Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.
Hi, I have a question. The other week I as at a bar with some friends and I ordered a drink which came with a straw. The straw was straight at the top end, but cut to a 45 degree angle on the bottom. To my surprise when I placed my finger over the top of the straw and removed it from the full glass, the straw failed to hold the liquid inside and it flowed out. Why does this work with normal straws but not straws cut at an angle? Thanks Flaming Ferrari (talk) 13:28, 26 December 2013 (UTC)[reply]
The trick depends on the pressure being equal in cross section at the bottom, holding up all the water equally. Since the tip is at an angle, surface tension will pull the water down the longer side, and the difference in pressure between the two sides will allow air to start flowing in the shorter side, emptying the straw. μηδείς (talk) 15:55, 26 December 2013 (UTC)[reply]
Uhmm... That would suggest, that if one leaned a capillary tube over, the water would flow out. Let us use Occam's razor. If this straw was about an 1/8 of an inch in diameter or more, then the surface tension will not over come gravity. Your drink presumably, held not pure water, so the surface tension could be even less than pure water. Any alcohol (a wetting agent) will lower surface tension by a lot, as will phosphoric acid (think Coke). What diameter straw was this?--Aspro (talk) 23:21, 26 December 2013 (UTC)[reply]
No, it would not suggest that at all, a capillary tube is much smaller in diameter. μηδείς (talk) 23:33, 26 December 2013 (UTC)[reply]
How much smaller? Give us a dimension rather than a pontification.--Aspro (talk) 23:54, 26 December 2013 (UTC)[reply]
My OR: plastic drinking straw + scissors = no such effect at all.--Digrpat (talk) 01:04, 27 December 2013 (UTC)[reply]
Yes, my research confirms that of Digrpat that there is no such effect (or that it is marginal). What does make a big difference is the accuracy of the cut at the top of the straw because this affects the seal you make with your finger. Perhaps Medeis was making another joke? Dbfirs 10:17, 27 December 2013 (UTC)[reply]
No joke. The result will depend on how far down the straw the water extends. If the water surface is not close to the angular cut, there will be no difference in surface tension between the sides, and no flow. μηδείς (talk) 19:54, 27 December 2013 (UTC)[reply]
I haven't checked this formula out, because I have no idea of the viscosity of the drink nor the diameter of the straw, therefore, I don't feel inclined to pull figures out-of-the-air to test it. Yet, it might yield the definitive answer. --Aspro (talk) 19:18, 27 December 2013 (UTC)[reply]
Why are we talking about surface tension? That's a total red-herring. When the OP puts his finger over the end of the straw...as the straw is lifted, it's air pressure that keeps the liquid in the straw. If the seal with the finger was perfect and the straw impermiable/incompressible, you could raise water to about 32 feet before a vacuum would form under your finger. This effect is totally independent of the angle of the straw to the liquid and to the angle of the ends to the cylinder. So if the straw can't hold up even a few inches of water then air is getting into the top of the straw somehow - holes in the straw, imperfect seal with finger or perhaps the straw is collapsing under the exterior air pressure...impossible to tell which without examining it carefully.
But surface tension, angle of cut, depth of straw in the liquid have (essentially) nothing to do with the answer...neither does what kind of liquid it is, because air pressure can hold up around 750mm of mercury - and there are no heavier liquids at room temp. So please ignore previous complete and utter BS from Medeis, Aspro and 71.20.250.51.
Oh for heaven’s sake SteveBaker, please read the OP's question before replying... The OP quotes:”and removed it from the full glass”. Get the point? You're referring to a Barometric. Note: the end of the tube is still in the bath and not in free air. It is the density and mass of the liquid (fighting against gravity and ambient pressure) that denotes its hight in the tube. type apparatus that does not fundamentally depend on surface tension but ambient atmospheric pressure. If you have come across these devises ( in real life rather than in just text books), you will know also, how to take surface tension 'level' into account in order to read them accurately. Please be civil and apologize for your BS comment.--Aspro (talk) 14:48, 28 December 2013 (UTC)[reply]
Yes and no. Any number of equivalent devices exist. An eye dropper works on that same principle. But let me ask you this: If a straw is straight, you can put it most of the way in, seal off the top and pull out a straw's worth of liquid. But if you tilt that straw, doesn't the liquid then run out? I'm thinking that's what's going on with the cocktail straw. Now, if you hold the sealed-off straw in such a way that the bent part is pointing straight down, would that fix the problem? ←Baseball BugsWhat's up, Doc?carrots→ 10:54, 28 December 2013 (UTC)[reply]
I was hanging out for the NYE fireworks, but I have a hunch they'll now be starting a couple of days earlier. -- Jack of Oz[pleasantries] 10:52, 28 December 2013 (UTC) [reply]
@SteveBaker: note indent for 71.20.250.51's reply. Response to previous question -- not to OP's question. ~IP:71.20.250.51 (talk) 10:57, 28 December 2013 (UTC)[reply]
Steve Baker: I believe you're mistaken. You do need surface tension to prevent all the water from running out of the straw because of Rayleigh-Taylor instability at the air-water interface. The diameter and cut will make a difference for this. If your reasoning were correct, you could do the same trick with a coffee cup. (You can't.) --Amble (talk) 13:58, 28 December 2013 (UTC)[reply]
Thanks for the interesting link, Amble. I still think that the angle of cut has only a marginal effect, even for a wide straw. I agree that it's impossible with coffee-cup diameter using water, but the trick is certainly possible up to hose-pipe diameter. Dbfirs 18:50, 28 December 2013 (UTC)[reply]
... later, after further experiments ... I made the mistake earlier of experimenting only with water. I now discover that there are indeed three variables to consider: the width of the straw, the surface tension of the fluid, and the angle of cut (plus, I suppose, the viscosity and density, though I haven't tried varying those). With water and a narrow or medium straw, the angle of cut didn't seem to be significant, but it certainly is with a wider straw and a lower surface tension, so Flaming Ferrari's observations were valid (I apologise for doubting them). I haven't experimented with different straw materials to see whether capillary effects are significant, but for normal-width straws, the capillary rise would be only a few millimetres. Oh, and BB's suggestion of holding the straw at an angle so that the angle cut part is horizontal does prevent fluid from flowing out, just as for a straight cut with the straw vertical. Dbfirs 23:00, 28 December 2013 (UTC)[reply]
I tested it, had to add a few drops of washing up liquid to the water because my straws were too narrow (or the surface tension too high), and even then it only emptied about 3 cm before it stopped. It's probably the cut causing asymmetric flow inside the straw: you need as much air to go up as there's water coming down; with a straight straw, if you take it out of the water or if you let some air in at the top, you won't get asymmetric flow: the water in the middle may move faster than the water closer to the wall of the straw, but you don't get a different speed between "left and right". When it's cut at an angle the water at the end will move to one side before dropping, pulling the water on that side down (or something similar, in any case, it's asymmetric), and in the right conditions every layer on that side pulls the next one down, keeping the flow going. A straight end makes it harder to initiate the flow.
If the straw is too narrow the surface tension will stop it eventually.
The stem of chemistry lab funnels are also cut at an angle, but that's probably only so you don't have to shake off the last drop every time you use them. During filtering you rather have the stem filled, the weight of the column would speed up the process, so the angle is more a disadvantage in that case. Ssscienccce (talk) 23:00, 29 December 2013 (UTC)[reply]
