Liesegang Rings (or Liesegang Bands). Pattern formation by Reaction/Diffusion

Recipe: (From the June 1969 issue of Scientific American, "Amateur Scientist" column)

a) Dissolve 4% by weight of ordinary gelatin into hot water.
Also dissolve into this water, 0.4 grams of potassium dichromate per 100 ml. of water.
WARNING: Dichromate is a serious cumulative poison; don't ever let any get on you.

b) Pour this hot liquid solution into thin layers in Petri dishes, or onto microscope slides.
(In this case, we put these gels between slides, with a one millimeter space between them.)

c) Leave your poured plates overnight to gel.

d) Either put crystals of silver nitrate directly into contact with the dichromate gel, or dissolve enough silver nitrate into water, and put drops of this solution in contact with the gel.

e) Watch precipitation of red silver dichromate crystals in the gel.

Instead of forming a continuous mass of silver dichromate crystals, rings of crystals form, separated by clear areas in which crystallization has not occurred. Depending on the geometric patterns of sources of diffusing dichromate and silver ions, various geometric patterns can form, including not just bands but also spots and dotted lines.


video of Liesegang bands forming, shown in class

computer simulation of bands forming, shown in class

Pascal program that generates this simulation