|MadSci Network: Physics|
Hi Jennifer, Wow, what a fun question! Before getting too far into the details, I have a fun experiment you can do for yourself. Empty out your beer bottle, and then fill it up with water. Use a clear bottle such as Corona, so you can see the water inside. If you are like me, and you think all beers are inferior to Guinness, then you can just dump it into the sink. I don't recommend drinking it yet, because you'll need your taste buds in a bit. What you are looking for is a bottle that you can see through easily, filled with a clear liquid that doesn't foam. Next, take some pepper and sprinkle it onto the surface. Then remove the pepper from the glass near the tip, so that the only pepper remaining is sitting on the surface of the water. The reason I use pepper is because it has a very sharp impact on your taste buds. You can taste even a single speck. On the other hand, the pepper will rest on the surface of the liquid and will not sink into the body of the bottle (until you take a sip). Now take a sip. Do you taste any pepper? That is the quick and dirty answer to your question. When I do the experiment, I do not taste any pepper on the first sip, but I do on the second. So what is happening? When you rotate the bottle of beer, you are moving the glass bottle itself. Most of the liquid inside, however, does not rotate with the bottle. To see how this is, take an ordinary glass and fill it with ice water. If you rotate the glass, the ice cubes remain nearly stationary. The only way you'll see the ice cubes rotate in unison with the glass is if you rotate it evenly for several seconds. Try it! There is a reason for this: If you are familiar with the concept of friction, then you probably know that you can move one object by rubbing it against another. For example, set up a stack of paper on a cloth surface, and then slide the top sheet with your hand. It helps to fan the paper first. This puts air between the pages, which reduces friction between the pages. What you will find is that the pages will slide in layers. The top page goes with your hand, but the pages immediately below do not travel so far. The pages at the bottom of the stack stay with the table. The same thing is happening in the beer bottle. As you rotate the bottle, the liquid inside remains oriented the same direction it was before you rotate the bottle. Only a very thin layer next to the glass rotates with the glass. In physics we call this the boundary layer. It is usually very thin. Therefore, when you take a sip, you will be getting mostly liquid that was earlier up against the side of the bottle, just at the base of the neck. For extra credit, you can watch the pepper flakes move along with the air bubble to the middle of the bottle while you take a sip, which tells you generally where the liquid goes. There is a caveat however! Once you disturb the liquid it starts to mix. It doesn't take much of a jostle to create mixing. Simply picking up and putting down a beer bottle is usually enough to trigger mixing; taking a full sip will *completely stir* the bottle. You would be absolutely shocked to find out how quickly the liquid in the bottle mixes. In fact, if you watch the pepper flakes immediately after taking your sip, you can learn a lot about this mixing and will have taken your first step into the study of what we physicists call "fluid dynamics". (Dynamics means the study of motion.) The first thing you will notice is that the pepper disperses evenly throughout the bottle within a second or two. The second thing you will notice is that the flecks often move in little spirals. While the detailed reasons for these motions are interesting (to physicists), the upshot is very simple: When you sip from the bottle, (without slurping or other chicanery), you get a sip from the side of the bottle, just slightly below the top. Once you've sipped, your next sip consists of beer mixed from all parts of the bottle. Finally, you should know that beer behaves almost identically to water in this experiment. The two are so very close, that the final answer is the same whichever you use. By contrast, if you were to redo the experiment with molasses, you would taste the pepper on the first sip, indicating that you are getting molasses off the top of the bottle. The primary reason I don't use beer in this demonstration is that the foam adds an additional layer of complexity (no pun intended), and frankly, beer and pepper do not taste well together. Hiccup!
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