Intrinsic Cavitation Research Articles

The asymptotic behavior of Riemann solutions for the Aw-Rascle-Zhang traffic flow model with the polytropic and logarithmic combined pressure term

The exact Riemann solutions for a specially designated Aw-Rascle-Zhang traffic flow model are explicitly obtained by using the combination of 1-rarefaction or 1-shock wave with 2-contact discontinuity. The vacuum and delta shock formation is carefully identified and analyzed in the Riemann solutions when both the two perturbed parameters in the combined pressure term drop to zero simultaneously, where the intrinsic cavitation and concentration phenomena are concretely surveyed and explored. Additionally, our theoretical analysis results are also validated through several representative numerical simulations.

The vanishing pressure limits of Riemann solutions for the Aw-Rascle hydrodynamic traffic flow model with the logarithmic equation of state

Two kinds of Riemann solutions for the Aw-Rascle hydrodynamic traffic flow model with the logarithmic equation of state are explicitly obtained by using the combination between 1-rarefaction or 1-shock wave along with 2-contact discontinuity. The formation of vacuum state and delta shock wave is identified and analyzed when the perturbation parameter in the pressure term drops to zero, where the intrinsic cavitation and concentration phenomena are surveyed and explored concretely. Additionally, several numerical results displaying the formation process of vacuum state and delta shock wave are also presented by taking three different perturbation parameters for comparison.

Enhancement of Boiling Histotripsy by Steering the Focus Axially During the Pulse Delivery.

Boiling histotripsy (BH) is a pulsed high-intensity focused ultrasound (HIFU) method relying on the generation of high-amplitude shocks at the focus, localized enhanced shock-wave heating, and bubble activity driven by shocks to induce tissue liquefaction. BH uses sequences of 1-20 ms long pulses with shock fronts of over 60 MPa amplitude, initiates boiling at the focus of the HIFU transducer within each pulse, and the remainder shocks of the pulse then interact with the boiling vapor cavities. One effect of this interaction is the creation of a prefocal bubble cloud due to reflection of shocks from the initially generated mm-sized cavities: the shocks are inverted when reflected from a pressure-release cavity wall resulting in sufficient negative pressure to reach intrinsic cavitation threshold in front of the cavity. Secondary clouds then form due to shock-wave scattering from the first one. Formation of such prefocal bubble clouds has been known as one of the mechanisms of tissue liquefaction in BH. Here, a methodology is proposed to enlarge the axial dimension of this bubble cloud by steering the HIFU focus toward the transducer after the initiation of boiling until the end of each BH pulse and thus to accelerate treatment. A BH system comprising a 1.5 MHz 256-element phased array connected to a Verasonics V1 system was used. High-speed photography of BH sonications in transparent gels was performed to observe the extension of the bubble cloud resulting from shock reflections and scattering. Volumetric BH lesions were then generated in ex vivo tissue using the proposed approach. Results showed up to almost threefold increase of the tissue ablation rate with axial focus steering during the BH pulse delivery compared to standard BH.

Extension of boiling histotripsy lesions by axial focus steering during pulse delivery

Boiling histotripsy (BH) is a pulsed high intensity focused ultrasound (HIFU) method relying on the generation of high amplitude shocks and bubble activity to induce tissues liquefaction. A sequence of pulses, 1–20 ms long, generates boiling bubbles at the focus of the HIFU transducer within each pulse, and the remainder of the pulse then interacts with those bubbles. One effect is the creation of a prefocal bubble cloud due to shock scattering: the shock is inverted when reflected from the bubble wall resulting in sufficient negative pressure to reach intrinsic cavitation threshold immediately proximally to these bubbles. Here, a methodology is proposed to extend the length of this prefocal bubble cloud by steering the focus toward the transducer during the BH pulse and thus accelerate treatment. A BH system comprising a 1.5 MHz 256-element phased array connected to a Verasonics V1 system was used. High-speed imaging in transparent gels was performed to observe the extension of the bubble cloud resulting from shock scattering. Volumetric BH lesions were generated in ex vivo tissue. Results showed a threefold increase of the volumetric ablation rate with focus steering compared to standard BH. [Work supported by NIH R01EB007643, R01GM122859, and R01EB25187.]

The interaction of shockwaves with a vapour bubble in boiling histotripsy: The shock scattering effect

Boiling histotripsy is a High Intensity Focused Ultrasound (HIFU) technique which uses a number of short pulses with high acoustic pressures at the HIFU focus to induce mechanical tissue fractionation. In boiling histotripsy, two different types of acoustic cavitation contribute towards mechanical tissue destruction: a boiling vapour bubble and cavitation clouds. An understanding of the mechanisms underpinning these phenomena and their dynamics is therefore paramount to predicting and controlling the overall size of a lesion produced for a given boiling histotripsy exposure condition. A number of studies have shown the effects of shockwave heating in generating a boiling bubble at the HIFU focus and have studied its dynamics under boiling histotripsy insonation. However, not much is known about the subsequent production of cavitation clouds that form between the HIFU transducer and the boiling bubble. The main objective of the present study is to examine what causes this bubble cluster formation after the generation of a boiling vapour bubble. A numerical simulation of 2D nonlinear wave propagation with the presence of a bubble at the focus of a HIFU field was performed using the k-Wave MATLAB toolbox for time domain ultrasound simulations, which numerically solves the generalised Westervelt equation. The numerical results clearly demonstrate the appearance of the constructive interference of a backscattered shockwave by a bubble with incoming incident shockwaves. This interaction (i.e., the reflected and inverted peak positive phase from the bubble with the incoming incident rarefactional phase) can eventually induce a greater peak negative pressure field compared to that without the bubble at the HIFU focus. In addition, the backscattered peak negative pressure magnitude gradually increased from 17.4 MPa to 31.6 MPa when increasing the bubble size from 0.2 mm to 1.5 mm. The latter value is above the intrinsic cavitation threshold of –28 MPa in soft tissue. Our results suggest that the formation of a cavitation cloud in boiling histotripsy is a threshold effect which primarily depends (a) the size and location of a boiling bubble, and (b) the sum of the incident field and that scattered by a bubble.

Histotripsy-enhanced thrombolysis

Deep vein thrombosis (DVT) is a major public health problem, affecting 600,000 Americans annually with a healthcare cost of $10 billion. Standard interventional techniques are not effective for the chronic thrombus components present in 27% to 43% of DVT cases. Histotripsy is a focused ultrasound therapy that employs the mechanical action of bubble clouds to ablate tissue and induce vigorous fluid mixing. We have demonstrated synergy between histotripsy and the thrombolytic recombinant tissue plasminogen activator (rt-PA) for dissolution of retracted venous clots in vitro. Here, the role of histotripsy-induced bubble activity in rt-PA clot dissolution will be discussed. The rt-PA thrombolytic efficacy of histotripsy pulses that nucleate bubble activity either via (1) shock wave scattering or (2) exceeding the intrinsic cavitation threshold of the clot will be presented. The impact of each histotripsy type on the formed elements that comprise the clot will be described, as well as the likelihood of containing mechanical activity within the target zone. Finally, the utility of passive cavitation imaging to quantify the bubble activity necessary for liquefaction of tissue-mimicking phantoms representative of acute and chronic pathologies will be presented.

Intrinsic threshold versus shock scattering histotripsy treatment of clots in vitro

Catheter-directed thrombolytics are the frontline therapy for iliofemoral deep vein thrombosis. We have demonstrated that histotripsy can enhance the efficacy of the thrombolytic recombinant tissue plasminogen activator (rt-PA) through the mechanical activity of bubble clouds. Depending on the histotripsy pulse duration, bubble activity can be initiated via shock wave scattering or exceeding the peak negative pressure of the intrinsic cavitation threshold. In this study, the thrombolytic efficacy and bubble activity of shock scattering and intrinsic threshold histotripsy were compared. The mass loss was assessed of retracted human whole blood clots exposed to rt-PA (0 or 2.68 μg/mL) and histotripsy pulses of 1 (intrinsic threshold) or 5 (shock scattering) cycles duration. During the histotripsy exposure, bubble cloud emissions were captured on a linear array and processed to form passive acoustic images. The combination of rt-PA and histotripsy was more efficacious than rt-PA alone at 25 MPa peak negative pressure for single-cycle pulses, and 20 MPa peak negative pressure for five-cycle pulses. The degree of thrombolytic efficacy correlated with bubble cloud emissions within the clot. Overall, these results indicate that rt-PA and shock scattering or intrinsic threshold histotripsy is a promising combination thrombolytic therapy.

Acoustic nonlinearity and the generation of large tensile pressures to explain atomization in drop-chain acoustic fountains

An ultrasound beam propagating upward in a liquid creates an acoustic fountain at a gas interface in the form of a drop chain. High-speed photography shows that one or several drops in such a fountain explode in less than a millisecond, resulting in liquid atomization [Simon et al. J. Fluid Mech., 2015, 766, pp. 129-146]. To explain this phenomenon, a nonlinear theory involving an isolated spherical drop is developed. The model considers an initial excitation in the form of a spherical standing acoustic wave at the lowest resonance frequency, i.e., when the drop diameter coincides with a wavelength. If higher harmonics are generated inside the drop due to acoustic nonlinearity, these harmonics will also have the form of standing spherical waves. At higher frequencies, more of the energy of each harmonic is localized near the drop center. Calculations demonstrate that harmonic generation can lead to large increases in both peak positive and peak negative acoustic pressure at the drop center. Such large tensile pressures may exceed the intrinsic cavitation threshold, leading to the nucleation of a bubble at the center and explosion of the drop as the bubble grows rapidly. [Work supported by RFBR 17-02-00261 and NIH R01EB007643.]

Evaporation from an Ink-Bottle Pore: Mechanisms of Adsorption and Desorption

We present a molecular simulation study of argon adsorption and desorption in ink-bottle pores in order to investigate the effects of pore width, length, and temperature on the form of the hysteresis loop, with particular emphasis on the pressure at which evaporation occurs. We show that intrinsic cavitation occurs only in an ink-bottle pore with a sufficiently long and narrow neck. The tension in the condensed fluid is characterized by the average value of the shortest distance between molecules, and when this reaches a critical value, cavitation occurs. As the neck width is increased or the neck length is decreased, the evaporation mechanism in the cavity switches from cavitation to evaporation at higher pressure than the cavitation pressure (which we call cavitation-like pore opening). For cavitation-like pore opening, the evaporation pressure is greater than the intrinsic cavitation pressure and the tensile strength just prior to evaporation is less than the critical tension. For a given temperature, the reduced pressure of cavitation, (P/P0)cav, is always lower than the cavitation-like pore opening reduced pressure, and (P/P0)cav follows a linear dependence on the temperature. The intersection between the cavitation curve and the cavitation-like pore opening curve for a given neck size is the transition temperature, Ttrans: for T < Ttrans, cavitation-like pore opening is the operating mechanism, while cavitation occurs for T > Ttrans.

Histotripsy beyond the intrinsic cavitation threshold using very short ultrasound pulses: microtripsy

Histotripsy produces tissue fractionation through dense energetic bubble clouds generated by short, high-pressure, ultrasound pulses. Conventional histotripsy treatments have used longer pulses from 3 to 10 cycles, wherein the lesion-producing bubble cloud generation depends on the pressure-release scattering of very high peak positive shock fronts from previously initiated, sparsely distributed bubbles (the shock-scattering mechanism). In our recent work, the peak negative pressure (P-) for generation of dense bubble clouds directly by a single negative half cycle, the intrinsic threshold, was measured. In this paper, the dense bubble clouds and resulting lesions (in red blood cell phantoms and canine tissues) generated by these supra-intrinsic threshold pulses were studied. A 32-element, PZT-8, 500-kHz therapy transducer was used to generate very short (<2 cycles) histotripsy pulses at a pulse repetition frequency (PRF) of 1 Hz and P- from 24.5 to 80.7 MPa. The results showed that the spatial extent of the histotripsy-induced lesions increased as the applied P- increased, and the sizes of these lesions corresponded well to the estimates of the focal regions above the intrinsic cavitation threshold, at least in the lower pressure regime (P- = 26 to 35 MPa). The average sizes for the smallest reproducible lesions were approximately 0.9 × 1.7 mm (lateral × axial), significantly smaller than the -6-dB beamwidth of the transducer (1.8 × 4.0 mm). These results suggest that, using the intrinsic threshold mechanism, well-confined and microscopic lesions can be precisely generated and their spatial extent can be estimated based on the fraction of the focal region exceeding the intrinsic cavitation threshold. Because the supra-threshold portion of the negative half cycle can be precisely controlled, lesions considerably less than a wavelength are easily produced, hence the term microtripsy.

Atomic Scale Fluctuations Govern Brittle Fracture and Cavitation Behavior in Metallic Glasses

We perform atomistic simulations on the fracture behavior of two typical metallic glasses, one brittle (FeP) and the other ductile (CuZr), and show that brittle fracture in the FeP glass is governed by an intrinsic cavitation mechanism near crack tips in contrast to extensive shear banding in the ductile CuZr glass. We show that a high degree of atomic scale spatial fluctuations in the local properties is the main reason for the observed cavitation behavior in the brittle metallic glass. Our study corroborates with recent experimental observations of nanoscale cavity nucleation found on the brittle fracture surfaces of metallic glasses and provides important insights into the root cause of the ductile versus brittle behavior in such materials.

Toughenability of polymers

We demonstrate that all solid polymers are intrinsically brittle and will undergo a ductile to brittle fracture transition based on the nature of their bonding alone. The most effective way of avoiding a ductile to brittle transition is to reduce the plastic resistance to delay reaching the brittle strength which in unoriented polymers is governed by intrinsic cavitation. While a number of possibilities for this exist, the most widely used techniques involve incorporation of rubbery particles that can cavitate or rigid particles that can debond prior to plastic flow. In both approaches the continuous homo-polymer is transformed into a quasi-regular cellular solid that is much more capable of undergoing large local plastic flow by ligament stretching between cavitated particles and is less susceptible to the propagation of brittle cracks under the usual conditions of tensile straining. Under impact conditions, however, in a notched sample which concentrates the strain rate at the notch root, the plastic resistance of the stretching ligaments rises sharply due to two separate but related effects. First, by an increase in the shear modulus due to the high frequency nature of the Izod impact test to fracture, viewed as a quarter cycle oscillation, which directly elevates the flow resistance and second, by the further effect of increase due to the much increased plastic strain rate. At the notch root then, the plastically stretching and strain hardening ligaments are left with a much reduced capacity to strain further before the cavitation stress is reached. While rubber particle-modified polymers can still exhibit considerable toughening, rigid-particle-modified polymers suffer severely from clustering of rigid particles into super critical flaws that trigger brittle response, much like what is encountered in structural steels.Based on their known mechanical response in neat form six, semi-crystalline polymers have been analyzed in detail to evaluate their potential for toughening under impact conditions. The results correlate very well with the experimental findings.