Why does coffee always spill disposable coffee cups even with a lid?

When you realize that you are going to be late for work, you may rush to buy a cup of coffee and rush to the office as fast as you can. It's just that by the time you get to the office, the coffee in the disposable cup in your hand has spilled the small hole in the lid, and maybe there will be some yellowish-brown spots on your shirt, after that, you have to start the day in this "spotted shirt".

The same thing happened to RobKaczmarek after he bought his favorite coffee. The director of marketing and sales at ConvergentScience in the United States was very interested in the question of why coffee spilled out of the cup, even if it was covered with a lid, and then created a model of it. People who want to use fluid mechanics to solve this problem think that building this model is a joke. "it's just because of the design of the lid," he explained. "this is because the coffee will hit the lid of the cup when it shakes, and then because there is a hole in the lid, the surging coffee will spill out through the hole, and the observation shows that the speed of the coffee increases as it passes through the hole in the lid."

Of course, not all coffee cups have holes in the lids. Some lids are reasonably designed. A concave surface is designed around the holes to counteract the strength of the liquid in the cup hitting the lid through this sunken structure.

If the cup lid is angled or hemispherical, maybe you don't have to worry about the coffee spilling. LisaJardine-Wright, a physicist at the University of Cambridge, has considered this problem. "I think the lid should be designed at an opposite angle to the level."

Of course, the coffee will spill depends in part on the speed at which you walk. A 2012 study found that most coffee spills occur between steps 7 and 10.

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