R. G. Holt

Shock‐driven growth of bubble clouds.

Laser‐nucleated bubble clouds in a high‐pressure spherical resonator have previously been reported to develop into tight clusters after many acoustic cycles. The formation of these organized clusters is largely driven by the reconvergence of shocks from earlier cloud collapses. Highly temporally (40 Mfps) and spatially (3 μm/pixel) resolved images reveal that the expansion of the shock‐nucleated clusters is initially very fast (> 8 km/s), much faster than the shocks themselves. However, this explosive growth of the cluster does not begin until hundreds of nanoseconds after the shock passes, and so the cluster never surpasses the shock itself. The phase diagrams of the cluster and shocks are mapped out and the shock‐driven nucleation is discussed. [Work supported by the Impulse Devices, Inc.]

Quasi-static acoustic tweezing thromboelastometry

Essentials Blood coagulation measurement during contact with an artificial surface leads to unreliable data. Acoustic tweezing thromboelastometry is a novel non‐contact method for coagulation monitoring. This method detects differences in the blood coagulation state within 10 min. Coagulation data were obtained using a much smaller sample volume (4 μL) than currently used.

Controlling a high intensity focused ultrasound induced cavitation field via duty cycle

Cavitation has been implicated in the lack of control over the shape of thermal lesions generated by high‐intensity focused ultrasound (HIFU). A coincident effect the decline in the acoustic emissions from cavitation at the focus suggests that the HIFU energy is shielded from the focal region, possibly by prefocal bubble activity. Most clinical techniques employ continuous‐wave (CW) ultrasound, which can exacerbate the problem depending on the acoustic intensities employed. This talk presents a series of experiments investigating techniques to control HIFU energy delivered to, and cavitation activity within, a tissue phantom. A passive cavitation detector (PCD) is employed as a sensor of cavitation activity. For 1.1‐MHz CW ultrasound at focal pressures above 3 MPa, bubble shielding was inferred from a steady decline in the PCD signal over time. By lowering the duty cycle the PCD output remained constant over time. Finally, driving the HIFU source initially with a CW signal and then switching to a pulsed s...

Micro-cavitation on demand via the nanoparticle mediated interaction of light and sound

The safe utilization of controlled cavitation for HIFU therapy and ultrasound assisted drug delivery requires nucleation sites for bubble formation. We consider the potential for nucleating transient vapor cavities using laser-illuminated gold nanoparticles combined with high-intensity focused ultrasound. An transparent polyacrylamide gel phantom was seeded with 82-nm diameter gold particles and exposed to 20 ns pulses from a 532 nm Nd:Yag laser. Laser firing was synchronized with the arrival of a burst of 1.1 MHz focused ultrasound. Acoustic emissions from ensuing inertial cavitation were detected passively using a 15 MHz focused transducer. At a laser energy of 0.10 mJ/pulse, the resulting inertial cavitation nucleation threshold pressure (peak-negative focal pressure) was as low as 0.92 MPa. In comparison, a peak-negative focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination (nano-particles were present in both cases). Experimental results agree well with a simple model for transient heating and cavity formation. Since the particles are durable, one can re-activate them as needed, essentially yielding cavitation nuclei “on demand.” [Work supported by the Dept. of the Army (Award No. DAMD17-02-2-0014) and the Center for Subsurface Sensing and Imaging Systems (NSF ERC Award No. EEC-9986821).]The safe utilization of controlled cavitation for HIFU therapy and ultrasound assisted drug delivery requires nucleation sites for bubble formation. We consider the potential for nucleating transient vapor cavities using laser-illuminated gold nanoparticles combined with high-intensity focused ultrasound. An transparent polyacrylamide gel phantom was seeded with 82-nm diameter gold particles and exposed to 20 ns pulses from a 532 nm Nd:Yag laser. Laser firing was synchronized with the arrival of a burst of 1.1 MHz focused ultrasound. Acoustic emissions from ensuing inertial cavitation were detected passively using a 15 MHz focused transducer. At a laser energy of 0.10 mJ/pulse, the resulting inertial cavitation nucleation threshold pressure (peak-negative focal pressure) was as low as 0.92 MPa. In comparison, a peak-negative focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination (nano-particles were present in both cases). Experimental results agree well with ...

Laser nucleation of single bubbles and clouds in an acoustic resonator via pressure-dependent dielectric breakdown

Obtaining bubbles on demand at precise times and locations in a non-contact fashion can be useful in a variety of applications. Of special importance is the combination of laser nucleation with acoustics, so that bubbles are only just nucleated by the optics but grown to macroscopic size solely by the acoustics. We present theory and experiment for the non-thermal laser nucleation of bubbles in an acoustic field in the absence of significant absorbing/scattering particles. First we present theory and experiment for the threshold for dielectric breakdown in water, resolving the distinct minimum at 20 bar. Then, we present a method and results for nucleating single and multiple bubbles with temporal uncertainty of 5 ns, and spatial uncertainty of 1 mm. Results for bubble number and first cycle expansion are reported as functions of the timing of the nucleating laser pulse with respect to the acoustic field. [Work supported by Impulse Devices, Inc.]

Void fraction inference in cavitating fuel injector flows

It has been shown that hydrodynamic cavitation within fuel injectors plays a significant role in their performance, with the desirable effect of broadening the resultant fuel spray. Experiments are challenging owing to the relatively small geometries, high pressure, and high Reynold’s number (Re) associated with such flows. Previous studies have observed cavitation in optically transparent nozzles at slower flows. By utilizing acoustic and vibration measurement techniques cavitation activity may be measured in a steel fuel injector at more practical Re flows used in applications. We report here experimental measurements taken using a laser vibrometer and a commercial fuel injector. Previous studies have demonstrated a resonant frequency shift as a function of injection pressure. Among competing hypotheses, our working hypothesis is that this shift is the result of mass unloading of cantilever mode oscillations of the fuel injector. The dynamic void fraction caused by cavitation activity within the fuel injector can then be inferred from the measured frequency shift. We report measurements of mode shapes and frequencies for static and flowing fuel injectors as functions of the flow rate.It has been shown that hydrodynamic cavitation within fuel injectors plays a significant role in their performance, with the desirable effect of broadening the resultant fuel spray. Experiments are challenging owing to the relatively small geometries, high pressure, and high Reynold’s number (Re) associated with such flows. Previous studies have observed cavitation in optically transparent nozzles at slower flows. By utilizing acoustic and vibration measurement techniques cavitation activity may be measured in a steel fuel injector at more practical Re flows used in applications. We report here experimental measurements taken using a laser vibrometer and a commercial fuel injector. Previous studies have demonstrated a resonant frequency shift as a function of injection pressure. Among competing hypotheses, our working hypothesis is that this shift is the result of mass unloading of cantilever mode oscillations of the fuel injector. The dynamic void fraction caused by cavitation activity within the fuel in...

Blood coagulation monitoring using acoustic levitation

Impaired blood coagulation can result from a variety of conditions including severe trauma, illness or surgery, and can cause life-threatening bleeding or thrombotic disorders. As a result, instruments which measure functional changes in blood properties upon activation of the clotting cascade are crucial to assess coagulopathies in various disease states. Thromboelastography (TEG) is the gold standard for whole blood coagulation monitoring. TEG requires the blood to be in contact with the sample holder and sensor, leading to a variety of potential artifacts in the results. Acoustic levitation provides non-contact containment and manipulation. We have levitated microliter drops of blood, and employed a drop eigenmodal oscillation technique in order to infer viscoelastic material properties of blood as it coagulates. By comparing our results with TEG results on the same samples, we are able to illustrate certain advantages of the levitation technique. [Work supported by NSF grant # 1438569.]

Cavitation detection in a nozzle flow

The presence of cavitation inside fuel injector nozzles has been linked not only to damage associated with cavity collapse near the walls, but also more intriguingly to improved spray atomization. Previous studies have shown that cavitation is associated with increased spray angle. Our goal is to investigate the underlying mechanics. In this talk we describe our initial efforts to employ both acoustic techniques (passive cavitation detection, or PCD) and optical techniques (optical cavitation detection, or OCD) to characterize nozzle cavitation. Experiments are conducted with acrylic nozzles of various geometry. Unfocused single element transducers are used for PCD, while digital imaging is used for OCD. Cavitation onset thresholds and development are studied as functions of flow rate, nozzle geometry, and upstream fluid preparation. Cavitation characterization results will be compared with an in-house computational code being developed to model cavitation in fuel injectors.

The effects of wind turbine wake turbulence on bat lungs

Bat mortality is known to increase near wind turbines. Recent studies are in disagreement as to the exact cause of death of these bats. Literature suggests that they are either killed upon direct contact with the turbine blades or by barotrauma. In barotrauma, a sudden change in the surrounding air-pressure causes tissue damage in biological structures that contain air, most notably the lungs. The present work develops a computational model of the bat lung, in which the lung is modeled as a gas bubble with an elastic shell immersed in a fluid, whose dynamics are governed by a Rayleigh-Plesset-like equation. Pressure gradients near the wind turbine are obtained using computational fluid dynamics. The lung’s response to pressure changes is attained by simulating the pressure’s effect on the gas bubble. The study allows for a greater understanding of bat barotrauma and its potential link to wind turbine pressure fields.

Quasi-static acoustic tweezing for low-volume blood coagulation analysis

Available contact assays for blood plasma and whole blood coagulation have low predictive power in patients with coagulopathy or take a significant amount of time and blood volume to obtain diagnostic data. We have developed an innovative low-volume non-contact technology for real-time assessment of blood coagulation, referred to as “Quasi-static Acoustic Tweezing Thromboelastometry”(QATT). In our method, human blood drops with volume less than 5 microliter (~100 times smaller than the volume required by current coagulation technologies) are levitated in air by acoustic radiation forces. The sample drop location and deformation are induced by a quasi-static change in the acoustic pressure. By extracting a linear regime slope, the samples exhibit a unique elasticity profile over time (tweezograph) less than 20 minutes, characterized by clot initiation time (CIT), time to firm clot formation (TFCF), and maximum clot strength (MCS). The exposure of blood samples to pro- or anti-thrombotic agents (Fibrinogen ...