Fastest way to cool down a cup of coffee?

You know the drill, your are desperate for some coffee but the temperature of the newly served coffee is molten lava. What’s a good way to quickly cool down a cup of coffee? Or tea off course — It’s just water anyway.

I know one fast way with Liquid Nitrogen;

This article is available in video format here.

It occurs from time to time i forget my Liquid Nitrogen at home. In that scenario, what’s the best way to cool down the black gold as past as possible, if you have nothing? No ice cubes, no saucer and so on? Let’s test the following 3 common cooling options:

  1. Stir the coffee around with a spoon
  2. Lift the coffee up and down with a spoon
  3. Blow on the coffee

These 3 different options are clarified in this image here, same order.

Well let’s test to see what is faster, so we can get our coffee as fast as possible - or even interrupt poor strangers on a cafe, telling them they are cooling their drinks wrong.

The setup

To test the 3 different methods of cooling, i will be using:

  • An Arduino with a digital thermometer, that logs the temperature every 2 seconds.
  • A water heater.
  • A spoon.
  • Lastly, a mercury thermometer to check my readings are on point.

For reference, I did a 40 minutes measurements of a cup of coffee just standing in room temperature. I’ve also marked a green area between 45–60 degrees Celsius, where I think the coffee ins in a good drinkable temperature. I’ve even been so nice to include Neanderthal/Imperial units on the right:

In the following gif I am doing the test. These are the 4 measurements:

  • Upper left (purple ) is the control test, where nothing is done.
  • Upper right (orange) is blowing on the coffee
  • Down left (red) is lifting spoon up and down
  • Down right (blue) is stirring with the spoon

All these measurements are then done over 20 minutes. Yes I blew on the coffee for 20 minutes.

Doing nothing, blowing on the coffee, lifting spoon up, and stirring the coffee

Mechanics of heat transfer

Before showing the result I’ll cover a few concepts from thermodynamics, and talk about how they affect our coffee system.

Radiation

One way an object can transfer heat, is by radiation. The wavelength of the radiation depends on the temperature of the object. Burning coal will emit visible light because they are quite hot. But both our bodies and a cup of coffee is too cold to emit in the visible regime — they emit light in the infrared regime so we can detect that with a infrared sensor.

Sources: Coal: pixabay, Girl dancing: Route 94 — My Love, infrared coffee: FLIR infrared camera

However, because the radiation in our coffee system is constant, this phenomenon will not effect our system. It also results in an insignificant small loss of heat.

Evaporative cooling

This one you know from sweating our going out of the shower — a cooling effect! As water molecule randomly collide width each other, some molecules will randomly gain enough energy to escape the boundary force of water. So they escape out of the system and takes a lot of energy with it, leaving the system colder on average. The graph shows the distribution of different speeds of a liquid, called a Boltzmann distribution. In a given moment a small percentage of molecules (red on graph) have enough energy to escape, but only the molecules on the surface escape, since molecules in the liquid bumps into another molecule.

Image by Author

This mechanic of heat transfer could be important for our coffee cooling, as it is a effective cooling method and it depends on some of the variable we are varying, like surface area of the liquid.

Convection

As liquid and gas have a different density depending on the temperature, it will lead to a natural movement of the fluid. You know heat rises to the top in the house. The same happens in the coffee liquid, and the heated air around the coffee.

Image by Author

Conduction

Well this one is easy; it’s simply the mechanics of touch. Some materials a better and some are worse. An example is why metal feels cool, compared to something like wood. It simply distributes the heat around the whole material, while wood only distribute the heat very locally — this means the spot you are touching in metal is never being heated up by your and, as cold metal nearby transfer its “coldness”.

Image by Author

A cool example of this is done by XMDEMO he shows the effect in action. An ice cube is quickly being melted on the material (Opposite effect as to the hand).

Result and explanation

Are you ready, because this is what the result showed

Image by Author

Blowing is an absolute winner! But why? It seems like a hard question to answer, since all 3 methods have a some cooling mechanism in common:

  • They all help to distribute the heat around in the coffee. It’s called forced convection, since we are forcing the liquid around. Density is not at play here).
  • They all increase the surface area, for bigger conduction and evaporative cooling.

It turns out the answer, to why blowing is much faster, has to do with evaporation cooling. Blowing is much more effective, since, in addition to the two mechanism mentioned above, when you blow you are removing hot and humid air. This means, the air is replaced colder and dry air. Now a lot more evaporation can occur — which is extremely effective! If you don’t believe me, try drying i n some wind after taking a shower, it’s even colder.

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Want to help me create interesting & free physics articles and videos? Support on Patreon and or have a look at my science gifs as NFTs.

Thanks for reading!

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