Simple Chemistry Makes Mercury Blob Oscillate Into Different Shapes

As part of my ongoing exploration for my complexity lab manual, for examples of simple energy flow producing complex patterns, I've found this

mercury blob oscillator. (The web page includes an explanation and has a video which i can't get to work anymore)

What's happening here is basically that potassium dichromate is rusting the nail. This is a spontaneous process in which energy is released, in fact it releases the energy we put into the iron nail when we refined it from its ore (another form of rust!). That release of energy is what runs the whole fascinating process.

The reaction takes place by putting a mercury blob in a dilute solution of sulfuric acid and placing an iron nail next to it. Then i think in the video, the experimenter drops some potassium dichromate into the solution to get it started. This simple setup of water, mercury blob, nail, sulfuric acid and potassium dichromate is enough to produce complex motion.

Basically the reaction is that the dichromate oxidizes the iron nail. This is similar to the process of rusting, which happens spontaneously in our oxygen filled atmosphere. However, this reaction happens slowly. By placing a mercury blob close to the iron nail, the reaction is sped up, and fun things happen. Mercury is a fluid with a high surface tension and this causes it to form round blobs, like water drops on a waxy surfaces (leaves, feathers..).

First, the dichromate in the solution oxidizes the mercury, forming a layer of mercury sulfate (like a layer of rust on iron). This layer of rust doesn't have as much surface tension as the pure mercury does, so the blob flattens out. As with any oxidation reaction, while the mercury is oxidized, the dichromate Cr2O7(2-) is reduced to the chromium (III) ion. Eventually the blob flattens out enough to touch the nail, now the mercury sulfate layer oxidizes the iron, and again, since the iron is oxidized, the mercury sulfate becomes reduced back to liquid mercury, the blob rounds up again, and the process repeats.

You see in the video that once the experimenter has adjusted the position of the nail just right, the blob then begins to oscillate by itself. It does so in very curious ways! Oscillating in a square shape and then it seems to switch to a triangular shape! Part of this process is mediated by the fact that oxidation/reduction involves a transfer of electrons, and I suppose that the mercury blob will conduct the electrons away from the point of contact with the nail and spread them to the whole surface of rust to reduce it back to mercury. Perhaps different patterns of oscillation would form if the nail were positioned above the center of the blob or even more than one nail were used.

The interesting part of the process is that while the net result of the reaction is that all of the dichromate is eventually reduced to chromium ions and then the process comes to a halt, on the way, the intermediate stage of mercury oxidation and reduction can oscillate.

(This is similar to how the BZ reaction works (another complexity lab) where the bromate oxidizes all the malate to carbon dioxide, but on the way, the intermediate reaction, oxidation/reduction of cerium or iron also oscillates and forms the colored patterns on the petri dish.)

This oxidation and reduction of metals is basically how your battery runs your ipod to make your earbuds oscillate in interesting patterns called music! One metal is oxidizing another metal inside the battery. Eventually the battery runs down. In essence this blob oscillator is the battery and motor combined!

In fact almost everything interesting that happens on earth happens in this manner. For instance we, and all living creatures, can move around and have oscillations (breathing, heart beating..) because these are intermediate reactions within the ultimate process of oxidizing food to carbon dioxide. Even the patterns of weather across the planet is a process of oscillations and cycles that mediate the flow of heat from the sun to outer space as it passes though the earth's atmosphere.

It is true that one day the sun will stop 'burning' and all energy flow will cease. This is what the second law of thermodynamics is all about, that, by itself, the entire universe is running down to a state of high entropy (randomness). The point of these labs, is to show that in the meanwhile, this process of running down drives the spontaneous formation of interesting stable patterns.