Sonoluminescence

Sonoluminescence is the occurrence of a flash of light during the collapse of cavitation bubbles generated in a liquid by a powerful ultrasonic wave. A typical experiment in observing sonoluminescence is as follows: a resonator is placed in a container with water and a standing spherical ultrasonic wave is created in it. With sufficient ultrasound power, a bright point source of bluish light appears in the very center of the tank - the sound turns into light.

Content

History and Early Research

Despite the fact that the phenomenon was first observed back in the 1930s , the mechanism of sonoluminescence was completely incomprehensible for a long time. This is due to the fact that in the first experiments only single and rather dim flashes were visible, that is, all this time it was not possible to select the optimal conditions for the occurrence of sonoluminescence.

From left to right: the appearance of a bubble, slow expansion, rapid and sudden collapse, the emission of light.

In the 1990s, installations appeared that gave a bright, continuous, stable sonoluminescent light. As a result, it became possible to study sonoluminescent light not with the help of photographic films (that is, accumulating light over a long period of time), but in real time, with excellent temporal and spatial resolution. The experiments showed that the sonoluminescent glow occurs as a result of the following cycle:

A standing ultrasonic wave in the rarefaction phase creates a very low pressure in water, which leads to local rupture of water and the formation of a cavitation bubble.
For about a quarter of the period of the ultrasonic wave (that is, as long as the pressure remains very low), the bubble grows, and if the standing sound wave is spherically symmetric, then the bubble remains spherical. In individual experiments, the diameter of the bubble reached fractions of a millimeter.
In the compression phase, the cavitation bubble collapses, and faster and faster. The collapse process also accelerates the surface tension force .
In the final lobes of the period, a very short and bright burst of light erupts from the center of the collapsed bubble. Since in the stationary mode a cavitation bubble is born and collapses millions of times per second, we see an averaged sonoluminescent light.

From the point of view of physical intuition, sonoluminescence has a number of paradoxical properties.

Sonoluminescence is most effective in ordinary water. Only in recent years ^{[ when?} ^] it was difficult to achieve the occurrence of sonoluminescence in other liquids.
A small concentration of inert gases dissolved in water significantly enhances the effect.
The brightness of sonoluminescent light increases dramatically with cooling water.
A bright sonoluminescent flash usually has a more or less smooth spectrum , without any separate spectral lines . This spectrum grows steeply in the violet direction and is approximately similar to the spectrum of the radiation of a completely black body with a temperature of the order of hundreds of thousands of Kelvin .

It was the spectrum that became the main stumbling block when trying to explain the phenomenon. If sonoluminescent light is of thermal origin, it is necessary to explain how ultrasound heats water to such temperatures. If high temperatures have nothing to do with it, then what is the origin of light in general?

Single Bubble and Multi Bubble Sonoluminescence

In the 1990s, the phenomenon of multi-bubble sonoluminescence was discovered. It arises if the conditions for cavitation are created not at a point, but in a rather large area, of the order of a centimeter or more. In this case, many individual bubbles are continuously born and collapse, which interact, unite, collide with each other. In contrast to this regime, the regime of the central vesicle described above began to be called single-bubble sonoluminescence .

With multibubble sonoluminescence, the glow is more dim and has a completely different spectrum. Namely, in the spectrum, individual emission lines are clearly traced and even dominated; for example, the emission line of the excited neutral radical OH * at 310 nm is clearly visible. In addition, if any substances are dissolved in water, then their emission lines also appear in the spectrum ^[1] . All this irrefutably testifies in favor of the fact that the luminescence during multi-bubble sonoluminescence is of thermal origin. Depending on the specific conditions, the temperature of the luminous region during multibubble sonoluminescence was 2000-5000 kelvin ^[2] .

The sharp difference in the spectra of single- and multi-bubble sonoluminescence led to the appearance of the point of view that we are talking about completely different phenomena. However, in the early 2000s, works appeared in which a smooth transition between these two modes of sonoluminescence was discovered ^[3] . After these studies, it became clear that single-bubble sonoluminescence is of a thermal nature, and its mysterious spectrum is explained by too high temperature and pressure during the collapse of one spherically symmetric bubble, so that individual excited radicals remove excitation in a collisional manner and do not have time to emit a photon ^[4] .

Theoretical Model

So, if the nature of light is thermal, then it is necessary to explain why such high temperatures are achieved.

Currently, it is believed that water heating occurs as follows.

With rapid compression of the cavitation bubble, water vapor experiences a process close to adiabatic compression. Moreover, since the radius of the bubble can decrease by tens of times, it is quite possible to heat water vapor by orders of magnitude, that is, up to several thousand Kelvin.
It is known that the heating efficiency in an adiabatic process is determined by the adiabatic index, which in turn strongly depends on which gas we are considering. Heating is most effective for monatomic gases, so that even small impurities of inert gases in water can significantly affect the heating efficiency.
The dependence of the sonoluminescence brightness on the water temperature is determined by the balance between water vapor and inert gases inside the bubble. With decreasing water temperature, the volatility of inert gas vapor almost does not change, while the pressure of saturated water vapor drops sharply. This leads to better vapor heating during compression of the bubble.
It is clear that the initial bubble has an irregularly spherical shape. When collapsing, these distortions of symmetry are amplified, and as a result, it is not possible to focus all the initial energy into a point. If with single-bubble cavitation, when the initial distortions are small, it is possible to reduce the radius of the bubble by an order of magnitude or more, then with multi-bubble sonoluminescence, the initial distortions do not allow the bubble to be compressed strongly, which affects the final temperature.
In the case of single-bubble sonoluminescence, at the last stage of the collapse of the cavitation bubble, the walls of the bubble develop a speed of 1–1.5 km / s, which is 3-4 times faster than the speed of sound in the gas mixture inside the bubble. As a result, during compression, a spherical converging shock wave arises, which then, reflected from the center, passes through the matter again. It is known that a shock wave effectively heats the medium: when passing through the front of the shock wave, the substance heats up M² times, where M is the Mach number . This, apparently, leads to an increase in temperature by another order of magnitude and allows you to reach hundreds of thousands of Kelvin.

Schwinger Model

An unusual explanation of the sonoluminescence effect, which belongs to Schwinger ^[5] , is based on the consideration of changes in the vacuum state of the electromagnetic field in the bubble during the rapid change in the shape of the latter, from a point of view close to what is usually used to describe the Casimir effect when the vacuum state of the electromagnetic field is considered in a flat capacitor, depending on the boundary conditions determined by the plates. (See also Unruh effect ). This approach was developed in more detail by Claudia Eberlein ^[6] ^[7] .

If this is true, then sonoluminescence is the first example in which radiation associated with a change in the vacuum state is directly experimentally observed.

Arguments were made in favor of the fact that sonoluminescence is associated with the conversion of too much energy in too little time to be consistent with the above explanation ^[8] . However, other credible sources argue that the explanation through vacuum energy may still be true ^[9] .