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Ask Dr. Knowledge

Why is drinking a milkshake through a straw harder than a soft drink?

May 10, 2010

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If I order a soft drink at a fast-food restaurant, I can drink it with a thin straw or a wide straw, with little difficulty either way. When it comes to a milkshake though, I can’t get anything at all through the narrow straw. Why?

The first thing to notice about the milkshake is that it’s viscous, which is the scientific way of saying that it’s sticky. Water is somewhat viscous, too, but much less so.

When you suck on a straw, you are removing air from inside your mouth to make a partial vacuum, so that the outside air pushes the liquid up the straw for you to drink. The rate at which it flows is higher the lower the viscosity of the liquid. Water, or a soft drink, is easier to suck up the straw because it sticks less to itself and the sides of the straw.

What comes as a surprise to most people is how the flow rate depends on the diameter of the straw. It is proportional to the fourth power of the diameter, so that a straw that is half the diameter of another (a factor of two smaller) is 2x2x2x2=16 times harder to suck through. Water flows so easily that you don’t really notice the difference — 16 times harder than “almost effortless’’ is still “easy.’’

If you can just manage to suck up a milkshake through the wide straw, however, you would need 16 times the suction to get it up the narrower one at the same rate.

Incidentally, blood is also viscous. This is why it is important not to clog your arteries — just a small amount of narrowing can require a huge extra effort to keep the blood flowing. It’s also why you get so little notice before a heart attack.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.