Tuesday, 8 November 2011

Surface Tension makes Lava lamps

When I think of stuff from the early 80's I think of Star Wars figurines and Laval lamps. Those lamps that you turn on and then turn the lights out sit in your bubble chair and watch in amazement. This was before internet, before cell phones and before good tv like Lost. This is what people did. They watched lava lamps. Granted in some cases the people might have been on drugs but that is what people did they watched lava lamps. Lava lamps are called so because the makers thought the material represented the balsatic smooth unbroken Pahoehoe lava from Hawaii.
Briton Edward Craven-Walker (WWII pilot, inventor and later nudist genre director) invented the lava lamp in 1960 and marketed it quickly after that as a display device. It was originally modeled after something he saw in a pub. What he saw was a jar with two immiscible liquids in them at a pub and decided to build his own (see link diy lavalamp). He went home and used a tall juice container of tapered glass to build his own. On the bottom of the lavalamp a standard bulb or halogen The lamp which heats the glass bottle attached. Inside Craven-Walker experimented with a number of solutions but found that a translucent mix of mineral oil, paraffin wax and carbon tetrachloried would achieve the desired effect when placed in an aqueous medium.

When looking at a lavalamp you notice the floating blobs that change after it heats up. You will see that the wax is more dense than water and float on top of the water at any temperature and when heated it expands, becomes less dense than that of water as well as more fluid. On the other hand the carbon tetrachloride is heavier than water but when heated the density is just slightly above that of water. The blobs move because they got to the top of the tapered glass and cool then go back down to the incandescent light bulb to get heated again. A wire coil in the base of the bottle acts as a surface tension breaker to recombine wax after it descends.
The lavalamp contains a standard incandescent bulb or halogen lamp which heats a tall (often tapered) glass bottle containing water and a transparent, translucent or opaque mix of mineral oil, paraffin wax and carbon tetrachloride.[1] The water and/or mineral oil can be colored with dyes. The density of common wax is much lower than that of water and would float on top under any temperature. However, the carbon tetrachloride is heavier than water (also nonflammable and miscible with wax), and is added to the wax to make its density at room temperature just little higher than that of the water. When heated, the wax mixture becomes less dense than the water because wax expands more than water when both are heated. It also becomes fluid, and blobs of wax ascend to the top of the device where they cool (which increases their density relative to that of the water's) and then descend.
The underlying mechanism is a form of Rayleigh–Taylor instability which is named after one of the fathers of understanding surface tension Lord Raleigh and G.I. Taylor. It is an instability of an interface between two fluids of different densities (like the wax, water and tetracycline), which occurs when the lighter fluid is pushing the heavier fluid.This is the case with an interstellar cloud and shock system. In the case of the lavalamps there is an equivalent situation since when the gravity acts on the two fluids of different density. The falling stream of liquid then breaks up due to the Plateau-Raleigh instability which explains why and how a falling stream of fluid breaks up into smaller droplets. The driving force in this Raleigh-Taylor instability is that liquids, by virtue of their surface tensions, tend to minimize their surface area. The lavalamp has a metallic wire coil in the base of the bottle acts as a surface tension breaker to recombine the cooled blobs of wax after they descend. Psychedelic!