Thursday, January 6, 2011

Why Does Water Stay In The Straw With Your Thumb Over It?

Preliminary discussion and experiments:

Question (possibly leading)

You know that straw trick where you stick a straw in your drink, put your finger over the top of it, and pull the straw out of the drink so your straw is now full of water? And then you remove your finger, and all the liquid falls out.

As I understand it, the liquid doesn't fall out while your finger is there because to do so, it would pull a vacuum in the space between your finger and the liquid.

But if you tried this in a vacuum, doesn't that change it? If you're already in a vacuum, then you can't create more vacuum, so there should be nothing stopping the straw from allowing the vacuum to form between finger and liquid.

i thought it was surface tension at the bottom that holds the water in place.

do some simple experiments, try it with longer and wider straws.

after a certain width the lower surface breaks and the water spills out, no?

ditto with a longer straw? the weight of the water above the lower surface will break it?

now do i have any tubing to try this with? now i'm not so sure about a longer straw...

maybe there is also the interface between the water and the straw that is holding the water in there.

You're a genius

I was thinking about this after my initial reactive post. And was thinking the real culprit is surface tension. What I was thinking was what if the straw was 3 ft in diameter. No way it would hold. Since a park is being built near by there was some waste PVC. I tried my experiments and I couldn't get water to stay in anything with a diameter of larger than .5 in.

I liked your strait forward logical approach.

wait, simpler experiment, try it with soap in the water. or another fluid with less surface tension.

now where is that staw...

HA experiment rules! just tried it, made a staw

out of a pen, got the water in it.

touched the bottom with my finger, nothing

touched it with water, nothing.

touched it with dish soap, boom! half of it poured out till it reached half way up the straw, where there was no longer soap and a miniscus formed again

I know that surface tension plays a part, but not the whole part. Otherwise, removing your finger at the top wouldn't cause the water to fall. Also, bulb pipettes would never work.

ok, did you see my post above? the next peice of teh puzzle is when you said the water doesn't fall cause you can't make a vacuum between the water and your thumb. but if the collumn of water is more than about 30 feet (at which the weight of the water is = to the weight of the atmosphere of similar cross section) the water DOES fall, and DOES leve a vacuum

ok, so further experimentation with my straw (4" long by 1/4" inside diameter) shows that when i lift my finger, surface tension WILL keep the water in the straw if the collumn is about 2 or 3mm high. that's the extent to the force due to surface tension.

it's confusing. with the finger, atmospheric pressure holds the water up, but only if the surface tension is there. once i break it, the interactions at the lower surface become complex and...


Now this is EXACTLY what I was getting at!

Atmospheric pressure HAS to play a role in siphons.

Surface tension definitely works to counteract gravity to some extent (see capillary action), but it also plays a role in preventing air from moving through the liquid to the top.

I wonder if the water in your experiments isn't forming bubbles when you add the soap, which move to the top and allow the water to fall without pulling a vacuum.

thinking about the geometry and the forces at the surface of the water with or without soap gave me a headache. maybe i'll think about it some more.

the surface tension allows the atmospheric pressure to act on the entire surface as a whole, i.e. the lower surface essentially becomes a rigid membrane. and thus the atmospheric pressure can exert its force on the entire column.

when i shake the tube when the surface is convex out, a drop will pinch off, and the surface will reform, flat or concave in.

if i shake it some more a bubble will form and will rise a certain distance, which i suppose means that the collumn of water above it is sinking a distance of the thickness of the bubble. it then usualy stops half way up the tube and gets stuck! maybe at this point the collumn of water above it is not heavy enough to exert enough force to move it and surface tension/adhesion holds it in place?

another puzzle. even with a column of air above a column of water, the water does not spill out. it's got atm pressure below pushing up. but what's the presure in the collumn of air above? i don't have the tools with which to measure it.

even if there is just a bubble, i don't know if the volume of air forming the bubble gets expanded i the formation of the bubble thus lowering the air pressure inside the bubble.

it's all very confusing.

It's still confusing to me when you use the phrase "pulling a vacuum"

For the Straw: A vacuum makes no difference. It isn't a pressure differential that is keeping the water "up" It is the seal (and therefore closed system) between the fluid, straw, and your finger, that prevent a bigger vacuum.

Now explain that one to me...

What scientific principle keeps the liquid from falling?

Can we draw a force diagram? What is acting on the liquid to counteract the force of gravity?

FWIW (probably not much), Wikipedia claims that atmospheric pressure plays a part.

force diagram... it's a fluid.. but, mg for the mass of liquid, down. air pressure times cross sectional area pushing up. now the complicated part: at the miniscus: it's i don't know what shape.. but it ISN'T horizontal! so that leads me to beleive that mg of the water is pushing down on it, and then there is the surface tension pulling with horizontal component towards the wall of the straw and vertical component up.

better observation: for my 4" water column in ~1/4 in. inner diameter plastic straw, i can make the bottom surface of the water convex, flat or concave.

that's curious. wait, that's with air between the top of the water and my thumb...

more experiments:

so i immerse the straw and my thumb in teh glass of water: pull it out and no space between top of water and my thumb. the miniscus is convex, bowing downward.

i shake it a little and a drop of water comes off, miniscus horizontal.

shake it again, and teh miniscus goes concave into the straw.

shake it again and another drop of water comes off and a BUBBLE sloooowly rises up the straw till it reaches finger, and now there's a region of AIR between the top of the water and my finger.

but the water STILL doesn't pour out! hmm what's the miniscus at the top look like?

lets see..

straw too cloudy at top to see, but here's something curious:

holding column of water in with finger at top.

shake it a little and drop of water comes out and bubble rises to the top of the collumn

shake it again and another bubble rises half way and gets STUCK in the column of water, so there is from top to bottom:

finger, thin layer of air, 2" column of water, 1/5" layer of air, 2" column of water!

You missed it.

1) We can ignore the meniscus and surface tension forces. This is obviously not sufficient to keep the liquid suspended, as demonstrated when the finger at the top of the straw is removed.

2) My question is regarding this situation in a vacuum, so air pressure is NOT what is pushing the water upward to counteract gravity.

So what is?

i know, my answer is only part of your question... but wait, you at least read the results of my experiment? with the finger in place the water stays in the tube. when i destroy the surface tension with soap, the water pours out!


lets start here: if i raise the straw out of the water, mg in the column of the water is down. weight of atmosphere transmitted to the cross section of the straw at the surface pushes up.

the water does not fall while the weight of the water is less than the weight of the atmosphere above that column.

when you lift the straw up about 30 feet, finally the weight of the water IS greater and the higher you lift it, the water WILL fall, and WILL leave a vacuum below your finger.

this is why we invented steam engines to work at the bottom of mines. pumps at the surface could not pull water up when it was below 30 feet!

3) also, i failed to point out that probably with a THIN enough straw the water will probably stay in even with finger off. but now maybe adhesive forces between water and the walls plays a part? it's similar to my result of a 2-3mm tall column of water will stick in the tube with air above and below it and no finger holding top.

4) We also had discussion of why siphoning water with a tube from one bucket to a lower bucket works. It's related but adds MORE complications.

5) Important note from another discussion:

Rayleigh-Taylor instabilities.

The real reason the water falls out of the tube.

why do you say these instablilties is why the water flows out of the tube? in the turbulent regime? or is it just what gets it initiating the flow?

It is what intiates the flow.

Remember the only reason we are talking about this problem at all is that the outside air pressure is pushing the water back into the closed pipe with more force than the water weight pusing out. This is the condition Of RT instability. A low density fluid, air, pushing on a high density fluid, water. The simulation picture in the wiki article, if you turn it upside down is, is exactly the OP's problem. This is for the pipe straight vertical.

One simple proof is the old playing card/ glass of water trick. Place a card over the open end of a glass of water. Hold it and invert the glass. Let go of the card and, magically, the water does not fall out! The card is a stiff solid and changes the conditions away from the RT condition. The card is not free to flow. But it shows the outside air pressure is great enough to hold the water (and card) in the glass. So gravity is insufficient to pull the water out of an inverted glass. You need RT to make it fall!

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