Cavitation pressure in water

Exploring operational boundaries for acoustic concentration of cell suspensions

The development of a standardized, generic method for concentrating suspensions in continuous flow is challenging. In this study, we developed and tested a device capable of concentrating suspensions with an already high cell concentration to meet diverse industrial requirements. To address typical multitasking needs, we concentrated suspensions with high solid content under a variety of conditions. Cells from Saccharomyces cerevisiae, Escherichia coli, and Chinese hamster ovary cells were effectively focused in the center of the main channel of a microfluidic device using acoustophoresis. The main channel bifurcates into three outlets, allowing cells to exit through the central outlet, while the liquid evenly exits through all outlets. Consequently, the treatment separates cells from two-thirds of the surrounding liquid. We investigated the complex interactions between parameters. Increasing the channel depth results in a decrease in process efficiency, attributed to a decline in acoustic energy density. The study also revealed that different cell strains exhibit distinct acoustic contrast factors, originating from differences in dimensions, compressibility, and density values. Finally, a combination of high solid content and flow rate leads to an increase in diffusion through a phenomenon known as shear-induced diffusion. KEY POINTS: • Acoustic focusing in a microchannel was used to concentrate cell suspensions • The parameters influencing focusing at high concentrations were studied • Three different cell strains were successfully concentrated.

Review on the impacts of external pressure on sonochemistry

The impact of hydrostatic pressure, commonly known as ambient or external pressure, on the phenomenon of sonochemistry and/or sonoluminescence has been extensively investigated through a multitude of experimental and computational studies, all of which have emphasized the crucial role played by this particular parameter. Numerous previous studies have successfully demonstrated the existence of an optimal static pressure for the occurrence of sonoluminescence and multi-bubble or single-bubble sonochemistry. However, despite these findings, a universally accepted value for this critical pressure has not yet been established. In addition, it has been found that the cavitation effect is completely inhibited when the static pressure is either too high or too low. This comprehensive review aims to delve into the primary experimental results and elucidate their significance in relation to hydrostatic pressure. We will then conduct an analysis of numerical calculations, focusing specifically on the influence of external pressure on single bubble sonochemistry. By delving into these calculations, we will be able to gain a deeper understanding of the experimental results and effectively interpret their implications.

A Review of Ultrasonic Treatment in Mineral Flotation: Mechanism and Recent Development

Ultrasonic treatment has been widely used in the mineral flotation process due to its advantages in terms of operational simplicity, no secondary pollutant formation, and safety. Currently, many studies have reported the effect of ultrasonic treatment on mineral flotation and shown excellent flotation performance. In this review, the ultrasonic mechanisms are classified into three types: the transient cavitation effect, stable cavitation effect, and acoustic radiation force effect. The effect of the main ultrasonic parameters, including ultrasonic power and ultrasonic frequency, on mineral flotation are discussed. This review highlights the uses of the application of ultrasonic treatment in minerals (such as the cleaning effect, ultrasonic corrosion, and desulfuration), flotation agents (such as dispersion and emulsification and change in properties and microstructure of pharmaceutical solution), and slurry (such formation of microbubbles and coalescence). Additionally, this review discusses the challenges and prospects of using ultrasonic approaches for mineral flotation. The findings demonstrate that the application of the ultrasonic effect yields diverse impacts on flotation, thereby enabling the regulation of flotation behavior through various treatment methods to enhance flotation indices and achieve the desired objectives.

Cavitation-induced pressure saturation: a mechanism governing bubble nucleation density in histotripsy

Objective.Histotripsy is a noninvasive focused ultrasound therapy that mechanically disintegrates tissue by acoustic cavitation clouds. In this study, we investigate a mechanism limiting the density of bubbles that can nucleate during a histotripsy pulse. In this mechanism, the pressure generated by the initial bubble expansion effectively negates the incident pressure in the vicinity of the bubble. From this effect, the immediately adjacent tissue is prevented from experiencing the transient tension to nucleate bubbles. Approach.A Keller-Miksis-type single-bubble model was employed to evaluate the dependency of this effect on ultrasound pressure amplitude and frequency, viscoelastic medium properties, bubble nucleus size, and transducer geometric focusing. This model was further combined with a spatial propagation model to predict the peak negative pressure field as a function of position from a cavitating bubble.Main results. The single-bubble model showed the peak negative pressure near the bubble surface is limited to the inertial cavitation threshold. The predicted bubble density increased with increasing frequency, tissue viscosity, and transducer focusing angle. The simulated results were consistent with the trends observed experimentally in prior studies, including changes in density with ultrasound frequency and transducerF-number.Significance.The efficacy of the therapy is dependent on several factors, including the density of bubbles nucleated within the cavitation cloud formed at the focus. These results provide insight into controlling the density of nucleated bubbles during histotripsy and the therapeutic efficacy.

A capillary-induced negative pressure is able to initiate heterogeneous cavitation

A capillarity-induced negative pressure is of general importance for understanding the phase behaviors of liquids in small pores and cracks. A unique example is the embolism in the xylem of plants and the cavitation at the limiting negative pressure generated by evaporation of water from nanocapillaries in the cell walls of leaves. In this work, by combining the effect of a capillary and cavitation together, we demonstrate with molecular dynamics (MD) simulations that capillarity is able to induce spontaneous cavitation in the presence of hydrophobic heterogeneities. Our simulation results reveal separately how the capillary generates a negative pressure and how the generated negative pressure affects the onset of cavitation. We then interpret the cavitation mechanism and determine the occurrence of cavitation as a function of the hydrophobicity of the nucleating substrates where the cavitation initiates and as a function of the hydrophilicity of the capillary tube from which the negative pressure generates. Our results reveal that the capillary-induced cavitation can be described well with a heterogeneous nucleation mechanism, within the framework of classical nucleation theory.

Biofilm removal from Difficult-to-Reach places via secondary cavitation within a constrained geometry mimicking a Periodontal/Peri-Implant pocket

Biofilm removal from the apical region of the periodontal or peri-implant pocket, which is very difficult to achieve with mechanical instruments, is a major unresolved issue in dentistry. Here, we propose the use of photoacoustically induced streaming and secondary cavitation to achieve superior cleaning efficacy in the apical region of the periodontal and peri-implant pocket. We have used a prefabricated narrow wedge system that mimics the consistency of periodontal and peri-implant pockets of both healthy and severely inflamed tissue. We studied the effect of single-pulse modality Er:YAG on Pseudomonas aeruginosa biofilm removal. We used different laser energies, fiber-tip positions, and laser treatment durations. The cleaning process was monitored in real-time with a high-speed camera after each individual laser pulse application. The obtained results suggest that biofilm cleaning efficacy in a difficult-to-reach place in healthy model tissue is directly related to the onset of secondary cavitation bubble formation, which correlates with a significant improvement of biofilm removal from the apical region of the periodontal or peri-implant pocket. In comparison to the healthy tissue model, the laser energy in inflamed tissue model had to be increased to obtain comparable biofilm cleaning efficacy. The advantage of photoacoustic cavitation compared to other methods is that laser-induced cavitation can trigger secondary cavitation at large distances from the point of laser application, which in principle allows biofilm removal at distant locations not reachable with a laser fiber tip or other mechanical instruments.

Energetics of the microsporidian polar tube invasion machinery

Microsporidia are eukaryotic, obligate intracellular parasites that infect a wide range of hosts, leading to health and economic burdens worldwide. Microsporidia use an unusual invasion organelle called the polar tube (PT), which is ejected from a dormant spore at ultra-fast speeds, to infect host cells. The mechanics of PT ejection are impressive. Anncaliia algerae microsporidia spores (3-4 μm in size) shoot out a 100-nm-wide PT at a speed of 300 μm/s, creating a shear rate of 3000 s-1. The infectious cargo, which contains two nuclei, is shot through this narrow tube for a distance of ∼60-140 μm (Jaroenlak et al, 2020) and into the host cell. Considering the large hydraulic resistance in an extremely thin tube and the low-Reynolds-number nature of the process, it is not known how microsporidia can achieve this ultrafast event. In this study, we use Serial Block-Face Scanning Electron Microscopy to capture 3-dimensional snapshots of A. algerae spores in different states of the PT ejection process. Grounded in these data, we propose a theoretical framework starting with a systematic exploration of possible topological connectivity amongst organelles, and assess the energy requirements of the resulting models. We perform PT firing experiments in media of varying viscosity, and use the results to rank our proposed hypotheses based on their predicted energy requirement. We also present a possible mechanism for cargo translocation, and quantitatively compare our predictions to experimental observations. Our study provides a comprehensive biophysical analysis of the energy dissipation of microsporidian infection process and demonstrates the extreme limits of cellular hydraulics.

Enhanced Performance of an Acoustofluidic Device by Integrating Temperature Control

Acoustofluidics is an emerging research field wherein either mixing or (bio)-particle separation is conducted. High-power acoustic streaming can produce more intense and rapid flow patterns, leading to faster and more efficient liquid mixing. However, without cooling, the temperature of the piezoelectric element that is used to supply acoustic power to the fluid could rise above 50% of the Curie point of the piezomaterial, thereby accelerating its aging degradation. In addition, the supply of excessive heat to a liquid may lead to irreproducible streaming effects and gas bubble formation. To control these phenomena, in this paper, we present a feedback temperature control system integrated into an acoustofluidic setup using bulk acoustic waves (BAWs) to elevate mass transfer and manipulation of particles. The system performance was tested by measuring mixing efficiency and determining the average velocity magnitude of acoustic streaming. The results show that the integrated temperature control system keeps the temperature at the set point even at high acoustic powers and improves the reproducibility of the acoustofluidic setup performance when the applied voltage is as high as 200 V.

Numerical investigation on the influence of dual-frequency coupling parameters on acoustic cavitation and its analysis of the enhancement and attenuation effect

To understand the effect of coupling parameters between two ultrasonic waves on acoustic cavitation, in this work, Keller-Miksis equation was introduced to built a bubble dynamics model that was used to describe the dynamic evolution of bubble and to discuss the effect of dual-frequency coupling parameters, such as frequency difference f (5 ∼ 280 kHz), phase difference φ (0 ∼ 7π/4 rad), and power allocation ratio β (0 ∼ 9), on acoustic cavitation in the presence of two ultrasonic waves irradiation. The enhancement and attenuation effect of cavitation have also been analyzed in detail by comparing the different dual-frequency combinations with single-frequency mode. It was found that all coupling parameters have a significant impact on acoustic cavitation, where the smaller values of f and φ were employed when β = 1, the stronger cavitation intensity was observed. Nevertheless, as the power allocation ratio is increased from 1 to 9 at φ = 0 for different frequency differences, the acoustic cavitation exhibits an attenuation trend. When the total acoustic power is evenly distributed, namely β = 1, the largest maximum expansion ratio (i.e. 12.96) was obtained at φ = 0 and f = 5 kHz, which represents a strongest cavitation effect. In addition, for different frequency combinations, the enhancement effect is found under the mixture of low and low frequency, whereas attenuation effect is generated easily by the combination of high and low frequency. Moreover, the effect become more pronounced as the proportion of high frequency component increases.

Energetics of the Microsporidian Polar Tube Invasion Machinery

Microsporidia are eukaryotic, obligate intracellular parasites that infect a wide range of hosts, leading to health and economic burdens worldwide. Microsporidia use an unusual invasion organelle called the polar tube (PT), which is ejected from a dormant spore at ultra-fast speeds, to infect host cells. The mechanics of PT ejection are impressive. Anncaliia algerae microsporidia spores (3-4 μm in size) shoot out a 100-nm-wide PT at a speed of 300 μm/sec, creating a shear rate of 3000 sec-1. The infectious cargo, which contains two nuclei, is shot through this narrow tube for a distance of ~60-140 μm (Jaroenlak et al., 2020) and into the host cell. Considering the large hydraulic resistance in an extremely thin tube and the low-Reynolds-number nature of the process, it is not known how microsporidia can achieve this ultrafast event. In this study, we use Serial Block-Face Scanning Electron Microscopy to capture 3-dimensional snapshots of A. algerae spores in different states of the PT ejection process. Grounded in these data, we propose a theoretical framework starting with a systematic exploration of possible topological connectivity amongst organelles, and assess the energy requirements of the resulting models. We perform PT firing experiments in media of varying viscosity, and use the results to rank our proposed hypotheses based on their predicted energy requirement. We also present a possible mechanism for cargo translocation, and quantitatively compare our predictions to experimental observations. Our study provides a comprehensive biophysical analysis of the energy dissipation of microsporidian infection process and demonstrates the extreme limits of cellular hydraulics.

In silico assessment of histotripsy-induced changes in catheter-directed thrombolytic delivery

Introduction: For venous thrombosis patients, catheter-directed thrombolytic therapy is the standard-of-care to recanalize the occluded vessel. Limitations with thrombolytic drugs make the development of adjuvant treatments an active area of research. One potential adjuvant is histotripsy, a focused ultrasound therapy that lyses red blood cells within thrombus via the spontaneous generation of bubbles. Histotripsy has also been shown to improve the efficacy of thrombolytic drugs, though the precise mechanism of enhancement has not been elucidated. In this study, in silico calculations were performed to determine the contribution of histotripsy-induced changes in thrombus diffusivity to alter catheter-directed therapy. Methods: An established and validated Monte Carlo calculation was used to predict the extent of histotripsy bubble activity. The distribution of thrombolytic drug was computed with a finite-difference time domain (FDTD) solution of the perfusion-diffusion equation. The FDTD calculation included changes in thrombus diffusivity based on outcomes of the Monte Carlo calculation. Fibrin degradation was determined using the known reaction rate of thrombolytic drug. Results: In the absence of histotripsy, thrombolytic delivery was restricted in close proximity to the catheter. Thrombolytic perfused throughout the focal region for calculations that included the effects of histotripsy, resulting in an increased degree of fibrinolysis. Discussion: These results were consistent with the outcomes of in vitro studies, suggesting histotripsy-induced changes in the thrombus diffusivity are a primary mechanism for enhancement of thrombolytic drugs.

The water cavitation line as predicted by the TIP4P/2005 model

The formation of vapor bubbles in a metastable liquid, cavitation, is an activated process due to the free energy cost of having both phases at contact. Such an energetic penalty enables the existence of the liquid beyond its thermodynamic borders. Establishing the stability limits of a liquid as ubiquitous as water has important practical implications and has thereby attracted a lot of attention. Different experimental strategies and theoretical analyses have been employed to measure and predict the cavitation line, or the pressure-temperature kinetic stability border of liquid water. Understanding the location of the cavitation line requires knowing the cavitation rate dependence on pressure and temperature. Such dependency is difficult to obtain in experiments, and we use molecular simulations with the TIP4P/2005 model to fill this gap. By deeply overstretching liquid water below the saturation pressure, we are able to observe and quantify spontaneous cavitation. To deal with a lower overstretching regime, we resort to the Seeding technique, which consists of analyzing simulations of a liquid containing a vapor bubble under the theoretical framework of Classical Nucleation Theory. Combining spontaneous cavitation with Seeding, we get a wide overview of the cavitation rate. We study two different temperatures (450 and 550 K) and complement our perspective with the results previously obtained at 296.4 K [Menzl et al., Proc. Natl. Acad. Sci. 113, 13582 (2016)] to establish a broad simulation-experiment comparison. We find a good agreement between simulations and both isobaric heating and isochoric cooling experiments using quartz inclusions. We are, however, unable to reconcile simulations with other experimental techniques. Our results predict a decrease in the solid-liquid interfacial free energy as the liquid becomes increasingly overstretched with a temperature independent Tolman length of 0.1 nm. Therefore, the capillarity approximation underestimates the nucleation rate. Nonetheless, it provides a fair indication of the location of the cavitation line given the steep rate vs pressure dependence. Overall, our work provides a comprehensive view of the water cavitation phenomenon and sets an efficient strategy to investigate it with molecular simulations.

The evolution of cavitation in narrow soft-solid wedge geometry mimicking periodontal and peri-implant pockets

In periodontology and implantology, laser-induced cavitation has not yet been used to treat biofilm-related problems. In this study we have checked how soft tissue affects the evolution of cavitation in a wedge model representing periodontal and peri-implant pocket geometry. One side of the wedge model was composed of PDMS mimicking soft periodontal or peri-implant biological tissue, the other side was composed of glass mimicking hard tooth root or implant surface, which allowed observations of the cavitation dynamics with an ultrafast camera. Different laser pulse modalities, PDMS stiffness, and irrigants were tested for their effect on the evolution of cavitation in the narrow wedge geometry. The PDMS stiffness varied in a range that corresponds to severely inflamed, moderately inflamed, or healthy gingival tissue as determined by a panel of dentists. The results imply that deformation of the soft boundary has a major effect on the Er:YAG laser-induced cavitation. The softer the boundary, the less effective the cavitation. We show that in a stiffer gingival tissues model, photoacoustic energy can be guided and focused at the tip of the wedge model, where it enables generation of secondary cavitation and more effective microstreaming. The secondary cavitation was absent in severely inflamed gingival model tissue, but could be induced with a dual-pulse AutoSWEEPS laser modality. This should in principle increase cleaning efficiency in the narrow geometries such as those found in the periodontal and peri-implant pockets and may lead to more predictable treatment outcomes.

Real-time measurement of stapes motion and intracochlear pressure during blast exposure

Blast-induced auditory injury is primarily caused by exposure to an overwhelming amount of energy transmitted into the external auditory canal, the middle ear, and then the cochlea. Quantification of this energy requires real-time measurement of stapes footplate (SFP) motion and intracochlear pressure in the scala vestibuli (Psv). To date, SFP and Psv have not been measured simultaneously during blast exposure, but a dual-laser experimental approach for detecting the movement of the SFP was reported by Jiang et al. (2021). In this study, we have incorporated the measurement of Psv with SFP motion and developed a novel approach to quantitatively measure the energy flux entering the cochlea during blast exposure. Five fresh human cadaveric temporal bones (TBs) were used in this study. A mastoidectomy and facial recess approach were performed to identify the SFP, followed by a cochleostomy into the scala vestibuli (SV). The TB was mounted to the "head block", a fixture to simulate a real human skull, with two pressure sensors - one inserted into the SV (Psv) and another in the ear canal near the tympanic membrane (P1). The TB was exposed to the blast overpressure (P0) around 4 psi or 28 kPa. Two laser Doppler vibrometers (LDVs) were used to measure the movements of the SFP and TB (as a reference). The LDVs, P1, and Psv signals were triggered by P0 and recorded simultaneously. The results include peak values for Psv of 100.8 ± 51.6 kPa (mean ± SD) and for SFP displacement of 72.6 ± 56.4 μm, which are consistent with published experimental results and finite element modeling data. Most of the P0 input energy flux into the cochlea occurred within 2 ms and resulted in 10-70 μJ total energy entering the cochlea. Although the middle ear pressure gain was close to that measured under acoustic stimulus conditions, the nonlinear behavior of the middle ear was observed from the elevated cochlear input impedance. For the first time, SFP movement and intracochlear pressure Psv have been successfully measured simultaneously during blast exposure. This study provides a new methodology and experimental data for determining the energy flux entering the cochlea during a blast, which serves as an injury index for quantifying blast-induced auditory damage.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Ultrasonic photoacoustic emitter of graphene-nanocomposites film on a flexible substrate

Photoacoustic devices generating high-amplitude and high-frequency ultrasounds are attractive candidates for medical therapies and on-chip bio-applications. Here, we report the photoacoustic response of graphene nanoflakes - Polydimethylsiloxane composite. A protocol was developed to obtain well-dispersed graphene into the polymer, without the need for surface functionalization, at different weight percentages successively spin-coated onto a Polydimethylsiloxane substrate. We found that the photoacoustic amplitude scales up with optical absorption reaching 11 MPa at ∼ 228 mJ/cm² laser fluence. We observed a deviation of the pressure amplitude from the linearity increasing the laser fluence, which indicates a decrease of the Grüneisen parameter. Spatial confinement of high amplitude (> 40 MPa, laser fluence > 55 mJ/cm²) and high frequency (Bw-6db ∼ 21.5 MHz) ultrasound was achieved by embedding the freestanding film in an optical lens. The acoustic gain promotes the formation of cavitation microbubbles for moderate fluence in water and in tissue-mimicking material. Our results pave the way for novel photoacoustic medical devices and integrated components.

Injection continuous liquid interface production of 3D objects

In additive manufacturing, it is imperative to increase print speeds, use higher-viscosity resins, and print with multiple different resins simultaneously. To this end, we introduce a previously unexplored ultraviolet-based photopolymerization three-dimensional printing process. The method exploits a continuous liquid interface-the dead zone-mechanically fed with resin at elevated pressures through microfluidic channels dynamically created and integral to the growing part. Through this mass transport control, injection continuous liquid interface production, or iCLIP, can accelerate printing speeds to 5- to 10-fold over current methods such as CLIP, can use resins an order of magnitude more viscous than CLIP, and can readily pattern a single heterogeneous object with different resins in all Cartesian coordinates. We characterize the process parameters governing iCLIP and demonstrate use cases for rapidly printing carbon nanotube-filled composites, multimaterial features with length scales spanning several orders of magnitude, and lattices with tunable moduli and energy absorption.

Laser-induced shock-wave-expanded nanobubbles in spherical geometry

The secondary cavitation generation following laser-induced breakdown in aqueous media in spherical geometry, mimicking the geometry of the frontal part of the human eye, was studied. A numerical simulation of the shock wave propagation was performed, yielding peak-pressure maps, correctly predicting the location of the secondary cavitation onset for different shock wave source positions. The comparison between the simulation results and the experiments, performed with a high-precision, multiple-illumination technique, supports the suggested description of the nature of the secondary cavitation onset. It is shown that large transient negative pressures are created at the location of the acoustic image of the shock wave source, which is different from the optical focus. After the passage of the shock wave, abundant secondary cavitation is generated there. Additionally, the existence of an important contributing factor to the reduction of the secondary cavitation threshold is supported by the experimental results, namely the pre-illumination of the water by the breakdown-generating laser pulse, playing a crucial role in conditioning the medium. There is strong experimental evidence of the existence of another mechanism of pre-conditioning the water for the secondary cavitation onset, namely in the form of repetitive negative pressure pulse passage through the same volume, an indication of a possible two- or multiple-stage process.

Amplification of high-intensity pressure waves and cavitation in water using a multi-pulsed laser excitation and black-TiOx optoacoustic lens

A method for amplification of high-intensity pressure waves generated with a multi-pulsed Nd:YAG laser coupled with a black-TiOx optoacoustic lens in the water is presented and characterized. The investigation was focused on determining how the multi-pulsed laser excitation with delays between 50 µs and 400 µs influences the dynamics of the bubbles formed by a laser-induced breakdown on the upper surface of the lens, the acoustic cavitation in the focal region of the lens, and the high-intensity pressure waves generation. A needle hydrophone and a high-speed camera were used to analyze the spatial distribution and time-dependent development of the above-mentioned phenomena. Our results show how different delays (t_d ) of the laser pulses influence optoacoustic dynamics. When t_d is equal to or greater than the bubble oscillation time, acoustic cavitation cloud size increases 10-fold after the fourth laser pulse, while the pressure amplitude increases by more than 75%. A quasi-deterministic creation of cavitation due to consecutive transient pressure waves is also discussed. This is relevant for localized ablative laser therapy.

Non-contact optical characterization of negative pressure in hydrogel voids and microchannels

Negative pressure in water under tension, as a thermodynamic non-equilibrium state, has facilitated the emergence of innovative technologies on microfluidics, desalination, and thermal management. However, the lack of a simple and accurate method to measure negative pressure hinders further in-depth understanding of the properties of water in such a state. In this work, we propose a non-contact optical method to quantify the negative pressure in micron-sized water voids of a hydrogel film based on the microscale mechanical deformation of the hydrogel itself. We tested three groups of hydrogel samples with different negative pressure inside, and the obtained results fit well with the theoretical prediction. Furthermore, we demonstrated that this method can characterize the distribution of negative pressure, and can thus provide the possibility of investigation of the flow behavior of water in negative pressure. These results prove this technique to be a promising approach to characterization of water under tension and for investigation of its properties under negative pressure.

Attachment of bioinspired microfibrils in fluids: transition from a hydrodynamic to hydrostatic mechanism

Reversible and switchable adhesion of elastomeric microstructures has attracted significant interest in the development of grippers for object manipulation. Their applications, however, have often been limited to dry conditions and adhesion of such deformable microfibrils in the fluid environment is less understood. In the present study, we performed adhesion tests in silicone oil using single cylindrical microfibrils of a flat-punch shape with a radius of 80 µm. Stiff fibrils were created using three-dimensional printing of an elastomeric resin with an elastic modulus of 500 MPa, and soft fibrils, with a modulus of 3.3 MPa, were moulded in polyurethane. Our results suggest that adhesion is dominated by hydrodynamic forces, which can be maximized by stiff materials and high retraction velocities, in line with theoretical predictions. The maximum pull-off stress of stiff cylindrical fibrils is 0.6 MPa, limited by cavitation and viscous fingering, occurring at retraction velocities greater than 2 µm s^-1. Next, we add a mushroom cap to the microfibrils, which, in the case of the softer material, deforms upon retraction and leads to a transition to a hydrostatic suction regime with higher pull-off stresses ranging from 0.7 to 0.9 MPa. The effects of elastic modulus, fibril size and viscosity for underwater applications are illustrated in a mechanism map to provide guidance for design optimization.

Water as a "glue": Elasticity-enhanced wet attachment of biomimetic microcup structures

Octopus, clingfish, and larva use soft cups to attach to surfaces under water. Recently, various bioinspired cups have been engineered. However, the mechanisms of their attachment and detachment remain elusive. Using a novel microcup, fabricated by two-photon lithography, coupled with in situ pressure sensor and observation cameras, we reveal the detailed nature of its attachment/detachment under water. It involves elasticity-enhanced hydrodynamics generating "self-sealing" and high suction at the cup-substrate interface, converting water into "glue." Detachment is mediated by seal breaking. Three distinct mechanisms of breaking are identified, including elastic buckling of the cup rim. A mathematical model describes the interplay between the attachment/detachment process, geometry, elasto-hydrodynamics, and cup retraction speed. If the speed is too slow, then the octopus cannot attach; if the tide is too gentle for the larva, then water cannot serve as a glue. The concept of "water glue" can innovate underwater transport and manufacturing strategies.

Radial oscillation and translational motion of a gas bubble in a micro-cavity

According to classical nucleation theory, a gas nucleus can grow into a cavitation bubble when the ambient pressure is negative. Here, the growth process of a gas nucleus in a micro-cavity was simplified to two "events", and the full confinement effect of the surrounding medium of the cavity was considered by including the bulk modulus in the equation of state. The Rayleigh-Plesset-like equation of the cavitation bubble in the cavity was derived to model the radial oscillation and translational motion of the cavitation bubble in the local acoustic field. The numerical results show that the nucleation time of the cavitation bubble is sensitive to the initial position of the gas nucleus. The cavity size affects the duration of the radial oscillation of the cavitation bubble, where the duration is shorter for smaller cavities. The equilibrium radius of a cavitation bubble grown from a gas nucleus increases with increasing size of the cavity. There are two possible types of translational motion: reciprocal motion around the center of the cavity and motion toward the cavity wall. The growth process of gas nuclei into cavitation bubbles is also dependent on the compressibility of the surrounding medium and the magnitude of the negative pressure. Therefore, gas nuclei in a liquid cavity can be excited by acoustic waves to form cavitation bubbles, and the translational motion of the cavitation bubbles can be easily observed owing to the confining influence of the medium outside the cavity.

Sulcal Cavitation in Linear Head Acceleration: Possible Correlation With Chronic Traumatic Encephalopathy

Traumatic Brain Injury (TBI) is a significant public health and financial concern that is affecting tens of thousands of people in the United States annually. There were over a million hospital visits related to TBI in 2017. Along with immediate and short-term morbidity from TBI, chronic traumatic encephalopathy (CTE) can have life-altering, chronic morbidity, yet the direct linkage of how head impacts lead to this pathology remains unknown. A possible clue is that chronic traumatic encephalopathy appears to initiate in the depths of the sulci. The purpose of this study was to isolate the injury mechanism/s associated with blunt force impact events. To this end, drop tower experiments were performed on a human head phantom. Our phantom was fabricated into a three-dimensional extruded ellipsoid geometry made out of Polyacrylamide gelatin that incorporated gyri-sulci interaction. The phantom was assembled into a polylactic acid 3D-printed skull, surrounded with deionized water, and enclosed between two optical windows. The phantom received repetitive low-force impacts on the order of magnitude of an average boxing punch. Intracranial pressure profiles were recorded in conjunction with high-speed imaging, 25 k frames-per-second. Cavitation was observed in all trials. Cavitation is the spontaneous formation of vapor bubbles in the liquid phase resulting from a pressure drop that reaches the vapor pressure of the liquid. The observed cavitation was predominately located in the contrecoup during negative pressure phases of local intracranial pressure. To further investigate the cavitation interaction with the brain tissue phantom, a 2D plane strain computational model was built to simulate the deformation of gyrated tissue as a result from the initiation of cavitation bubbles seen in the phantom experiments. These computational experiments demonstrated a focusing of strain at the depths of the sulci from bubble expansion. Our results add further evidence that mechanical interactions could contribute to the development of chronic traumatic encephalopathy and also that fluid cavitation may play a role in this interaction.

Gas-stabilizing nanoparticles for ultrasound imaging and therapy of cancer

The use of ultrasound in the clinic has been long established for cancer detection and image-guided tissue biopsies. In addition, ultrasound-based methods have been widely explored to develop more effective cancer therapies such as localized drug delivery, sonodynamic therapy, and focused ultrasound surgery. Stabilized fluorocarbon microbubbles have been in use as contrast agents for ultrasound imaging in the clinic for several decades. It is also known that microbubble cavitation could generate thermal, mechanical, and chemical effects in the tissue to improve ultrasound-based therapies. However, the large size, poor stability, and short-term cavitation activity of microbubbles limit their applications in cancer imaging and therapy. This review will focus on an alternative type of ultrasound responsive material; gas-stabilizing nanoparticles, which can address the limitations of microbubbles with their nanoscale size, robustness, and high cavitation activity. This review will be of interest to researchers who wish to explore new agents to develop improved methods for molecular ultrasound imaging and therapy of cancer.

Water cavitation from ambient to high temperatures

Predicting cavitation has proved a formidable task, particularly for water. Despite the experimental difficulty of controlling the sample purity, there is nowadays substantial consensus on the remarkable tensile strength of water, on the order of -120 MPa at ambient conditions. Recent progress significantly advanced our predictive capability which, however, still considerably depends on elaborate fitting procedures based on the input of external data. Here a self-contained model is discussed which is shown able to accurately reproduce cavitation data for water over the most extended range of temperatures for which accurate experiments are available. The computations are based on a diffuse interface model which, as only inputs, requires a reliable equation of state for the bulk free energy and the interfacial tension. A rare event technique, namely the string method, is used to evaluate the free-energy barrier as the base for determining the nucleation rate and the cavitation pressure. The data allow discussing the role of the Tolman length in determining the nucleation barrier, confirming that, when the size of the cavitation nuclei exceed the thickness of the interfacial layer, the Tolman correction effectively improves the predictions of the plain Classical Nucleation Theory.

Experimental Demonstration of a Stacked Hybrid Optoacoustic-Piezoelectric Transducer for Localized Heating and Enhanced Cavitation

Laser-generated focused ultrasound (LGFU) is an emerging modality for cavitation-based therapy. However, focal pressure amplitudes by LGFU alone to achieve pulsed cavitation are often lacking as a treatment depth increases. This requires a higher pressure from a transmitter surface and more laser energies that even approach to a damage threshold of transmitter. To mitigate the requirement for LGFU-induced cavitation, we propose LGFU configurations with a locally heated focal zone using an additional high-intensity focused ultrasound (HIFU) transmitter. After confirming heat-induced cavitation enhancement using two separate transmitters, we then developed a stacked hybrid optoacoustic-piezoelectric transmitter, which is a unique configuration made by coating an optoacoustic layer directly onto a piezoelectric substrate. This shared curvature design has great practical advantage without requiring the complex alignment of two focal zones. Moreover, this enabled the amplification of cavitation bubble density by 18.5-fold compared to the LGFU operation alone. Finally, the feasibility of tissue fragmentation was confirmed through a tissue-mimicking gel, using the combination of LGFU and HIFU (not via a stacked structure). We expect that the stacked transmitter can be effectively used for stronger and faster tissue fragmentation than the LGFU transmitter alone.

Modeling the Physics of Bubble Nucleation in Histotripsy

This work aims to establish a theoretical framework for the modeling of bubble nucleation in histotripsy. A phenomenological version of the classical nucleation theory was parametrized with histotripsy experimental data, fitting a temperature-dependent activity factor that harmonizes theoretical predictions and experimental data for bubble nucleation at both high and low temperatures. Simulations of histotripsy pressure and temperature fields are then used in order to understand spatial and temporal properties of bubble nucleation at varying sonication conditions. This modeling framework offers a thermodynamic understanding on the role of the ultrasound frequency, waveforms, peak focal pressures, and duty cycle on patterns of ultrasound-induced bubble nucleation. It was found that at temperatures lower than 50 °C, nucleation rates are more appreciable at very large negative pressures such as -30 MPa. For focal peak-negative pressures of -15 MPa, characteristic of boiling histotripsy, nucleation rates grow by 20 orders of magnitude in the temperature interval 60 °C-100 °C.

The Influence of Background Ultrasonic Field on the Strength of Adhesive Zones under Dynamic Impact Loads

The influence of background ultrasonic field on the ultimate dynamic strength of adhesive joints is studied using fracture mechanics analysis. Winkler foundation-type models are applied to describe the cohesion zone, and the incubation time fracture criterion is used. The challenging task is to study whether relatively weak ultrasound is able to decrease the threshold values of the external impact load depending on a joint model, such as an "elastic membrane" or "beam" approximation, and various boundary conditions at the ends. The specific task was to investigate the case of short pulse loading through application of time-dependent fracture criterion instead of the conventional principle of critical stress. Three different load cases, namely, step constant force, dynamic pulse, and their combination with ultrasonic vibrations, were also studied. The analytical solution to the problem demonstrates that background vibrations at certain frequencies can significantly decrease threshold values of fracture impact load. Specific calculations indicate that even a weak background sonic field is enough to cause a significant reduction in the threshold amplitude of a dynamic short pulse load. Additionally, non-monotonic dependency of threshold amplitude on pulse duration for weak background field was observed, which demonstrates the existence of optimal regimes of impact energy input. Moreover, this phenomenon does not depend on the way in which the beam edges mount, whether they are clamped or hinged, and it could be applied for micro-electro-mechanical switch design processes as an additional tool to control operational regimes.

Acoustic streaming induced by MHz-frequency ultrasound extends the volume limit of cell suspension culture

Large-scale cell suspension culture technology opens up opportunities for numerous medical and bioengineering applications. For these purposes, scale-up of the culture system is paramount. For initial small-scale culture, a simple static suspension culture (SSC) is generally employed. However, cell sedimentation due to the lack of agitation limits the culture volume feasible for SSC. Thus, when scaling up, cell suspensions must be manually transferred from the culture flask to another vessel suitable for agitation, which increases the risk of contamination and human error. Ideally, the number of culture transfer steps should be kept to a minimum. The present study describes the fabrication of an ultrasonic suspension culture system that stirs cell suspensions with the use of acoustic streaming generated by ultrasound irradiation at a MHz frequency. This system was applied to 100-mL suspension cultures of Chinese hamster ovary cells-a volume ten-fold larger than that generally used. The cell proliferation rate in this system was 1.88/day when applying an input voltage of 40 V to the ultrasonic transducer, while that of the SSC was 1.14/day. Hence, the proposed method can extend the volume limit of static cell suspension cultures, thereby reducing the number of cell culture transfer steps.

Acoustic Measurements of Nucleus Size Distribution at the Cavitation Threshold

An understanding of the acoustic cavitation threshold is essential for minimizing cavitation bio-effects in diagnostic ultrasound and for controlling cavitation-mediated tissue ablation in focused ultrasound procedures. The homogeneous cavitation threshold is an intrinsic material property of recognized importance to biomedical ultrasound as well as a variety of other applications requiring cavitation control. However, measurements of the acoustic cavitation threshold in water differ from those predicted by classic nucleation theories. This persistent discrepancy is explained by combining recently developed methods for acoustically nucleating single bubbles at threshold with numerical modeling to obtain a nucleus size distribution consistent with first-principles estimates for ion-stabilized nuclei. We identify acoustic cavitation at threshold as a reproducible subtype of heterogeneous cavitation with a characteristic nucleus size distribution. Knowledge of the nucleus size distribution could inspire new approaches to achieving cavitation control in water, tissue and a variety of other media.

An Analysis of Acoustic Cavitation Thresholds of Water Based on the Incubation Time Criterion Approach

Researchers are still working on the development of models that facilitate the accurate estimation of acoustic cavitation threshold. In this paper, we have analyzed the possibility of using the incubation time criterion to calculate the threshold of the onset of acoustic cavitation depending on the ultrasound frequency, hydrostatic pressure, and temperature of a liquid. This criterion has been successfully used by earlier studies to calculate the dynamic strength of solids and has recently been proposed in an adapted version for calculating the cavitation threshold. The analysis is carried out for various experimental data for water presented in the literature. Although the criterion assumes the use of macroparameters of a liquid, we also considered the possibility of taking into account the size of cavitation nuclei and its influence on the calculation result. We compared the results of cavitation threshold calculations done using the incubation time criterion of cavitation and the classical nucleation theory. Our results showed that the incubation time criterion more qualitatively models the results of experiments using only three parameters of the liquid. We then discussed a possible relationship between the parameters of the two approaches. The results of our study showed that the criterion under consideration has a good potential and can be conveniently used for applications where there are special requirements for ultrasound parameters, maximum negative pressure, and liquid temperature.

Dimensional engineering of a topological insulating phase in Half-Heusler LiMgAs

We propose a novel technique of dimensional engineering to realize low dimensional topological insulator from a trivial three dimensional parent. This is achieved by confining the bulk system to one dimension along a particular crystal direction, thus enhancing the quantum confinement effects in the system. We investigate this mechanism in the Half-Heusler compound LiMgAs with face-centered cubic (FCC) structure. At ambient conditions the bulk FCC structure exhibits a semi-conducting nature. But, under the influence of high volume expansive pressure (VEP) the system undergoes a topological phase transition (TPT) from semi-conducting to semi-metallic forming a Dirac cone. At a critical VEP we observe that, spin-orbit coupling (SOC) effects introduce a gap of [Formula: see text] 1.5 meV in the Dirac cone at high symmetry point [Formula: see text] in the Brillouin zone. This phase of bulk LiMgAs exhibits a trivial nature characterized by the [Formula: see text] invariants as (0,000). By further performing dimensional engineering, we cleave [111] plane from the bulk FCC structure and confine the system in one dimension. This low-dimensional phase of LiMgAs has structure similar to the two dimensional [Formula: see text] system. Under a relatively lower compressive strain, the low-dimensional system undergoes a TPT and exhibits a non-trivial topological nature characterized by the SOC gap of [Formula: see text] 55 meV and [Formula: see text] invariant [Formula: see text] = 1. Although both, the low-dimensional and bulk phase exhibit edge and surface states, the low-dimensional phase is far more superior and exceptional as compared to the bulk parent in terms of the velocity of Fermions ([Formula: see text]) across the surface states. Such a system has promising applications in nano-electronics.

Simulation of the Strain Amplification in Sulci Due to Blunt Impact to the Head

Traumatic brain injury (TBI) has become a concern in sports, automobile accidents and combat operations. A better understanding of the mechanics leading to a TBI is required to cope with both the short-term life-threatening effects and long-term effects of TBIs, such as the development chronic traumatic encephalopathy (CTE). Kornguth et al. (1) proposed that an inflammatory and autoimmune process initiated by a water hammer effect at the bases of the sulci of the brain is a mechanism of TBI leading to CTE. A major objective of this study is to investigate whether the water hammer effect is present due to blunt impacts through the use of computational models. Frontal blunt impacts were simulated with 2D finite element models developed to capture the biofidelic geometry of a human head. The models utilized the Arbitrary Lagrangian Eulerian (ALE) method to model a layer of cerebrospinal fluid (CSF) as a deforming fluid allowing for CSF to move in and out of sulci. During the simulated impacts, CSF was not observed to be driven into the sulci during the transient response. However, elevated shear strain levels near the base of the sulci were exhibited. Further, increased shear strain was present when differentiation between white and gray matter was taken into account. Both of the results support clinical observations of (1).

Characterization of Astrocytic Response after Experiencing Cavitation In Vitro

When a traumatic brain injury (TBI) occurs, low-pressure regions inside the skull can cause vapor contents in the cerebral spinal fluid (CSF) to expand and collapse, a phenomenon known as cavitation. When these microbubbles (MBs) collapse, shock waves are radiated outward and are known to damage surrounding materials in other applications, like the steel foundation of boat propellers, so it is alarming to realize the damage that cavitation inflicts on vulnerable brain tissue. Using cell-laden microfibers, the longitudinal morphological response that mouse astrocytes have to surrounding cavitation in vitro is visually analyzed. Astrocytic damage is evident immediately after cavitation when compared to a control sample, as their processes retract. Forty-eight hours later, the astrocytes appeared to spread across the fibers, as normal. This study also analyzes the gene expression changes that occur post-cavitation via quantitative polymerase chain reaction (qPCR) methods. After cavitation a number of pro-inflammatory genes are upregulated, including TNFα, IL-1β, C1q, Serping1, NOS1, IL-6, and JMJD3. Taken together, these results confirm that surrounding cavitation is detrimental to astrocytic function, and yield opportunities to further the understanding of how protective headgear can minimize or eliminate the occurrence of cavitation.

Recovery of Diamond and Cobalt Powder from Polycrystalline Drawing Die Blanks via Ultrasound-Assisted Leaching Process—Part 1: Process Design and Efficiencies

The treatment of industrial polycrystalline diamond (PCD) blanks in aqua regia at atmospheric pressure between 333 K and 353 K was performed via the ultrasound-assisted leaching process to investigate whether the influence of ultrasound is beneficial. Cobalt content in the solution and in the blanks was monitored as well as the effects of leaching temperature, solid-to-liquid ratio, and PCD blank size. The use of intermittent and permanent ultrasound helped reduce the leaching time and thus energy consumption by up to 50%. In all trials with ultrasound, higher temperature only has a slight effect. Solid-to-liquid ratio does not have a positive or negative impact. A new process design was tested using an innovative experimental setup for ultrasound-assisted leaching aiming at maximum cobalt and diamond recovery from PCD and final reuse of fine PCD for cutting and polishing other hard materials in different important industrial applications.

Selective aggregation by ultrasonic standing waves through gas nuclei on the particle surface

Gas nuclei in water are usually too small to be directly observed. They will grow into bubbles under the negative pressure, which is called cavitation (heterogeneous cavitation). In this study, the gas nuclei in the hydrophilic and hydrophobic silica particle suspension were investigated using the transient cavitation threshold measured by a high-intensity focused ultrasound (HIFU). The transient cavitation bubbles were also observed by a high-speed camera. The results showed that the nuclei only exist on the surface of hydrophobic particles. Furthermore, the aggregation experiments revealed that the aggregates were only formed in the hydrophobic silica suspension by ultrasonic standing waves (USW) at 200 kHz. This distinct difference was mainly due to the formation of gas nuclei on hydrophobic silica particles, which grew and coalesced into stable bubbles under the 200 kHz USW. The aggregation process in suspension was observed by a CCD camera. Moreover, the cavitation thresholds and acoustic radiation forces were analyzed to explain the mechanism of the acoustic aggregation. This study showed a very promising acoustic method for the selective aggregation of hydrophobic particles, which might be efficiently used in the mineral separation industry.

Volume expansive pressure (VEP) driven non-trivial topological phase transition in LiMgBi

Topological Insulators (TI) exhibit robust spin-locked dissipationless Fermion transport along the surface states. In the current study, we use first-principles calculations to investigate a Topological Phase Transition (TPT) in a Half-Heusler (HH) compound LiMgBi driven by a Volume Expansive Pressure (VEP) which is attributed to the presence of, intrinsic voids, thermal perturbations and/or due to a phenomena known as cavity nuclei. We find that, the dynamically stable face-centred cubic (FCC) structure of LiMgBi (which belongs to the F4[combining macron]3m[216] space group), undergoes TPT beyond a critical VEP (at 4.0%). The continuous application of VEP from 0.0% to 8.0% results in a phase transition from a, band insulator to a Dirac semi-metal nature. Qualitatively, the Dirac cone formation and band inversion along the high symmetry point Γ in the Brillouin Zone (BZ) are analysed in terms of Electronic Band Structure (EBS) and Projected Local Density of States (LDOS). The TPT is further characterised by the [Doublestruck Z]₂ invariant, (ν₀, ν₁ν₂ν₃) ≡ (1, 0 0 0) along the (0001) surface which indicates quantitatively that, HH LiMgBi is a strong TI. We hence propose, HH LiMgBi (known for its piezoelectric, thermo-electric and semi-conducting applications) as a strong TI with potential multi-purpose application in the field of electronics, spintronics and quantum computation.

Cavitation inception pressure and bubble cloud formation due to the backscattering of high-intensity focused ultrasound from a laser-induced bubble

Cavitation bubble cloud formation due to the backscattering of high-intensity focused ultrasound (HIFU) from a laser-induced bubble in various water temperatures and dissolved oxygen (DO) has been investigated. A laser-induced bubble generated near the geometrical focus of HIFU is utilized to yield intense negative pressure by the backscattering. Optical observation with a high-speed video camera and pressure measurement with a fiber-optic probe hydrophone are conducted simultaneously to understand the forming process of a bubble cloud and corresponding pressure field by the backscattering. Optical observation shows that a bubble cloud grows stepwise forming multiple layers composed of tiny cavitation bubbles, and the cavitation inception position is consistent with the local minimum pressure position simulated with the ghost fluid method. The bubble cloud grows larger in the opposite direction of HIFU propagation, and the absolute value of the cavitation inception pressure decreases with an increase in water temperature. The linear correlation between cavitation inception pressure and water temperature agrees with that given by Vlaisavljevich, Xu, Maxwell, Mancia, Zhang, Lin, Duryea, Sukovich, Hall, Johnsen, and Cain [IEEE Trans. Ultrason. Ferroelectr. Freq. Control 63, 1064-1077 (2016)]. However DO has minor dependence on the cavitation inception pressure when DO is degassed sufficiently. Furthermore, the gas nucleus size that might exist in the experiment has been estimated by using bubble dynamics.

Cavitation Nuclei Regeneration in a Water-Particle Suspension

Bubble nucleation in water induced by boiling, gas supersaturation, or cavitation usually originates from preexisting gas cavities trapped into solid defects. Even though the destabilization of such gas pockets, called nuclei, has been extensively studied, little is known on the nuclei dynamic. Here, nuclei of water-particle suspensions are excited by acoustic cavitation, and their dynamic is investigated by monitoring the cavitation probability over several thousand pulses. A stable and reproducible cavitation probability emerges after a few thousand pulses and depends on particle concentration, hydrophobicity, and dissolved gas content. Our observations indicate that a stable nuclei distribution is reached at a later time, different from previously reported nuclei depletion in early time. This apparent paradox is elucidated by varying the excitation rate, where the cavitation activity increases with the repetition period, indicating that the nuclei depletion is balanced by spontaneous nucleation or growth of nuclei. A model of this self-supporting generation of nuclei suggests an origin from dissolved gas adsorption on surfaces. The method developed can be utilized to further understand the spontaneous formation and distribution of nanosized bubbles on heterogeneous surfaces.

A review of effects and applications of ultrasound in mineral flotation

Ultrasound technology is widely applied in the flotation process. From the perspective of the theory of ultrasound, this article explains the effects and applications of ultrasound in the flotation process. To obtain a clear understanding of ultrasonic effects, we observe the phenomena of ultrasound using a high-speed camera and a CCD camera, and investigate potential applications in flotation. From these different phenomena, the ultrasonic effects are classified into three types of effect: the transient cavitation effect, stable cavitation effect, and acoustic radiation force effect. Based on these effects, the applications of ultrasound to mineral flotation are reviewed, including slime coating removal, oxidation film removal, desulfuration, tiny bubble generation, flotation reagent dispersion, and aggregation. In addition, the ultrasonic equipment and treatment methods applied in flotation are classified and compared based on their characteristics. Finally, we propose some potential directions in the study of the stable cavitation effect and acoustic radiation force effect, which are important, but are seldom mentioned in previous reports.

Physics of suction cups

We have developed a theory of air leakage at interfaces between two elastic solids with application to suction cups in contact with randomly rough surfaces. We present an equation for the airflow in narrow constrictions which interpolates between the diffusive and ballistic (Knudsen) air-flow limits. To test the theory, we performed experiments using two different suction cups, made from soft polyvinylchloride (PVC), in contact with sandblasted polymethylmethacrylate (PMMA) plates. We found that the measured time to detach (lifetime) of the suction cups was in good agreement with theory, except for surfaces with a root-mean-square (rms) roughness below ≈1 μm, where diffusion of plasticizer from the PVC to the PMMA surface caused blockage of critical constrictions. The suction cup volume, stiffness, and elastic modulus have a huge influence on the air leakage and hence the failure time of the cups. Based on our research we propose an improved biomimetic design of suction cups that could show improved failure times with varying degrees of roughness under dry and wet environments.

Jet direction in bubble collapse within rectangular and triangular channels

A vapor bubble collapsing near a solid boundary in a liquid produces a liquid jet that points toward the boundary. The direction of this jet has been studied for boundaries such as flat planes and parallel walls enclosing a channel. Extending these investigations to enclosed polygonal boundaries, we experimentally measure jet direction for collapsing bubbles inside a square and an equilateral triangular channel. Following the method of Tagawa and Peters [Phys. Rev. Fluids 3, 081601 (2018)10.1103/PhysRevFluids.3.081601] for predicting the jet direction in corners, we model the bubble as a sink in a potential flow and demonstrate by experiment that analytical solutions accurately predict jet direction within an equilateral triangle and square. We further use the method to develop predictions for several other polygons, specifically, a rectangle, an isosceles right triangle, and a 30^{∘}-60^{∘}-90^{∘} right triangle.

Turning Drops into Bubbles: Cavitation by Vapor Diffusion through Elastic Networks

Some members of the vegetal kingdom can achieve surprisingly fast movements making use of a clever combination of evaporation, elasticity, and cavitation. In this process, enthalpic energy is transformed into elastic energy and suddenly released in a cavitation event which produces kinetic energy. Here, we study this unusual energy transformation by a model system: A droplet in an elastic medium shrinks slowly by diffusion and eventually transforms into a bubble by a rapid cavitation event. The experiments reveal the cavity dynamics over the extremely disparate timescales of the process, spanning 9 orders of magnitude. We model the initial shrinkage as a classical diffusive process, while the sudden bubble growth and oscillations are described using an inertial-(visco)elastic model, in excellent agreement with the experiments. Such a model system could serve as a new paradigm for motile synthetic materials.

How water flow, geometry, and material properties drive plant movements

Plants are dynamic. They adjust their shape for feeding, defence, and reproduction. Such plant movements are critical for their survival. We present selected examples covering a range of movements from single cell to tissue level and over a range of time scales. We focus on reversible turgor-driven shape changes. Recent insights into the mechanisms of stomata, bladderwort, the waterwheel, and the Venus flytrap are presented. The underlying physical principles (turgor, osmosis, membrane permeability, wall stress, snap buckling, and elastic instability) are highlighted, and advances in our understanding of these processes are summarized.

The effects of ultrasound pressure and temperature fields in millisecond bubble nucleation

A phenomenological implementation of Classical Nucleation Theory (CNT) is employed to investigate the connection between high intensity focused ultrasound (HIFU) pressure and temperature fields with the energetic requirements of bubble nucleation. As a case study, boiling histotripsy in tissue-mimicking phantoms is modelled. The physics of key components in the implementation of CNT in HIFU conditions such as the derivation of nucleation pressure thresholds and approximations regarding the surface tension of the liquid are reviewed and discussed. Simulations show that the acoustic pressure is the ultimate trigger for millisecond bubble nucleation in boiling histotripsy, however, HIFU heat deposition facilitates nucleation by lowering nucleation pressure thresholds. Nucleation thus occurs preferentially at the regions of highest heat deposition within the HIFU field. This implies that bubble nucleation subsequent to millisecond HIFU heat deposition can take place at temperatures below 100 °C as long as the focal HIFU peak negative pressure exceeds the temperature-dependent nucleation threshold. It is also found that the magnitude of nucleation pressure thresholds decreases with decreasing frequencies. Overall, results indicate that it is not possible to separate thermal and mechanical effects of HIFU in the nucleation of bubbles for timescales of a few milliseconds. This methodology provides a promising framework for studying time and space dependencies of the energetics of bubble nucleation within a HIFU field.