Suction Pressure Locked Question

[kander]

If we take the case of a drinking straw in a glass of water. Placing you thumb over the top of the straw and lifting it from the water will lift the water in the straw, until your thumb is removed.

Question. How large a diameter can the straw go to before this no longer happens? Is this phonomena something to do with surface tension of the water?

 

[hyposmurf]

Theorical the inch water column equal to the atmospheric pressure at the point where are you.
It is about 10.033 meter or 33 ft .
If the straw is filled with mercury you can lift up about
29.52999 inchs
The phenomema has to do with the atmospheric presure.

Search for TORRICHELLI and how it demostrate it.

http://www.cadtutorforum.net

 

[JStephen]

Yes, it has to do with surface tension. Get some tubing and experiment to see how big you can go.

 

[BRIS]

I am afraid it has nothing to do with surface tension (or practically nothing). As Hyposmurf says it is to do with atmospheric pressure and you can go to any diameter (the only limit is the diameter of your thumb and your strength to lift a 10 m high column of water). You may best visualise the phenomena as the water column hanging on a vacuum at the top of the straw. As the water column drops under its weight the vacuum is stretched (until it reaches vapour pressure). The vapour pressure is the limit to which you can hold up a column of water (about 8.0m). You will need a 8.0 m long straw to test this.

Scientifically we visualise the phenomena the other way round with atmospheric pressure (10.33 m head of water) pushing up on the bottom of the water column. Since all you have at the top of the water column is a vacuum the only force pushing down is the weight of the water and since you have a pressure of 10.33 m pushing up then you can theoretically support a 10.33 m high water column. (In practice you don’t get a true vacuum – the water vaporises at the top of the column and the minimum pressure is vapour pressure – but don’t worry about that).

Brian

 

[Montemayor]

The question was:

"How large a diameter can the straw go to before this no longer happens?"

"This" refers to the ability to raise the straw without the captive water draining out of the straw.

I believe hyposmurf and BRIS don't address the issue. They merely reflect on the well-known ability to raise a column of water using vacuum as a driving force. The diameter of the straw does make a difference on the ability to be able to raise the straw out of the water (with the vacuum intact) and not have the water drain out.

JStephen is closer to answering the question.

 

[BRIS]

Well that is a question of how steady you can old the straw without allowing air to enter the straw and break the vacuum and that is a function of straw length as much as diameter. Hold it perfectly steady and there is no limit to the diameter. Asfor surface tension you will find that you can lift mercury in a class straw with negative surface tension.

 

[CRG]

BRIS, excuse my limited knowledge. Is negative surface tension called surface compression?

 

[BRIS]

CRG - I was simply referring to the fact that water is attracted to glass whereas mercury is repelled.
Since the surface of the mercury - when in a glass straw is concave - it must be in compression !.

 

[abeltio]

of course there is a limit!
otherwise, all the water in the oceans and the atmosphere would be sucked into space (full vacuum)
Thanks to Newton, who invented gravity, we can live in this planet. þ


this has to do with capillarity and surface tension vs. the weight of the column of liquid.

see:

http://www.du.edu/~jcalvert/math/hyperb.htm

a rough (very rough) experiment is this:
fill a glass with water.
place a plate or a piece of cardboard on top.
carefully turn it upside down.
if you did not leak, there is no air between the glass and the cover.
lift the glass.
clean the mess.

cheers.

saludos.
a.

 

[CRG]

BRIS, I don’t follow your surface compression explanation. I think that mercury would have a convex surface at the air/liquid interface whereas water would have a concave profile. Both profiles would have the surface in tension due to the balance of cohesion and adhesion.

Regarding the diameter of the straw, it is surface tension in combination with cohesion and adhesion that maintains the stability of the water, air, and glass interface at the bottom of the straw. I don’t think that you can have surface tension without cohesion??? Any instability in the fluid that is greater than the bottom surface tension of the fluid/air interface will cause the column of water to collapse by air entering the column. Hence, no matter how still you hold the straw when you raise it, there are still molecular motions that can “upset the cart” when the straw diameter gets large.

 

[katmar]

This is more than just an interesting party trick - it has very real applications in the process world. Vacuum systems often have what is known as a "barometric leg" which works just they way kander described the drinking straw, as a way of releasing liquids while holding the vacuum.

Surface tension definitely does play a part, but it is applicable to the bottom of the straw or pipe, and only when the bottom end is *not* submerged. As long as the bottom of the pipe is *submerged* in the main body of liquid, the diameter of the pipe or straw does not matter. The straw/pipe would remain filled with liquid water until its length exceeded 10 m (33 ft) and then if it was longer you would get water vapor forming in the volume above 10 m.

Things change as soon as the bottom of the straw/pipe is pulled out of the water. Now surface tension becomes important. If the surface tension is sufficient to stop droplets of water falling from the bottom surface, and to stop bubbles of air rising, then the water will remain in the pipe. Obviously as the pipe diameter increases there will be a greater likelihood of droplets falling.

There was a time when I could have done the force balance on the bottom surface, but those days are long since past. You can read it up in your old physics books as well as I can, so I will leave it to you to calculate the maximum diameter.

 

[Montemayor]

Thank you abelito and katmar for bringing the logic and truth of fluid statics back into reality.

abelito describes something similar to which I've done countless times in the lab and out in the field - like reading the pressure drop across an atmospheric packed column. However, we Chemical Engineers do it the classical way: we construct a barometric Mercury gauge (a "U" tube) by submerging it totally in Mercury until all internal air is displaced by the liquid Mercury. We then raise the filled tube out of the water and let gravity take it's effect on the sealed end of the "gauge" and we have a barometer! It can also be done with water as the fluid. I don't agree with abelito's description. This is not a "rough" experiment; it is very exact.

The same can be done with a conventional glass of water: submerge the empty glass in a tub of water, turning it right-side up beneath the water level and expeling all the air. Now, turn the glass upside-down (under water) and slowly raise it up and out of the water (without taking it totally out of the water) and you will have created a "perfect" vacuum in the top of the glass (due to displacement by water). A perfect vacuum will raise water to a height of 33.89866 ft (according to katmar's excellent conversion program that every engineer should have and run) so the glass should be longer than that to show a water level.

Bris' static theory falls simultaneously with the contained water when the conventional full water glass (with a pure vacuum) is slowly raised (upside down) out of the parent water tub. As abelito infers, the truth of the theory is revealed when the wet mess has to be mopped up. The fact that one can do this operation successfully with a straw, but not with a glass of water proves that the diameter of the circular apparatus is an important factor on whether the operation works or not.

 

[BRIS]

Abelto - Atmospheric pressure is 10.0m head of water - if you remove atmospheric pressure from the top surface of a liquid (create a vacuum) but leave it pressing on the bottom then the water will rise 10.0 m. If you are holding your glass of water upside down atmospheric pressure is pushing up on the bottom of he liquid. the weight of liquid is pushing down due to gravity and there is a partial vacuum at the top of the liquid. The oceans would disappear into space only if you removed gravity - we are discussing removing atmospheric pressure from part of the surface.

If your jar is 100 mm tall, atmospheric pressure is 10.0m the pressure of the partial vacuum will be 9.9m (or - 0.1 m gauge)..

It is the Van Der Waals forces between the molecules that prevent molecules dropping out of the straw and also prevent the air, that is pressing on the bottom of the liquid, from permeating up through the liquid - surface tension is a phenomenon of the intermolecular Van Der Waals forces. But the water is not held by adehesive forces forces between the liqued and the walls of the straw and this is what appeared to be being implied by soome of the answers that it is surface tension forces that is holding the liquid in the straw.

 

[BRIS]

Getting back to the original question - 1) it is atmospheric pressure that supports the water in the straw not surface tension or capillary action. 2) the maximum diameter is also dependent on the length of the straw. 3) There is no theoretical limit to the maximum diameter but as noted by montemayor try lifting an upturned glass of water you will likely end up with wet feet. The limit is fixed by how steady you can lift the straw out of the water. 4) As the straw is lifted out of the water the cohesive forces (Van Der Waals forces) between the water in the straw and the water in the bowl are broken and surface tension forces form over the surface of the water in the bowl and the surface of the water at the bottom of the straw. As you withdraw the straw from the bowl you will feel a the suction and subsequent release of the cohesive forces as the bond is broken. Unless this break is made perfectly horizontal (i.e the straw is lifted perfectly vertical and the surface of the water is perfectly horizontal) then a wedge air surrounded by surface tension forces will be formed at the break point. The dynamic forces will cause waves in the two water surfaces and this will cause the two air water surfaces to form a bubble. The air bubble rises to the top of the straw reducing the vacuum and increasing the dynamic instability - this in turn results in the formation of larger air bubbles and you have wet feet.

In conclusion the limit depends on how steady you can hold the straw and how smoothly you can withdraw it from the bowl. The difficulty is maintaining a steady withdrawal as the cohesive bonds are broken. To really get to the limit you would need to set up a rig to give a constant rate of extraction.

Brian