In today’s VERITAS we will look at some of the properties of bubbles. We will look at some phenomenon that are caused by bubbles and their collapse.
Almost all the information presented in this VERITAS has been gathered from the article “Bubble Puzzles” ( Physics Today Feb 2003 issue). If any of you wants to read the complete article, you can take the magazine from me.
Bubbles occur everywhere. But there are several phenomenon regarding bubbles that have still not been explained. For example: bubbles with a radius larger than .8mm do not rise up straight in water. They zig zag when they rise through water. Leonardo da Vinci was the first to notice this. The Physics of this is so complicated that we still do not have the exact equation for this.
Bubbles can be injected in fluids. Bubbles can also form spontaneously in fluids. Such fluid bubbles contain the fluid’s vapour. This process is called cavitation( or nucleation).
Cavitation occurs in a liquid when the local pressure drops below the vapor pressure of the liquid. One way to achieve this is to heat the liquid to a level when its vapor pressure becomes higher than the local pressure. For water it happens at about 100 degrees Celsius. At this point bubbles are formed everywhere in the water. These bubbles contain water vapour and not normal air. So in water cavitation is more commonly known as boiling. Also see the VERITAS on Super Heating ( VERITAS: Superheated Water 20 Mar 2003 http://unvarnished-veritas.posterous.com/superheated-water)
Another way to produce cavitation is by increasing the flow of a small part of the liquid with respect to the other parts. In world war 1 the Royal Navy approached Lord Rayleigh with a problem: the propellers of fast moving boats and submarines used to get damaged without an apparent reason. Lord Rayleigh analysed this and found the reason:
the propellers of these boats used to cause cavitation in water because of their high speeds. This cavitation produced bubbles of large sizes next to the propeller blades. When these bubbles collapsed they damaged the propeller blades!!!
Even today cavitation is a limiting factor for ship and submarine speeds.
There is an animal which uses this property of bubbles to kill its prey. The snapping Shrimp has two claws: one normal size and the other giant size. To catch its prey it closes its giant size claw so rapidly that a jet of water orignates from it.
This high speed water causes cavitation bubbles and when these bubbles collapse they emit sound in form of shock waves which kill the prey. The shrimp then uses its normal size claw to grap and eat its prey. These snapping shrimps live in huge colonies( in San Diego bay and around Florida). They cause so much noise that it disturbs submarine communication.
Another interesting phenomenon associated with collapsing bubbles is Sonoluminescence: the generation of light flashes when sound waves cause bubbles to collapse. We will not discuss this here because it has been covered in detail in a earlier VERITAS( VERITAS : Sonoluminescence Sept 1999 http://unvarnished-veritas.posterous.com/sonoluminescence) Sonoluminescence is still an unexplained phenomenon. The amount of light emitted from such bubbles is huge( 10,000 degrees Celsius types).
Go, wondrous creature! mount where Science guides:
Go, measure earth, weigh air, and state the tides:
Instruct the planets in what orbs to run,
Correct old time and regulate the Sun;