Oscillating mercury drop (08/02/04)

I forget now exactly where I found this - I have written down "from an old Russian Nauka i Zhisne (Science & Life) journal" but there are many references on the web to the oscillating mercury drop phenomenon.

Take a watchglass and place a small drop of mercury in it. Fill the watchglass with 50% sulphuric acid, to which has been added a little potassium dichromate. Take an iron rod [mild steel also works fine] and touch the mercury drop with it; the drop will be seen to contract [decrease in diameter; increases in thickness i.e. "bunches up"].

When the iron rod is placed in the solution (which is essentially an electrolyte), an e.m.f. is generated between the mercury and the iron. One will become charged +ve, the other -ve. Charge has an effect on the surface tension of the mercury and the drop spreads out. When the iron rod is touched to the mercury, the cell is shorted out and the drop contracts.

Now the clever bit. If the iron rod is touched to the side of the drop, the drop will contract. As it contracts, it breaks contact with the rod and thus expands again, only to make contact yet again with the rod. If the rod is suitably positioned, oscilliations of the mercury drop can be sustained for some period of time [around 5 seconds if you're really lucky].

Try it out and see - it is really beautiful to watch!

I later found the following two excellent articles. The first is on the oscillating drop, the second on a MEMS pump based on electrowetting.

Castillo-Rojas02 - Complex oscillatory behavior of the mercury beating heart system

Yun01 - A micropump driven by continuous electrowetting actuation for low voltage and low power operations