Your standard lava lamp, also called a liquid motion lamp, show cases coloured globs of one liquid immersed in a second liquid that's usually transparent. Both are contained within a tall, see-through housing. The key to creating the characteristic undulating movements is in the relative properties of these liquids.
Did you know? Immiscible liquids, like oil and water, don't mix with one another and, therefore, produce multiphasic mixtures. Miscible liquids, like chocolate syrup and milk, can mix to form a single phase solution that is completely homogeneous.
First, to exist as different moving entities in the lamp, the liquids are immiscible.
Second, the densities of the two liquids are very similar, but their rate of expansion upon heating is different. So when you turn on a lava lamp, the two liquids are gently warmed by a heating lamp at the base of the unit. Eventually,the heat causes the coloured globs to expand faster, and become slightly less dense, than the transparent liquid it's immersed in. The result? The globs slowly float to the top.
But when watching your lava lamps, you may have noticed that not only do the globs float to the top, they drop to the bottom too! This is achieved by making sure that the housing containing the two liquids is sufficiently tall enough to create a temperature gradient. That is, the temperature at the top of the housing is a bit cooler than at the bottom. So as the globs rise, they cool down, contract, become denser than the transparent liquid, and start tsink. At the bottom,the cycle starts all over again. And there you have it: perpetual motion.
Did you know? Hot air balloons use these very same principles to rise and descend.The air in a balloon expands when it's heated by propane burners at its base. This warmer air inside the balloon is the less dense than the colder air outside the balloon, making it rise into the sky.
Did you know that nature takes advantage of similar principles? Some fish have swim bladders which they can fill up with gas (decreasing their density relative to surrounding water) whenever they want to rise to the surface. Modern technology also applies this idea to control the
buoyancy of submarines, using large internal ballast tanks that can be filled with either water or air to change the overall density of the sub. Just goes to show you how the simple principles of the nifty little lava lamp can be seen in action in the world around us.
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