Surface Tension vs. Air Pressure

Which is stronger, liquid surface tension or ambient air pressure? How are these forces related? How do they affect the movement of air and water? These are not questions you’re likely to encounter out of the blue, but they can lead to a pretty fun little mini experiment. Give it a try if you’ve got a few minutes and a few interested students (or maybe just randos who dig science).

The Spills & Thrills of Science

For this Demo Science science demo, you’ll need two identical drinking glasses, preferably with thick sides (they don’t actually have to be glass—plastic ones will work just fine), a drinking straw, a vessel filled with water deep enough to submerge the glasses, and somewhere where you can spill water without making too huge of a mess. Side by side sinks work perfectly for those last two requirements.

Dunk your drinking glasses in the sink/tub/whatever of water and make sure they fill up completely (no air bubbles). Turn them sideways and align their mouths. Turn them vertical again—one glass should be upside down on top of the other, and they should be lined up so that the water is contained within (a few drips down the sides are inevitable, but if it comes pouring out, you’ve done something wrong and need to start over).

Lift them out of the water-filled basin and place them in your spill catching container. Be very careful as you set them down so you don’t jar their alignment and spill. Set the right side up glass on the floor of the sink (or whatever), keeping your hand on the upside down glass to maintain its balance.

With your observers watching closely, carefully lift the top glass to make a small, barely visible opening between the rims of the glasses. You’ll know right away if you’ve pulled them too far apart because SPLOOSH! All the water will pour out of your top glass. If you do it just right, you’ll see a tiny gap with a bead of water running all the way around it, filling the space between the glasses.

At this point, there actually is enough space for the water to come out, but its surface tension will hold it in place. (It’s similar to when you fill a glass to the brim and just a little bit beyond, when you can see a bubble of liquid that’s sticking up above the top of the glass, but it doesn’t spill over the sides.)

Like this!
Photo credit: owly9 / Foter / CC BY-NC

Now, take your straw and aim it at the opening. Blow through it into the space. SPLOOSH! The water will pour out of the top glass.

Breaking the Tension

When you pulled the glasses apart, the air pressure outside of them was greater than the air pressure inside. This, combined with the water’s surface tension, kept the water from spilling out. When you blew air into the glasses with your straw, you disturbed this equilibrium as the air going in broke the surface tension and displaced the water. That, coupled with the unavoidable pull of gravity, allowed the water to escape.

If you want to up the stakes and try again, perform this demonstration with a different, thicker liquid, such as gravy or semi-cooled Jell-O. These fluids will require more air pressure to break their surface tension, making further experimentation both informative and delicious!