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Physics and Science of Manufactured Goods: Drinking Straws

AAEAAQAAAAAAAAOfAAAAJGUwZTk5Y2E4LWVjOTktNGE1YS04YWI2LWU2Yjg2MzYxZDY4NAWithout some fundamental science to draw upon we all go about our day oblivious to what’s happening around us, from the micro to macro level there are phenomena at work that seamlessly blend into our experiences and transform our lives. With a new appreciation of some basic scientific concepts much more of the world opens up and is clearly evident day-to-day. It can really be life-changing.

Normal drinking straws, for example, are made from a food grade plastic of polypropylene (or in some cases bio resins of corn, wheat, or potatoes), mixed in powder form with other fillers, pigments, stabilizers, and anti-oxidants. The mixture is heated, melted, and shaped into thin rods by extruding the hot mixture through a die. Extruding, if you’re unfamiliar with the process, is “pushing” a solid through a shape to form a continuous profile. Remember the Play Dough that “grew” hair, same principal. The rods are cooled, cut into pieces, then extruded again through another die that hollows out the center. Lastly they’re cooled again, cut, wrapped, and packaged.

So we can make a straw but have you ever really stopped to think about how a straw works? You probably use one everyday without giving it a second thought. I guess perhaps you simply suck the liquid up the straw? Sounds simple enough, right? But how can you “suck” the liquid? What is suction exactly? Is there a string tied to the molecules in your Coke? Are there magnets added to your drink and in the straw? No, no convenient handles on the Coke molecules either. The process actually starts inside your mouth.

The straw is at rest in the glass with a balanced pressure outside the glass and inside the straw. To create a “suction” effect you decrease the pressure in your mouth by increasing it’s volume. To demonstate this use your finger and pretend you’re sucking on a straw. To start the process you usually have to drop your jaw to increase the volume in your mouth, therefore decreasing the inside pressure. It’s related to the mechanics of breathing, however if you had to breath to use a straw you’d likely drown in your beverage.

Boyle’s law states that at a constant temperature for a fixed mass the absolute pressure and the volume of a gas are inversely proportional…thank you Wikipedia. So what does this mean? Blow a balloon half way and tie it (our fixed mass at constant temperature). Next gently squeeze one side of the balloon thereby decreasing it’s volume, what happens? It gets tighter on the other side, or rather the pressure inside is increased. Decrease the pressure by letting go and thereby increasing it’s volume. The values are inversely (reversed in position or order) proportional. Get it?

OK back to our straw. Decrease the pressure by dropping your jaw and thereby increasing the volume inside your mouth. In an attempt to balance the pressure inside and outside of your mouth the air in the straw is forced or pushed up into your mouth by the surrounding atmospheric pressure. If you don’t get this picture that balloon again, blow it up and then let the air out. Why does the air escape? Because the pressure is greater in the balloon than it is outside. Air moves from areas of high pressure to areas of low pressure, don’t argue, it just does.

Now we have created a vacuum inside the straw, the air at the surface of the liquid (or the atmospheric pressure if you will) is now enticed to try to get air up into the straw which has no air in it. So the atmosphere now pushes on the surface of the liquid in the container, increases the pressure inside, and the liquid has no outlet except to rush up the straw. Fascinating right?

A few last interesting details, like for example straws don’t work where there is no air, like in the vacuum of space. And there is a limit to how long a drinking straw can be, it’s based upon the local atmospheric pressure. With zero pressure in the straw the atmospheric pressure (at sea level) can only push the water to about 34 feet high. We did this experiment in school once and most people could only apply enough negative pressure to get the height to ten feet or so. With higher altitudes of course there is less atmospheric pressure so the maximum height would be less.

If you have any examples of products based upon the consequence of pressure, or have any questions, suggestions, or are interested in having other plastic products manufactured please drop me a line…see us at www.natlusurd.com.

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