts: 3,538
Cecil never did answer that question about sucking spaghetti. But just because the air pressure is the same on either side of the straw does not mean that the total pressure is. To get the gfluid to go up in the first place, the sucking pressure has to exceed that of gravity and friction (which works against the fluid, moving up in the straw). Fluid goes up the straw to begin with because the pressure by sucking on the end gets transferred down the straw... if you drew a diagram of the bottom half of the straw the fluid would still rise because the pressure in the middle of the straw exceeds that at the fluid side even though the sucking is at the top end.

Since the pressure gradient is inside the straw, it must exceed the strength of gravity when the straw is covered by the finger (this pressure is in the air at the top of the straw and can't escape through the bottom hole). Friction losses are not negligible in a small straw (or with any fluid, this is why engineers calculate the Reynold's number), and of course friction acts up when the fluid is trying to flow down.
#4
11-21-2002, 11:33 PM
 UncleBill Guest Join Date: Jan 2001 Location: Central Joisey Posts: 4,527
Ahh, but when you place a straw in a full glass without sucking, cover the end, and remove, the fluid still remains. Think vacuum, surface tension, and capillary style action.
#5
11-21-2002, 11:37 PM
 UncleBill Guest Join Date: Jan 2001 Location: Central Joisey Posts: 4,527
And try this, I just did. Place straw in full glass so that fluid (water) enters up halfway. Cover open end with finger and remove straw. Water remains in straw. Turn straw 90 degrees to horizontal. Water remains in straw, same as before. Turn more so finger end is below water, remove finger for an instant and then cover again quickly, so that the column of water progresses toward the finger end (so you have air inside the starw at both ends of the water). Return straw to horizontal. Remove finger from end. The water remains in the columnar shape, although both ends are open to the air.
#6
11-22-2002, 12:22 AM
 Ringo Charter Member Join Date: Apr 1999 Location: Houston, TX, USA Posts: 11,261
And the surface tension thought makes me wonder - what happens if you touch the water on the bottom end of the straw, when a finger is in place topside, with a piece of soap?
#7
11-22-2002, 04:13 AM
 Mangetout Charter Member Join Date: May 2001 Location: England Posts: 56,990
Try it with a less viscous fluid and it doesn't work.
#8
11-22-2002, 07:54 AM
 UncleBill Guest Join Date: Jan 2001 Location: Central Joisey Posts: 4,527
True, that is why I specified water in this case. Probably would n't work with a less viscous fluid, or one with a lower surface tension. Interesting on the soap angle. It appears the water flowed out at a faster rate using this method than it did touching the straw with a non-soapy finger.
#9
11-22-2002, 09:00 AM
 Mangetout Charter Member Join Date: May 2001 Location: England Posts: 56,990
I think what is essentially happening is that the viscosity/surface tension of the liquid is such that air won't naturally form a small enough bubble to enter the bottom of the column.
#10
11-22-2002, 10:14 AM
 slortar Guest Join Date: Feb 2001 Location: Kalamazoo, MI Posts: 5,142
You people must be a blast to go to restaurants with...
#11
11-22-2002, 01:55 PM
 Jinx Member Join Date: Dec 1999 Location: Lost In Space Posts: 7,797
Quote:
 Originally posted by mangeorge Hint: If the straw is big enough, the water will flow. Peace, mangeorge
Mangeorge, how big? Do you mean big enough such that my finger could no longer cover the opening? Anyhow, this doesn't really explain what really stops the fluid from flowing out of the straw... Thanks, Jinx
#12
11-22-2002, 02:20 PM
 Jinx Member Join Date: Dec 1999 Location: Lost In Space Posts: 7,797
Quote:
 Originally posted by Dr_Paprika Cecil never did answer that question about sucking spaghetti. But just because the air pressure is the same on either side of the straw does not mean that the total pressure is. To get the gfluid to go up in the first place, the sucking pressure has to exceed that of gravity and friction (which works against the fluid, moving up in the straw). Fluid goes up the straw to begin with because the pressure by sucking on the end gets transferred down the straw...
Wait! I was talking about free-flowing under gravity...not sucking. Yes, I know you're talking about total suction head which is the pressure needed to lift a fluid flowing through a pipe, or straw, in this case.

Quote:
 Friction losses are not negligible in a small straw (or with any fluid, this is why engineers calculate the Reynold's number), and of course friction acts up when the fluid is trying to flow down.
Yes, I am aware that, typically, the friction losses for flow in a pipe are not to be neglected. And, the frictional losses increase with flowrate. But, instinctively, my first thought is to say that the fricitional losses can be ignored. Hmm...I will have to give this a little more thought.

The typical hydraulics problem either gives you: (a) a fixed flow rate and one end pressure as inputs (plus the physical piping configuration itself) OR (b) the two end pressures...and you solve for the flow rate. Regardless, the flow is assumed to be a constant value.

But, in this case, I'm talking about free fall of a liquid which would, therefore, not have a constant flowrate. Hmm...this shouldn't really matter too much...

Again, I'll have to give this matter more thought.
- Jinx
#13
11-22-2002, 02:58 PM
 zut Charter Member Join Date: Apr 2000 Location: Detroit, MI Posts: 3,725
Perhaps this isn't the question you're asking, Jinx, but you realize that the air pressure in the top side of the straw (between the water and your thumb) is less than atmospheric? As you lift the straw up, the increasing head of the water column causes the water to flow downward a tiny bit, which makes a larger air volume at the top, which reduces the air pressure. When you lift the straw clear of the water, you have different air pressures at each end of the straw: the pressures are different, automatically, by the weight of the column of water. As long as the surface tension is enough to prevent an air bubble from working inwards, this is stable, and there is no flow.

Or is that not what you're asking?
#14
11-22-2002, 03:17 PM
 mangeorge Guest Join Date: Mar 1999 Location: Berkeley, CA Posts: 14,089
Quote:
 Originally posted by Jinx Mangeorge, how big? Do you mean big enough such that my finger could no longer cover the opening? Anyhow, this doesn't really explain what really stops the fluid from flowing out of the straw... Thanks, Jinx
Big enough so that the outside air can form a bubble, as mentioned by Mangetout, and break the vacuum above the water column.
Take any cylinder that's open on one end, a water glass will do, and immerse it in a bowl of water. Turn the closed end up, lift the cylinder up slowly out of the water, and the column will stay in the glass until the open end breaks the surface of the water in the bowl. You can see the bubble of air rise to the top, releasing the water column.
What keeps the water in the straw is obvious, because when you remove your finger, the water flows out. The only thing you've changed is the vacuum at the top of the water column.
Peace,
mangeorge
#15
11-22-2002, 03:58 PM
 bbeaty Guest Join Date: Aug 1999 Location: Seattle WA USA Posts: 888
The water is heavy, so it should flow out... but as it starts moving down, it stretches out and the water pressure at the top gets less. (Or if there are any tiny bubbles up at the top near your finger, they expand as the water column moves down, and again the pressure there gets less.) So the pressure between the top and bottom of the water is not equal as the OP thought.

That lowered pressure is too strong. The pressure difference between the outside air and the top of the water column is more than enough to keep the column lifted up against your finger.

So, make the straw longer. That will make the water column heavier. How tall must it be before the water column finally starts falling? That would depend on the outside air pressure, so results will be different on a mountaintop, versus at sea level, versus down in a pressurized submersible.

At 15PSI pressure, the water column ideally starts falling when the column is 33 feet tall.

But if you actually try performing this experiment (with 50ft of aquarium tubing in a stairwell!) you'll find that you don't need 33 ft. This is because water is full of dissolved air, and if the pressure starts falling, the water "effervesces." The top of your aquarium tubing will fill with bubbles and the water column will start descending. If you want to see the "33 feet effect," you have to use degassed water.

The classic water-column experiment doesn't use water, it uses a much heavier liquid: mercury. If you dunk a glass tube into a container of mercury, seal the top of the tube, then lift it upwards, the mercury column will stay up there unless the column is taller than about 29 inches. Ever hear the phrase "twentynine inches of mercury?" Probably, since you can use a mercury-filled plugged-straw setup to measure the pressure of outside air.

PS
No matter how hard you suck, you can't create more than 15PSI of "suction" at sea level. Put a vacuum hose on your neck and it will produce a mild hickey. But if you take yourself deep underwater where the pressure is higher, the hickey-producing ability of vacuum pumps becomes proportionally greater. At the bottom of an ocean trench, a vacuum pump could suck flesh into a hose like it was whipcream, or produce a hickey in a steel plate. I've always wondered if that's why giant squids are able to leave scars on whales. At 1 ATM pressure I wouldn't think a 1" squid sucker could make much of a mark. But 33ft underwater the pressure is two ATM, and 66ft deep it's three.
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#16
11-22-2002, 04:33 PM
 PosterChild Guest Join Date: Jan 2001 Posts: 524
Re: No Flow in the Soda Straw, But Why?

Quote:
 Originally posted by Jinx I've studied fluids, but where is it written that flow depends on displacement of air? (Esp. when the air is behind the column, not ahead to block it from flowing downwards, freely.) - Jinx
This may be a sticking point. How could the fluid start to move without displacing air? Air has to go up if the water goes down.
Go to the extreme: catsup. Hold it upside down and it won't flow out until you stick a knife up one side to let air in, otherwise it won't flow. If your straw had a path for air to flow from the bottom to the top then the air would flow into where the water was- which it can't with your finger there.

PC
#17
11-22-2002, 05:37 PM
 ZenBeam Guest Join Date: Oct 1999 Location: I'm right here! Posts: 8,858
Quote:
 Mangeorge, how big? Do you mean big enough such that my finger could no longer cover the opening?
How about bigger than a drop of water? If you get your finger wet, and let the water drip off, the drops are about as big (within a factor of two, anyway) as a fat straw. And a factor of two makes sense: a drop forms on one side of the straw, and a bubble on the other side, and the water can pour out.
#18
11-22-2002, 07:51 PM
 manhattan Charter Member Charter Member Join Date: Aug 1999 Posts: 9,132
Quote:
 Originally posted by slortar You people must be a blast to go to restaurants with...
UncleBill pays. So yes.
#19
11-25-2002, 08:24 AM
 slortar Guest Join Date: Feb 2001 Location: Kalamazoo, MI Posts: 5,142
Actually, I don't think it's that big a deal to pay, since y'all would just sit around playing with your drinks until the wait staff gets sick of you and throws everybody out.

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