# Capillary Action **URL:** \ **Category:** Factual Questions\ **Created:** [October 13, 2005, 3:08am UTC](https://boards.straightdope.com/t/capillary-action/326014 "2005-10-13T03:08:48Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![copperwindow](https://avatars.discourse-cdn.com/v4/letter/c/4af34b/32.png) [@copperwindow](https://boards.straightdope.com/u/copperwindow)\ **Post date:** [October 13, 2005, 3:08am UTC](https://boards.straightdope.com/t/capillary-action/326014/1 "2005-10-13T03:08:48Z")
When I put soda in a straw and take the straw out of the soda with my finger covering the top of he straw, the soda stays in. I asked my girlfriend why and she told me it’s because of “Capillary Action”, but when I asked her what it was she gave me a full scientific explenation with all of these terms I don’t understand. Can anyone explain this to ne in layman’s terms? ---
**Author:** ![Rhubarb](https://avatars.discourse-cdn.com/v4/letter/r/59ef9b/32.png) [@Rhubarb](https://boards.straightdope.com/u/Rhubarb)\ **Post date:** [October 13, 2005, 3:28am UTC](https://boards.straightdope.com/t/capillary-action/326014/2 "2005-10-13T03:28:20Z")
That’s not capillary action. Capillary action is what causes fluid to rise up through a very thin straw spontaneously. IIRC, this is due to surface tension. I’ve mostly forgatten the longer explanation involving wetted surfaces and molecular adhesion (I think). What you are doing when you hold your finger over the end of the straw is creating a partial vacuum at the top of the straw as you pull it from the liquid. Then the normal atmospheric pressure (14.7 psig or 101.5 kPa) at the bottom of the straw will hold the liquid in place BTW, this will not work in a total vacuum, even if you managed to find a liquid that did not boil away in a vacuum. Not to mention the damage to the capillaries in your finger. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [October 13, 2005, 3:38am UTC](https://boards.straightdope.com/t/capillary-action/326014/3 "2005-10-13T03:38:13Z")
> [@copperwindow](#): > > When I put soda in a straw and take the straw out of the soda with my finger covering the top of he straw, the soda stays in. I asked my girlfriend why and she told me it’s because of “Capillary Action”, but when I asked her what it was she gave me a full scientific explenation with all of these terms I don’t understand. Can anyone explain this to ne in layman’s terms? Liquid surfaces tend to form a film (called surface tension) because the molecules of the liquid adhere to each other and to the liquid below and not to the molecules of the gas above the surface. When the liquid wets the walls of a thin tube inserted into it the adhesion of the liquid to the material of the tube exerts an upward force and the molecules in contact with the tube transfer this upward force to the adjacent molecules because of surface thension. The liquid rises until the upward force is just balanced by the weight of the column of liquid pulled up in the tube. ---
**Author:** ![NinjaChick](https://avatars.discourse-cdn.com/v4/letter/n/71c47a/32.png) [@NinjaChick](https://boards.straightdope.com/u/NinjaChick)\ **Post date:** [October 13, 2005, 3:49am UTC](https://boards.straightdope.com/t/capillary-action/326014/4 "2005-10-13T03:49:25Z")
The straw thing isn’t entirely surface tension. It’s also basic pressure: The straw is partly filled with liquid when you take it out of the glass. Keeping your finger over one end means that, if the soda were to fall out, for an instant there would be a vacuum (no air, no soda) inside the straw. Air can’t get in through your finger, and nor can it get in the bottom while the soda is getting out. Therefore, the soda stays put. ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [October 13, 2005, 8:11pm UTC](https://boards.straightdope.com/t/capillary-action/326014/5 "2005-10-13T20:11:31Z")
If it were capillary action, not only would the fluid stay in the straw if you removed your finger, it would always spontaneously rise to the top of the straw as soon as you put the straw in the liquid. It doesn’t do either of those things, therefore it’s very unlikely to be capillary action. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 13, 2005, 8:29pm UTC](https://boards.straightdope.com/t/capillary-action/326014/6 "2005-10-13T20:29:58Z")
Have you ever donated blood? For one of the tests beforehand, the nurse pricks your finger, and touches a thin plastic tube to the drop of blood. The tube draws the blood up into it some distance. That’s a true example of capilary action. In order for capilary action to be noticeable, you generally need a very thin tube, generally much thinner than a drinking straw (these tubes are in fact called capillary tubes).