Every one of us has seen formation and collapsing of air bubbles in water during rain or while blowing air in the water. There are many reasons why a bubble pops. Evaporation of its water content, air turbulence, and, most commonly, dryness - contact with a dry surface or dry air. The bubbles formed in the sun evaporate quickly. We know that hitting water with sound waves also forms the bubbles.  Bubbles are low-pressure regions surrounded by high pressure. In a bubble, the outer pressure pushes in on the lower-pressure air, and the bubbles quickly collapse. Difficult to observe commonly but when the bubbles collapse; they emit light, in flashes that last trillionths of a second. Theories range from tiny nuclear fusion reactions to some type of electrical discharge, or even compression heating of the gases inside the bubbles.

One of the leading theories is that it is caused by adiabatic heating of the bubble at collapse, leading to partial ionization of the gas inside the bubble and to thermal emission such as bremsstrahlung.  Many researchers thought that the rapid collapse of the bubble heat the gas inside and create a glowing plasma.  Physicists have measured high temperatures inside these bubbles, on the order of tens of thousands of degrees Fahrenheit, and taken numerous pictures of the light they produce. It is far from clear, what is the source of the light produced on bubble collapsing.  However, bubble power is considered as promising method of generating energy.  It works under the principle of sonofusion, which is technically known as acoustic inertial confinement fusion.

Sonoluminescence

Sonoluminescence can occur when a sound wave of sufficient intensity induces a gaseous cavity within a liquid to collapse quickly. This cavity may take the form of a pre-existing bubble, or may be generated through a process known as cavitation. Sonoluminescence in the laboratory can be made to be stable, so that a single bubble will expand and collapse over and over again in a periodic fashion, emitting a burst of light each time it collapses. For this to occur, a standing acoustic wave is set up within a liquid, and the bubble will sit at a pressure anti-node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained. In this case, the bubble collapses into a very small volume, creating extremely high focal pressures and temperatures exceeding 5000⁰C. If there is nothing in the immediate vicinity of the bubble, the collapse is radially symmetric, and the bubble implosion creates a shock wave. The shock wave is capable of destroying both soft (cellular structure) and hard (bone, calculi) tissues. If there is a hard boundary within a few radii of the bubble during collapse, the collapse is asymmetric and causes a high-speed jet of fluid that has sufficient force to affect even metal surfaces. This is the mechanism by which end effectors become eroded with use and the mechanism for efficient tissue emulsification.

Tiny bubbles imploded by sound waves can make hydrogen nuclei fuse—and may one day become a revolutionary new energy source. It is derived from a related phenomenon, sonluminescence, where a source of sound, which is attached to a liquid-filled flask, sends pressure waves through the fluid, exciting the motion of tiny gas bubbles. The bubbles periodically grow and collapse due to extreme temperatures inside the bubble, producing visible flashes of light lasting less than 50 picoseconds. Chemical reactions occur during cavitations of a single, isolated bubble and yield photons, radicals and ions. That means, gas bubbles in a liquid can convert sound energy into light. It is hard to imagine that mere sound waves can possibly produce in the bubbles, even briefly, the extreme temperatures and pressures created by the lasers or magnetic fields, which themselves replicate the interior conditions of stars like our sun, where fusion occurs steadily. The bubbles’ violent collapse can cause some of the deuterium nuclei to undergo fusion. About 20 years ago, researchers studying these light-emitting bubbles speculated that their interiors might reach such high temperatures and pressures that they could trigger fusion reactions. Since then, several groups have been trying to achieve fusion using sound waves, most of them with a kind of enhanced sonoluminescence. This method, called single-bubble sonoluminescence, involves a single gas bubble that is trapped inside the flask by a pressure field and yields light flashes during repetitive implosions. Research teams from various organizations have joined forces to create acoustic fusion to promote the development of sonofusion which may one day become a revolutionary source of energy.

Industrial applications

The process of bubble formation in a fluid like water is called cavitation. These bubbles eventually collapse under the pressure of the surrounding fluid, sending out pressure waves that can affect anything nearby. Power ultrasound generates a number of effects resulting from cavitation bubble collapse that are beneficial in food processing. Some of the most useful applications are ultrasonically assisted extraction, emulsification, viscosity changes, protein modification, crystallization, sterilization, drying, and the tenderization and marination of meat.

• Shrimp use cavitation bubbles to hunt because the waves can kill small fish.
• Cavitation bubbles can also damage boat propellers, which produce the bubbles when they rapidly slice through the water, lowering the pressure.
• Cavitation has useful medical and industrial applications, like breaking up kidney stones and shattering clumps of dirt during wastewater processing.

In these cases, the objects affected are much larger than the collapsing bubble but cavitation could have promising applications for smaller particles, too, but there are a lot of unknowns about what happens to small particles near the bubble.

• The effect of cavitations on nearby particles that are about the same size as the bubble itself may be useful for manufacturing cleaning substances without having to use chemicals.
• Cavitations could also be used as a method for cleaning agricultural produce without chemical agents. Small bubbles could draw microbes and dirt away from produce without damaging the surface of delicate fruits like tomatoes and strawberries.