D. Felipe Gaitan

Sonoluminescence and bubble dynamics for a single, stable, cavitation bubble

High‐amplitude radial pulsations of a single gas bubble in several glycerine and water mixtures have been observed in an acoustic stationary wave system at acoustic pressure amplitudes on the order of 150 kPa (1.5 atm) at 21–25 kHz. Sonoluminescence (SL), a phenomenon generally attributed to the high temperatures generated during the collapse of cavitation bubbles, was observed as short light pulses occurring once every acoustic period. These emissions can be seen to originate at the geometric center of the bubble when observed through a microscope. It was observed that the light emissions occurred simultaneously with the bubble collapse. Using a laser scattering technique, experimental radius‐time curves have been obtained which confirm the absence of surface waves, which are expected at pressure amplitudes above 100 kPa. [S. Horsburgh, Ph.D. dissertation, University of Mississippi (1990)]. Also from these radius‐time curves, measurements of the pulsation amplitude, the timing of the major bubble collaps...

Transient cavitation in high-quality-factor resonators at high static pressures.

It is well known that cavitation collapse can generate intense concentrations of mechanical energy, sufficient to erode even the hardest metals and to generate light emissions visible to the naked eye [sonoluminescence (SL)]. Considerable attention has been devoted to the phenomenon of single bubble sonoluminescence (SBSL) in which a single stable cavitation bubble radiates light flashes each and every acoustic cycle. Most of these studies involve acoustic resonators in which the ambient pressure is near 0.1 MPa (1 bar), and with acoustic driving pressures on the order of 0.1 MPa. This study describes a high-quality factor, spherical resonator capable of achieving acoustic cavitation at ambient pressures in excess of 30 MPa (300 bars). This system generates bursts of violent inertial cavitation events lasting only a few milliseconds (hundreds of acoustic cycles), in contrast with the repetitive cavitation events (lasting several minutes) observed in SBSL; accordingly, these events are described as inertial transient cavitation. Cavitation observed in this high pressure resonator is characterized by flashes of light with intensities up to 1000 times brighter than SBSL flashes, as well as spherical shock waves with amplitudes exceeding 30 MPa at the resonator wall. Both SL and shock amplitudes increase with static pressure.

Sonoluminescence from single bubbles

Sonoluminescence (SL) is generally attributed to the radiative recombination of hydroxyl radicals produced by the high temperatures and pressures associated with cavitation bubble collapse. SL from a single bubble has been observed in a levitation cell at 20 kHz, which supports this assumption. The pressure amplitudes ranged from 110 kPa for water to 170 kPa for 60% glycerin in water. Pulsation amplitudes were measured to be between five and ten times the equilibium radius. Such large amplitude steady‐state pulsations have never been observed before. The possible mechanisms by which stability is maintained will be discussed and theoretical calculations of temperatures and pressures inside the bubble will be presented. [Work supported by ONR.]

The effect of static pressure on the inertial cavitation threshold

The amplitude of the acoustic pressure required to nucleate a gas or vapor bubble in a fluid, and to have that bubble undergo an inertial collapse, is termed the inertial cavitation threshold. The magnitude of the inertial cavitation threshold is typically limited by mechanisms other than homogeneous nucleation such that the theoretical maximum is never achieved. However, the onset of inertial cavitation can be suppressed by increasing the static pressure of the fluid. The inertial cavitation threshold was measured in ultrapure water at static pressures up to 30 MPa (300 bars) by exciting a radially symmetric standing wave field in a spherical resonator driven at a resonant frequency of 25.5 kHz. The threshold was found to increase linearly with the static pressure; an exponentially decaying temperature dependence was also found. The nature and properties of the nucleating mechanisms were investigated by comparing the measured thresholds to an independent analysis of the particulate content and available models for nucleation.

The effect of static pressure on the strength of inertial cavitation events

Recent investigations of cavitation in fluids pressurized up to 30 MPa found that the intensity of light emissions increased by 1000-fold over that measured for single bubble sonoluminescence. A series of measurements is reported here to extend this original work by resolving the static pressure dependence of the shock wave and light emissions from the first and the most energetic collapses, along with the total shock wave energy and light emissions for the event. Each of these parameters was found to increase with the static pressure of the fluid. Furthermore, the energy of these shock wave and light emissions was found to increase in proportion to the stored acoustic energy in the system. These findings were corroborated using the Gilmore equation to numerically compute the work done by the liquid during the bubble collapse. The overall findings suggest that the increased collapse strength at high static pressure is due to the increased tension required to generate inertial cavitation, and not an increased pressure gradient between the interior of the vaporous bubble and the surrounding liquid.

Simultaneous measurements of shock waves, sonoluminescence flashes, and high‐speed video of cavitation in high‐quality factor resonators at high static pressures.

Violent cavitation activity has been observed in water under hundreds of bars static pressure using Impulse Devices spherical resonators. To better understand the extreme conditions inside and in the immediate vicinity of the collapsing bubbles, we simultaneously recorded multi‐frame shadowgraph images, acoustic pressure, and sonoluminescence (SL) flashes from the bubbles. Images of bubbles and shock waves were captured using a V710 Phantom high‐speed camera (400 000 frames/s). The tip of a fiber‐optic probe hydrophone was positioned in the field of view of the camera to correlate acoustic pressure with shadowgraph images of shock waves and bubble dynamics. SL flashes were collected with two photomultiplier tubes (PMTs, Hamamatsu, 1‐ns rise time). The PMTs had identical ultraviolet filters but different sensitivities to extend the dynamic range from a single to thousands of photons. Typically a single bubble was spontaneously nucleated at the center of the sphere. After the first collapse, the bubble reem...

The effects of hydrostatic pressure on conditions in and near a collapsing cavitation bubble.

It has long been understood that the conditions within a collapsing cavitation bubble become more extreme as hydrostatic pressure increases, but quantification of these conditions requires estimating the temperature, pressure, and density of the plasma in the bubble, a difficult task. To provide this information, we conducted numerical simulations using the plasma physics hydrocode HYADES, a 1‐D, three‐temperature, Lagrangean hydrodynamics, and energy transport code. The contents of a bubble at the center of a sphere of water at 1–3000 bars were specified at time = 0, and the bubble was driven by a spherically converging pressure wave of various frequencies (2.5–26 kHz) and amplitudes (10–3000 bars). Results were obtained for temperature, pressure, and density within and immediately outside the bubble. Calculations for bubble radius and the velocity and amplitude of the radiated shock wave compared well with experimental measurements at modest hydrostatic pressures (1–300 bars). At higher pressures, the m...

Optical nucleation of bubble clouds in a high pressure spherical resonator

An experimental setup for nucleating clouds of bubbles in a high-pressure spherical resonator is described. Using nanosecond laser pulses and multiple phase gratings, bubble clouds are optically nucleated in an acoustic field. Dynamics of the clouds are captured using a high-speed CCD camera. The images reveal cloud nucleation, growth, and collapse and the resulting emission of radially expanding shockwaves. These shockwaves are reflected at the interior surface of the resonator and then reconverge to the center of the resonator. As the shocks reconverge upon the center of the resonator, they renucleate and grow the bubble cloud. This process is repeated over many acoustic cycles and with each successive shock reconvergence, the bubble cloud becomes more organized and centralized so that subsequent collapses give rise to stronger, better defined shockwaves. After many acoustic cycles individual bubbles cannot be distinguished and the cloud is then referred to as a cluster. Sustainability of the process is ultimately limited by the detuning of the acoustic field inside the resonator. The nucleation parameter space is studied in terms of laser firing phase, laser energy, and acoustic power used.

Suppression of an acoustic mode by an elastic mode of a liquid-filled spherical shell resonator.

The purpose of this paper is to report on the suppression of an approximately radial (radially symmetric) acoustic mode by an elastic mode of a water-filled, spherical shell resonator. The resonator, which has a 1-in. wall thickness and a 9.5-in. outer diameter, was externally driven by a small transducer bolted to the external wall. Experiments showed that for the range of drive frequencies (19.7-20.6 kHz) and sound speeds in water (1520-1570 m/s) considered in this paper, a nonradial (radially nonsymmetric) mode was also excited, in addition to the radial mode. Furthermore, as the sound speed in the liquid was changed, the resonance frequency of the nonradial mode crossed with that of the radial one and the amplitude of the latter was greatly reduced near the crossing point. The crossing of the eigenfrequency curves of these two modes was also predicted theoretically. Further calculations demonstrated that while the radial mode is an acoustic one associated with the interior fluid, the nonradial mode is an elastic one associated with the shell. Thus, the suppression of the radial acoustic mode is apparently caused by the overlapping with the nonradial elastic mode near the crossing point.

Sonoluminescence And Its Application To Medical Ultrasound Risk Assessment

Ultrasound is used extensively in medical applications for diagnosis, therapy and even surgery. For these applications, the acoustic pressure may be delivered in short pulses at low duty cycles, in long pulses at high duty cycles, in continuous waves, or in the form of high intensity shock waves. The acoustic frequencies vary from about 20 kHz to higher than 10 MHz. If the acoustic pressure amplitude exceeds about 1 MPa, even for microsecond length pulses, then acoustic cavitation can occur in aqueous liquids. The inception of acoustic cavitation has traditionally been detected by measuring acoustic emissions from the cavitation field, such as a shock wave or a subharmonic. However, for short pulses and for high frequencies the acoustic emissions are difficult to detect. In most instances of cavitation bubble collapse, light is also emitted from the cavitation complex via a process called sonoluminescence, in which the internal temperature of the gas is elevated to incandescent levels. One of the classic papers in this area is one coauthored by George Reynolds 1, the subject of this memorial session. This paper extends his pioneering work, describes the phenomenon of sonoluminescence, introduces some new information intending to clarify its physics, and demonstrates how it can be used to assess possible risks associated with the use of medical ultrasound.