Rainbow Coloured Chocolate

Age Group: 9-12 years

Duration: 30 minutes

Biomimicry Connection:

In nature, one can find many examples of so-called structural colouring caused by the structure of the surface (e.g. peacock or kingfisher feathers, Apatura iris butterfly wings, metallic glitter of a Anoplotrupes stercorosus beetle, etc.). The use of this method can be very beneficial - for example, reducing the consumption of chemical dyes. Pupils will see that with the help of a simple action they can achieve interesting results.

Activity Details:

Tools and Materials

• diffraction grating (available here)
• 100 g of chocolate
• spoon
• thermometer
• access to a stove
• access to a fridge

Preparation

• Watch an example of this experiment here to familiarise yourself with the process. The experiment starts around 6-minutes into the clip, but it is really useful to watch from the start.

Description

• Explain to pupils that they will try to make chocolate with similar color properties to the peacock feather.
• Take the diffraction grating, in our case a film with a special microstructure that will help create a structure that scatters white light ) and chocolate for the experiment.
• Encourage pupils to view and play with the diffraction grating in light to understand that the grating is made of tiny spaces arranged in a regular pattern which make the light refract into rainbow colours. When passing through the grating, different wavelengths (of different colours) of white light are reflected at a different angle, and therefore break down into individual colours.
• Start melting some of the chocolate in a water bath at a temperature of about 45°C. Add a few more pieces of unmelted chocolate until it has all been melted. Remove from the heat and, stirring constantly, whisk to a spreadable consistency.
• Spread it on the diffraction grating in a thin layer (the thicker the layer, the longer the setting time).
• Let it solidify for about 10 minutes in a fridge. It is important to let the chocolate solidify thoroughly and then handle it preferably with gloves (so that you do not disturb its surface with warm fingers).
• Whilst waiting for the chocolate to solidify, show pupils some other examples of structural colouration from nature (there are lots of examples online).
• Ask pupils to observe the colour change of iridescent chocolate. Break off a piece of the ‘miracle’ chocolate for each pupil to examine and then eat

What has happened to the chocolate so that it now reflects colours of a rainbow? What did we spread the chocolate on? And does the peacock use the same trick we just used?

We spread the melted chocolate on a seemingly smooth surface. But this is no ordinary surface. On one side (where we spread the chocolate) there is a special texture, a kind of regular scratching, so small you can’t see it. We laid our soft chocolate on this textured surface, and when it hardened, it has this texture imprinted on it too. Since the texture is what causes the iridescent reflections, now our chocolate has this wonderful ability too.