Chocolate Milkshake Day

What the day is

Chocolate Milkshake Day is observed on September 12. The interesting thing about a shake is not the flavor but the fact that drinking one is a fluid mechanics problem with a hard limit.

Origin and background

Drinking through a straw is not sucking liquid up. It is lowering the pressure inside the straw so that the ordinary weight of the atmosphere pushes the liquid up for you. This matters because it caps what is possible: no matter how hard you pull, the largest pressure difference available is one atmosphere, since the best you can do is remove all the air inside.

How much flow that buys depends on the straw and the drink. The resistance of a narrow tube rises extremely steeply as it narrows, roughly with the fourth power of the radius, so halving the width of a straw makes it about sixteen times harder to draw through. Multiply that by a shake many times more viscous than water and you reach a genuine wall. A sufficiently thick shake in a sufficiently narrow straw cannot be drunk at all, however determined you are, which is the entire reason milkshake straws are noticeably wider than soda straws.

The thickness itself comes from three things working together. Ice cream is already a foam, whipped full of air during freezing, and that air makes it stiff. Fat globules crowd the mixture and resist flow. Ice crystals remain suspended rather than melting, and a shake blended too long turns thin because those crystals disappear and the foam collapses.

Many commercial shakes contain no ice cream at all, being made from a soft-serve mix with stabilizers that hold the texture. A malted version adds malted barley powder, which contributes the distinctly savory note.

How to take part

• Use a wide straw, or accept a spoon, which is not a defeat.
• Blend briefly. Over-blending melts the crystals and thins it out.
• Chill the glass first, which buys several minutes.
• Try a malted one if you never have. The difference is larger than it sounds.