Dancing droplets in the pan

You don’t always need a big laboratory for exciting experiments. You can measure the temperature coefficient of resistance with a multimeter from the DIY store. That means you’ll know exactly when to cook your pancake.

Everyone knows that the first pancake is always a failure. But why? Opinions vary on the internet (they always do) but the simplest and most attractive explanation is that the frying pan is just not hot enough at first. So how can you know when the pan is hot enough? By using the sharpest knife in the drawer – science. (You’re studying at university, so you should know what a metaphor is by now.)

Put the frying pan on the heat, let it get hot and throw in a few drops of water. If the pan is hot enough, the droplets will skitter over the surface without appearing to evaporate. That’s known as the Leidenfrost effect, described in De Aquae Communis Nonnullis Qualitatibus Tractatus (A tract about some qualities of common water) by the German doctor and theologian Johann Gottlob Leidenfrost (1715-1794).

So why don’t the droplets evaporate and what makes them skitter over the surface? As soon as the droplet touches the red-hot surface, a tiny amount of water on the undersurface evaporates. The steam that’s created forms a protective layer between the surface and the droplet, which means that the heat of the pan is transferred much less efficiently. The droplets skitter over the surface because the steam forms a sort of cushion on which the droplets float.

There are films on the internet that demonstrate the Leidenfrost effect in a different way. They show a red-hot steel ball being dunked into a beaker of water. You’d expect the water to boil instantly, or at least start to hiss and spit, but it doesn’t. The layer of steam around the ball provides insulation so the water only boils after a large delay.

There are other ways to check whether your frying pan is hot enough. Manufacturers now make frying pans with a spot that changes colour as the pan heats up. It’s a nice idea, but doesn’t come cheap. You can also use an infrared thermometer to measure the temperature of an object from close up but without touching it. Once again it’s a nice idea, but expensive, and ready-made solutions aren’t nearly as much fun. We want to use DIY.

After a great deal of thought, the BOOM editors came up with the temperature coefficient of resistance: metal has a higher electrical resistance and you can measure that.

Attach the probes of your multimeter (five euros at the DIY store) with adhesive tape or an elastic band to a ruler so that they are equidistant for every measurement. Set the multimeter to the highest sensitivity (usually 2000 ohm) and measure the resistance of the cooking surface of the frying pan at room temperature. Heat up the pan and measure the resistance again. If you’ve done it right, then the resistance will have increased

So what’s happened? Iron conducts electricity because the electrons are more or less ‘loosely’ attached to the atoms. If you charge the metal, the electrons flow from the negative to the positive pole. Add heat and the atoms begin to vibrate, resulting in the electrons having to jump over higher hurdles on their way – a higher resistance, in other words.

You can work out that change in resistance with simple addition: R2 = R1(1 + α(T2 - T1)), where R1 and R2 are the resistance at temperature T1 and T2, respectively, and α is the temperature resistance coefficient. That’s something you just look up in Table 8 of the so-called Binas book, the Dutch secondary school science compendium. Now you know why the BOOM editor keeps his Binas among the cookbooks…

Auteur: Ernst Arbouw