The circle of felt tip ink is spread out into its component colours when the paper is slowly fed with water from a wick.

Click to enlarge


Click to enlarge
   Description of the Experiment   

The visitor inserts a piece of absorbent paper into the plastic flap and draws coloured circles with felt tip pens on the paper that shows through the three holes. The flap is then swung over so that the centres of the three holes are pressed against the three wicks whose ends dip into the dish of water.

As water spreads from the centre of the circles, it carries the felt tip ink with it. However, different component dyes in the ink are carried by the water at different rates so that they are separated out in the process. After about a minute, the paper can be removed and the number of component colours in the inks easily seen. Dark coloured inks have the most components.

   Duration   

The Experiments lasts about 2-3-minutes.

   Conclusion   

The explanation of why different sorts of dye travel at different rates through a mat of cellulose fibres (paper) when a current of water flows through it is roughly as follows.

The molecules of a particular dye in water and in contact with cellulose fibres will "partition themselves between the two phases" in a particular proportion. E.g. some dye molecules attach strongly to the wet fibres so that, at the surface of a fibre, 90% of the dye molecules attach to the fibre and 10% of them are in the water.

Another type of dye molecule might distribute themselves with 50% on the fibre surface and 50% in the water.

   Detailed Conclusion   

As the water containing the dye molecules flows past the fibres, those molecules which are strongly attached to the fibres will take much longer to flow than those of which a much greater proportion tends to stay in the water.

So the fibres exert a different drag on the different components of the ink.

None of the components travels as fast as the water itself - you can see that the paper is damp much further than any part of the ink has reached.