Phillip A. Anderson

Shock‐controlled bubble cloud dynamics and light emission.

Cavitation bubble 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. In this talk we describe cavitation bubble cloud experiments carried out in spherical resonators at ambient and acoustic pressures up to 30 MPa. Key to our system is the ability to nucleate with temporal and spatial controls, which we achieve using dielectric breakdown in water from pulsed focused laser beams. Our observations show that the cloud dynamics are controlled by the repetitive emission of shock waves, which propagate outward from the inertial cloud collapse, reflect off of the sphere wall, and then converge on the resonator center. Shock convergence phenomena and light emission from compact cloud collapse will be discussed. [Work supported by the Impulse Devices, Inc.]

Experimental characterisation of light emission during shock-driven cavity collapse

The authors describe experimental work examining the collapse of a cavity by a strong shockwave. A millimeter size cavity is cast in Phytagel, which is then impacted by a metallic projectile accelerated by a compressed gas gun, reaching velocities up to 500 m/s. The impact generates a strong shockwave that propagates into the gel at greater than sonic velocity. Schlieren images are presented that illustrate both this process and the subsequent cavity collapse at a sub-microsecond timescale. As the shockwave reaches the cavity, it is shown to cause a rapid asymmetric collapse process characterized by the formation of a high-speed transverse jet. The pressure of the shockwave is found to be 100+ MPa as measured via a custom-built fiber-optic probe hydrophone. Previous work examining shock-driven cavity collapse observed luminescence, postulated to be due to the high-speed impact of the transverse jet on the far bubble wall; this experimental observation is replicated. Further, the light emission is characte...

Characterizing shock waves in hydrogel using high speed imaging and a fiber-optic probe hydrophone

The impact of a stainless steel disk-shaped projectile launched by a single-stage light gas gun is used to generate planar shock waves with amplitudes on the order of 102MPa in a hydrogel target material. These shock waves are characterized using ultra-high-speed imaging as well as a fiber-optic probe hydrophone. Although the hydrogel equation of state (EOS) is unknown, the combination of these measurements with conservation of mass and momentum allows us to calculate pressure. It is also shown that although the hydrogel behaves similarly to water, the use of a water EOS underpredicts pressure amplitudes in the hydrogel by ∼10% at the shock front. Further, the water EOS predicts pressures approximately 2% higher than those determined by conservation laws for a given value of the shock velocity. Shot to shot repeatability is controlled to within 10%, with the shock speed and pressure increasing as a function of the velocity of the projectile at impact. Thus the projectile velocity may be used as an adequat...

Spatially and temporally resolved single bubble sonoluminescence and its entrainment in Rayleigh-Taylor jets

Previous investigations of the temporal and spatial evolution of single bubble sonoluminescence (SBSL) have shown events to last on the order of tens to hundreds of picoseconds with spatial extents of less than 1 um. Here we present observations of the temporal and spatial evolution of laser-nucleated SBSL events in a high-pressure spherical resonator. Using high-speed imaging, we observe large, long-lived SBSL events reaching diameters of up to 50 um and lasting on the order of 30 ns. Observations of events entrained in Rayleigh-Taylor jets resulting from instabilities in the final stages of the bubbles collapses will also be presented. We observe the light emitting region entrained in these jets to reach velocities in excess of 4500 m/s and to travel up to 100 um before being extinguished. The size and duration of events, and the velocity of those entrained in Rayleigh-Taylor jets, will be compared to the maximum radius and collapse velocity of the bubbles responsible for generating them to develop a better understanding of the dynamics leading to, and the mechanisms responsible for light emissions during highly energetic collapse events. [Work supported by Impulse Devices, Inc.]

High pressure phase transitions in the fluid region surrounding the collapse point of large single bubbles in water

Observations from imaging experiments will be presented which have shown persistent, long-lived spherical objects to form in the fluid region surrounding large, single bubbles in highly over-pressured water. Objects have been observed to form in a region of fluid where pressures are first predicted to exceed 0.8 GPa, and to extend radially inward to where fluid pressures are predicted to reach 6 GPa. These pressures bound those requisite for transitions in water to the crystalline phases of Ice-VI and Ice-VII, at 1.1 GPa and 2.1 GPa, respectively. The objects have been observed to behave in a fashion more consistent with a highly viscous fluid. They support and recover from large shape deformations, as well as support fluid flows within them. While water does have phases which are known to exhibit properties of highly viscous fluids, they have only been observed to form at or near cryogenic temperatures, typically via hyperquenching or quasi-static pressurization at low temperatures. Here, we present evidence for a high pressure liquid-liquid phase transition in water surrounding collapsing bubbles at room temperature. [Work supported by Impulse Devices, Inc.]

Temporal evolution of laser‐nucleated bubble clouds in an acoustic resonator.

Using high‐speed digital imaging, the evolution of laser‐nucleated bubble clouds over multiple acoustic cycles is observed. Five 7×7 custom phase gratings are employed to produce 245 beams, which are then focused into the center of a spherical resonator capable of high static and acoustic pressure. The dependence of the evolution of the bubble cloud on several system parameters is measured. Notably the resulting cloud(s) depend strongly on the phase of the laser firing, the laser energy per pulse, and the acoustic pressure. Number of bubbles, radius of individual bubbles, and effective radius of clouds will be reported as functions of time. The interplay between cloud dynamics and shock waves will also be discussed. [Work supported by Impulse Devices, Inc.]

Collapse dynamics of laser‐nucleated bubble clusters in a spherical resonator.

The collective collapse of bubble clouds or clusters is important to many applications, including underwater sound propagation, shock‐wave lithotripsy, and focused ultrasound therapy. Energetic collective cluster collapses appear to happen only when there is sufficient nuclei density. In order to study this phenomenon, we have implemented a laser‐based method for creating clusters consisting of planar or cubic arrays of bubbles with controllable interbubble spacing. The growth and collapse of the arrays are observed using ultrahigh‐speed photography and other diagnostics. Effects of interbubble spacing and nucleation phase with respect to the acoustic period are studied. [Work supported by the US Army Space and Missile Command.]

Laser nucleation of bubble clusters in a spherical resonator.

Due to the strong energy focusing found in sonoluminescence, there is much interest in high‐energy bubble collapses. There is evidence in the literature that the collective collapse of a bubble cluster is more violent than a single bubble collapse under similar conditions. Precise control over the spatial and temporal placement of bubbles in an acoustic field would seem critical for achieving the strongest collapses, but nucleating bubbles acoustically is not precise or repeatable and does not allow for the creation of arrays of bubbles. A method is presented for creating two‐ and three‐dimensional bubble arrays in a controllable way. A pulsed Nd:YAG laser is used with custom diffraction gratings and focusing optics to nucleate bubble arrays in a spherical resonator at high static pressure. A specific implementation will be presented, along with results addressing uniformity and repeatability. [Work supported by the US Army Space and Missile Command.]