Sonoluminescence and Simulations

What is Sonoluminescence?

Sonoluminescence happens when small gas bubbles are frozen in liquid solutions at ultrasonic frequencies. They are held in place through acoustic waves. When the state (solid/liquid) that the bubbles reside in is broken, or the bubbles collapse, light is produced. The light is produced by thermal energy from the bubble collapse.

You can commonly see this effect in mentos when they are crushed: https://www.youtube.com/watch?v=tW8q_JfmcbU&t=126s However, this is called Triboluminescence.

Sonoluminescence is very interesting. Mainly because many properties about it are unknown. Even though we know that the light is produced by thermal energy, we still don’t know how the entire mechanism works.

There are some theories about Bremsstrahlung radiation, the argon rectification hypothesis, and hotspots that could possibly cause the thermal energy that creates the light. There are other theories that suggest temperature differences in a solution could also cause Sonoluminescence. This is still being studied however, and scientists are using spectral analysis to prove it.

Some other theories involve collision-induced radiation and corona discharges, nonclassical light, proton tunneling,and electrodynamic jets.

Sonoluminescence was first discovered at The University of Cologne in 1934 through sonar research. Two scientists, Hermann Frenzel and H. Schultes were experimenting with placing ultrasound transducers in tanks of photographic developer fluid. Seeking to speed up the process of photo development. However, they soon noticed bubbles appearing on the film, and saw the fluid was emitting light when the ultrasound was turned on. They didn’t have the equipment at the time to study the bubbles, but the bubbles in their experiment are called multi-bubble sonoluminescence (MBSL).

In the 1960s, more experiments on Sonoluminescence were conducted. Peter Jarman from the Imperial College of London proposed that these brief flashes of light were thermal in origin and could arise from micro shocks while the cavities from the bubbles are collapsing.

In 1989 experimental devices started to be used to track how this effect happens. Particularly for stable single-bubble sonoluminescence (SBSL). This effect happens when a single bubble is trapped in an acoustic wave. While it is trapped in this wave it emits a pulse of light every time it is compressed by the wave. Studying the acoustics the bubbles are trapped in allowed for a more comprehensive study of sonoluminescence. Especially for SBSL, as the scientists were only observing one single bubble.

It was also observed that under certain conditions the bubble in SBSL had an internal temperature that could be hot enough to melt steel. These observations were also noted in 2012 in similar experiments. In 2012, scientists were able to collapse a bubble and note that it contained a temperature of 12,000 Kelvins. However, most have temperatures that range from 2300-5100 Kelvins. The temperature of the bubble is very dependent on the gas that forms the bubble, and the composition of the state it is suspended in.
Scientists often use spectral measurements to calculate the temperature.

Sonoluminescence happens very fast. So it is not very stable. However, it can be made so in laboratory settings. When it is in a stable condition , the bubble can be made to collapse over and over again. It will also emit light everytime it collapses. On average, the light that flashes from the bubbles lasts between 35 and a few hundred picoseconds. The average size of the bubbles are around 1 micrometer in diameter depending on the state it is suspended in.

SBSL can have very stable periods and positions when it comes to emitting light. The frequency of the flashes can be more stable than the acoustic waves creating them. However, geometry wise the bubble has many instabilities. Because it is being crushed over and over again.

Depending on the gas that is creating the bubbles in Sonoluminescence, it can result in a brighter burst of light. Gasses such as helium, argon, and xenon can be responsible for this.

In 1937, researchers started looking into why these bubbles emit light.They could just collapse or naturally release their gas overtime, however they don’t. They first suggested that the light from these bubbles were caused by charge separation in the bubble’s cavity. For example, one wall of the bubble might have a negative charge, and the other a positive. When they collide, they emit light. However, there is some evidence to suggest that this is not the case.

If this charge separation was to happen, then a breakdown of the charges should happen during the expansion phase of the bubble. But instead the bubble collapses in an asymmetric way, and this breakdown stage does not occur.

Quantum Explanations

So because of the complications of explaining why this effect exists. A Lot of recent theories propose that there is a quantum explanation for everything. One of them being the Casimir Energy Hypothesis.

Casimir Energy Hypothesis describes something called the Casimir Effect. This describes a situation when a physical force is applied to macroscopic boundaries of a confined space. Which in turn creates quantum fluctuations of the field. It is named after Dutch physicist Hendrik Casimir, who coined this effect in 1948.

This theory was suggested as a cause for Sonoluminescence by Julian Schwinger, but wasn’t acted on until Claudia Eberlein of the University of Sussex looked into it. She suggests that the light from Sonoluminescence is generated by a vacuum within the bubble, a process similar to Hawking radiation. This is the radiation that is emitted from the event horizon of black holes.

Quantum theory operates with the understanding that a vacuum contains virtual particles.So this theory fits in quite nicely. When the energy is released from sonoluminescence it is usually very fast and large. The motion of this energy is consistent with the vacuum energy explanation.

There are some suggestions in the scientific community that sonoluminescence could be used for fusion.

Currently, the main source of fusion we have in the world right now is nuclear fusion. Nuclear fusion is the process where two light hydrogen based atomic nuclei combine to form a heavier atom. The result is usually Helium. Then the resulting energy is released as heat. This energy scatters particles such as neutrons through the air.

For fusion to occur, a certain amount of energy must be applied to the atoms to prevent them from repelling each other. For nuclear fusion this means heating up hydrogen plasma. Most forms of energy can’t be used for fusion because they cannot overcome something called the coulomb barrier. This is the force of repulsion that is felt by atoms as they get closer to each other. Each element has a different threshold of crossing this barrier.

Nuclear fusion also must work according to the Lawson Criterion in order to be self-sustaining. This can be figured out mathematically. The density of the fission reactants is multiplied against how long the reaction lasts. The resulting number must be over 1014 for fusion to happen.

If Fusion with sonoluminescence is possible, it must follow these rules as well. So bubble dynamics is a huge part of this study as well.

(This insert is for maybe a tiny group of people who will get this. But it sounds like I need to enlighten bubble guy bois.Should I encourage him? lol)

Understanding the compression of the gasses inside of the sonoluminescence bubbles is a key aspect to see if it could be used for fusion in the future. As well as the radius of the bubble, the chemical composition, initial pressure the bubble is under, and the starting temperature.

Knowing that some sonoluminescence can result in the output of extremely high temperatures, this would be quite useful if it could work. Or if this sonoluminescence fusion could be stable. However, the initial data from the studies have shown under most cases of sonoluminescence, that fusion is not possible. The amount of energy produced from a bubble collapsing would have to be 10 times the amount it is to form fusion. So it was a nice try.

Biological Sonoluminescence

In nature, Sonoluminescence sometimes happens naturally.

Pistol shrimp, or snapping shrimp as they are also known, produce luminescence from collapsing bubbles when they snap their claws together. When their claws close, it generates an acoustic pressure up to 80kPa. This in turn creates a bubble that speeds out from the claw, and reaches speeds of 60 miles per hour(97km/h). The sound released from the bubble bursting is around 218 decibels. This is enough to kill any small fish that come into contact with it. Which is how this shrimp catches its prey.

The produced from this shrimp’s effects is lower than in regular sonoluminescence, and is only visible through infrared viewing. However, scientists have playfully dubbed this effect "shrimpoluminescence" when they discovered it in 2001.

Sonoluminescence has also been observed in other crustaceans, such as the mantis shrimp. This shrimp once again uses its forelimbs to strike with such force that it creates cavitation bubbles.

These species have inspired many biodesigns in engineering.

Sonolumiencent Research Groups

This phenomenon is being studied by many different groups across the US, and around the world. The UCLA, Department of Physics & Astronomy,Acoustics Research Group is one of them.

They are currently looking at the energy of sound waves in fluids, So far they have found that sound waves in fluids can concentrate by 12 orders of magnitude to create flashes of light that last for less than 50 picoseconds.

These flashes come from hotspots inside bubbles that expand, and crash into the surrounding sound wave. These hotspots range in sizes from 10 nanometers to 100 microns.

They are also looking into something called Drop Tower Sonoluminescence. This occurs when a container of water is dropped by one inch and the impact causes a gas bubble to form. This inturn creates a flash of light that lasts for a microsecond and with an intensity around 150 Watts. The interior of the bubble formed becomes twice as hot as the surface of the sun.

Some of the other types of this effect they are looking into are:

Shake Tube Sonoluminescence:

- This effect generates a single flash of light from a 50 μm radius bubble contained in a tube of phosphoric acid. This tube is then shaken around 40 Hz.

Sulfuric Acid Sonoluminescence:

- This effect can produce multiple flashes of light from imploding bubbles in sulfuric acid. The bubbles generated can be around 90 μm maximum radius

Water Sonoluminescence:

- This effect is seen in water when it has a standing sound wave with a frequency of 40,000 cycles per second. This in turn generates 40,000 flashes of light per second.

Sonoluminescence in Liquid Metals:

- Multi Bubble Sonoluminescence has been observed in liquid metals. Mainly in mercury and alloys of Ga, In and Sn. The origin of these bubbles are thought to be associated with electrical micro discharges.

3D Visualizations

There are a few awesome visualizations of Sonoluminescence out there. So let’s explore them.

Recently, Sonochemistry and Sonoluminescenece fields are being combined to improve ultrasound technology when it comes to lipoplasty. Lipoplasty is also known as liposuction. There are certain health and safety concerns when it comes to liposuction procedures. Especially with ultrasound-assisted lipoplasty (UAL). But there are very few studies to confirm its effects on tissues in the body beyond fat cells.

Scientists are looking into making this technology safer by integrating it with aqueous solutions that are similar to Sonoluminescence solutions. The idea is that we could create a safer form of UAL by using the bubbles from Sonoluminescence to burst fat cells in localized areas. Therefore less of the surrounding tissues are damaged.

So far the physical results of the study are showing that traditional liposuction treatments are unsafe in their long term effects, and need to be updated going forward.

There are a few molecular dynamic simulations and models of Sonoluminescence out there as well. Since they are pretty complicated to set up and simulate, some clever researchers decided to start making computational models of the effect. Starting with molecular dynamics, they focused on clarifying the energy mechanisms of the system, and then moved on to the light and emission mechnisisms. All with the focus of replicating the interior of a collapsing gas bubble following the Rayleigh-Plesset equation.

Their results showcased the strong energy within the bubble including the formation of it’s shockwaves, ionization, and temperature ranges. Which averaged around 50,000-500,000 degrees Kelvin. It also showcased that the gas-liquid boundary had a huge effect on how the internal gas dynamics affected the bubble. They have also shown that the radius of the bubble correlates to the size of the light that emits from the burst of energy.

Hydrodynamic models are also being used to showcase bubble collapse. These simulations are very useful for showcasing sonoluminescence that happens under a few picoseconds. They showcase some of the same results as discussed above, while also considering options for air and water. As well as considering them as compressible fluids in the simulation.