Why is it hard to pour from a mug?

It’s a lazy Saturday morning and you’ve just finished pouring the last of the coffee into your mug when your significant other informs you they were hoping to have a little more coffee, too. Being the generous person you are, you pour half of your fresh coffee into their mug … only to dribble coffee all over the counter and the floor. Why is it so hard to pour liquid from a mug? A physics principle known as the Coanda effect is to blame for the mess in your kitchen, but it’s not all bad — it’s also the reason airplanes can fly.

You’ve probably never heard of Henri Coanda, which is a shame. After all, he was the first person to build a jet-powered aircraft (and according to some accounts, fly one) way back in 1910. His invention was less than successful, but in subsequent years, he did make a big contribution to our knowledge of how airplane wings produce lift.

In 1934, he filed a patent for a device that works on what’s now known as the Coanda effect: On a curved surface, a moving stream of fluid will create internal pressure that keeps it moving along that surface. Because air is considered a fluid just like water, an airplane wing can use this effect to generate lift. How? Faster moving molecules have lower pressure than lower moving molecules (according to a rule called Bernoulli’s principle), so a jet of air is basically a low-pressure stream surrounded on all sides by high-pressure areas. Place a surface on one side of that jet, and you remove the high-pressure area pushing up on it, leaving the pressure on the other side to push the entire jet down onto that surface. Voila, the stream of fluid stays “stuck” to the surface, even if that surface curves.

An airplane wing is curved on top and straight on the bottom. Because air’s natural tendency is to go in a straight line, the air molecules curving around the top of a wing are in conflict. Those closest to the wing stay “stuck” to the wing, but those furthest escape its pull and move straight. The bottom of the wing, meanwhile, is straight, so the air molecules stay packed together as they slide along its surface.

The result is that you have the same number of air molecules on the top of the wing as on the bottom, but on top, they’re stretched out into a larger area, which creates lower pressure. The air pushes up, and ladies and gentlemen, we have liftoff.

What does this have to do with coffee? The molecules in your coffee also experience pressure from the ambient air, and when it flows along the surface of your mug, that pressure keeps it “stuck.” And as the Coanda effect assures us, that coffee will stay stuck even as it curves around the lip of the mug. But the Coanda effect isn’t infinite. If the curve is sharp enough, as on a wine glass or the spout of a pitcher, the fluid becomes “unstuck” and flows freely. That’s why the coffee pitcher doesn’t dribble everywhere, but your mug does.

Checking out my related post: What is Luckin Coffee?

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