The mechanism for the formation of slug flow in vertical gas–liquid two-phase flow

This paper studies the mechanism for the formation of a slug flow in vertical gas–liquid two-phase flow. By analyzing void fraction waves and their instability, it is proved that the formation of a slug flow regime is due to the increase of void fraction waves, which causes the conglomeration of gas bubbles and the coalescence of bubble clusters in unstable bubbly flow. Experiments and analysis show that intense turbulence can restrain the formation of Taylor bubbles. Therefore, in a large diameter vertical pipe, a Taylor bubble can form under a condition of low continuous volume flux due to the action of void fraction waves. However, the coalescence effect of void fraction waves as it affects bubbles is suppressed in high continuous volume flux, and therefore, a slug flow regime cannot be observed in the evolution of flow patterns. Under a condition of high continuous volume flux (V_L=0.15 m/s) described in the paper, the flow pattern evolution is from cap bubbly flow to cap churn flow, and then gradually to churn flow with the increase of void fraction.     

Finite-element analysis of hepatic cryoablation around a large blood vessel

Cryoablation is a minimally invasive ablation technique for primary and metastatic hepatic tumors. Inadequate freezing around large blood vessels due to the warm blood flow can lead to local recurrence, and thus, necessitates close application of a cryoprobe to the large blood vessels. In this study, we constructed a perfusion model with an ex vivo bovine liver and ablated the tissue around a large blood vessel with one or two cryoprobes applied to the side of the vessel. The finite-element computer model developed in our previous study was modified to include a blood vessel and its convective heat transfer to the vicinity of the blood vessel. We compared the predicted simulation results to those acquired from this ex vivo perfusion model. The results indicate that blood vessels act as a heat source and generate steep temperature profiles in the area next to the large blood vessel. After validation, the maximum allowable distance between the cryoprobe and the large blood vessel for successful cryoablation was presented. The results of this study should be considered when placing cryoprobes in the vicinity of large blood vessels.     

An Innovative Design by Single‐Layer Superaerophobic Mesh: Continuous Underwater Bubble Antibuoyancy Collection and Transportation

Underwater bubbles are unavoidable in the natural world and industrial production. Understanding the behavior of underwater bubbles and manipulating gas bubbles are vital important to both fundamental scientific research and industrial application. Although there has been some progress in controlling underwater bubbles, continuous underwater bubble collection and transportation remain challenging targets. Herein, inspired by the mechanism of water spider's gas storage, a strategy to collect and transport underwater gas bubble is demonstrated by design of a single‐layer underwater superaerophobic mesh (USM) assembled with a quartz tube. Gas bubbles supplied by a syringe pump penetrate the mesh pore and then gather to form a gas column in the quartz tube. Collapse occurs when the gas column reach the maximum storage height/pressure. Under a continuous supply of gas bubbles, the change of pressure becomes a cyclic process, which acts in a pump‐like manner to transport bubbles continuously from the water to the gas phase in the USM device assembled with an asymmetric U‐tube. This novel gas collection and transport system provides a new inspiration for developing new technologies for applications in pipes, sensors, gas collection, and environment protection.     

Pulse Doppler ultrasound detection, characterization and sizeestimation of emboli in flowing blood

A theory describing pulse Doppler ultrasound signals due to backscattering due to emboli in flowing blood is presented. From this theory, the minimum detectable size of a formed-element embolus can be established as a function of carrier frequency and vessel size. Emboli can be sized and characterized, based on the ratio of the amplitude of the Doppler signal during embolus passage through the sample volume to background bloodflow Doppler signal when no embolus is present. This ratio is defined as the “embolus to blood ratio” (EBR). Size estimation of emboli can be done by insonating an embolus with a single frequency and measuring the EBR, only if the embolus does not exceed a certain size, and if the vessel diameter and per cent hematocrit are known. Using two different frequencies, the vessel geometry (diameter and sample volume length) and per cent hematocrit can be eliminated from calculation of embolus size. Sources of uncertainty in the EBR and their effect on embolus size estimation are discussed. Discrimination between gas and formed-element emboli is described, given a detector with sufficient dynamic range, and use of three carrier frequencies. The theory presented here is in agreement with experimental findings of other investigators     

Morphology‐Control Strategy of the Superhydrophobic Poly(Methyl Methacrylate) Surface for Efficient Bubble Adhesion and Wastewater Remediation

As one common form of gas existing in aqueous environment, gas bubbles have attracted considerable worldwide attention, owing to their promising applications in industrial production and daily life, such as pressure sensors, the recovery of valuable minerals from ores, aeration process, and water treatment. Usually, the behaviors of gas bubbles in aqueous environment are mainly dominated by buoyancy force. It drives the gas bubbles out of aqueous medium rapidly, which is unfavorable in various processes, especially in wastewater treatment. In this paper, various types of superhydrophobic poly(methyl methacrylate) (PMMA) sheets are facilely fabricated, such as five‐pointed star, triangle, circular, and ellipse. Compared with other shapes of superhydrophobic PMMA sheets, the prepared superhydrophobic PMMA circular sheet is capable of efficiently adhering gas bubbles and subsequently elongating their retention time in an aqueous environment. Furthermore, superhydrophobic PMMA circular sheet arrays are prepared, which can greatly improve the degradation efficiency of methyl blue by ozone (O₃). The investigations indicate that the present approach will find wild applications in bubble‐related fields and provide people with inspirations to develop efficient methods to manipulate gas bubbles in aqueous environment.     

Mean blood velocity measurement with a narrow ultrasound beam and an asymmetric velocity profile

Previous discussions of the measurement of spatial mean blood velocity using Doppler ultrasound with a narrow beam have required the velocity profile in the cross section to be symmetric about the vessel axis. A type of asymmetry is put forward which incurs no error in mean velocity measurement when the beam is directed through the point of maximum velocity. Assuming correct alignment of the beam, errors due to profile asymmetry are, therefore, related to the degree of departure of the asymmetry from this acceptable form.     

Computational Patient-Specific Models Based on 3-D Ultrasound Data to Quantify Uterine Arterial Flow During Pregnancy

Information on uterine blood flow rate during pregnancy would widely improve our knowledge on feto-placental patho-physiology. Ultrasonographic flow rate evaluation requires the knowledge of the spatial velocity profiles throughout the investigated vessel; these data may be obtained from hemodynamic simulations with accurate computational models. Recently, computational models of superficial vessels have been created using 3-D ultrasound data; unfortunately, common reconstruction methods are unsuitable for the uterine arteries due to the low quality achievable of imaged deep vessels. In this paper a simplified spline-based technique was applied to create computational models for patient-specific simulations of uterine arterial heamodynamics. Moreover, a novel method to quantify the uterine flow rates was developed based on echo-Doppler measurements and computational data. Preliminary results obtained for four patients indicated a quite narrow range for the blood flow rate through the main uterine artery with large variability in the flow split between corporal and cervical branches. Furthermore, parabolic-like velocity profiles were obtained in the branching region of the different patients, suggesting a clinical use of averaged, not patient-specific, spatial velocity distribution coefficients for the blood flow rate calculation. The developed reconstruction method based on 3-D ultrasound imaging is efficient for creating realistic custom models of the uterine arteries. The results of the fluid dynamic simulations allowed us to quantify the uterine arterial flow and its repartition in normal pregnancies.      

Decompression Studies Using Ultrasonic Imaging of Bubbles

Single bubbles ranging down in size to under 1 ?m (less than capillary size) can be noticed, localized, and measured in ultrasonic images of intact subjects using 7.5 MHz ultrasound for which the wavelength is 200 ?m. Subjects included humans, fish, and guinea pigs. A combined brightness modulation and deflection display was most effective. Bubble reality during decompression and association with symptoms has been demonstrated, as have asymptomatic bubbles, a tendency for bubble formation in fat, recompression bubble showers, and decompression without diving tables. In guinea pigs there were age and male-female differences in susceptibility. Adjacent tissue inert gas pressure, supersaturation, and time constant can be measured by adjusting ambient pressure until bubbles cease to grow. Present data generally favor a supersaturation rather than a phase equilibration model for bends onset. An increase in allowable supersaturation was observed when decompression was to altitude rather than to sea level. Goldfish were seen to survive bubbling that would kill the mammals studied, and some simultaneous observations by light and sound were made in transparent fish.     

Magnetocontrollable Droplet and Bubble Manipulation on a Stable Amphibious Slippery Gel Surface

Smart manipulation of liquid/bubble transport has garnered widespread attention due to its potential applications in many fields. Designing a responsive surface has emerged as an effective strategy for achieving controllable transport of liquids/bubbles. However, it is still challenging to fabricate stable amphibious responsive surfaces that can be used for the smart manipulation of liquid in air and bubbles underwater. Here, amphibious slippery surfaces are fabricated using magnetically responsive soft poly(dimethylsiloxane) doped with iron powder and silicone oil. The slippery gel surface retains its magnetic responsiveness and demonstrates strong affinity for bubbles underwater but shows small and switching resistance forces with the water droplets in air and bubbles underwater, which is the key factor for achieving the controllable transport of liquids/bubbles. On the slippery gel surface, the sliding behaviors of water droplets and bubbles can be reversibly controlled by alternately applying/removing an external magnetic field. Notably, compared with slippery liquid‐infused porous surfaces, the slippery gel surface demonstrates outstanding stability, whether in air or underwater, even after 100 cycles of alternately applying/removing the magnetic field. This surface shows potential applications in gas/liquid microreactors, gas–liquid mixed fluid transportation, bubble/droplet manipulation, etc.     

Versatile,Stable, and Scalable Gel-Like Aerophobic Surface System (GLASS) for Hydrogen Production

Facile removal of adsorbed gas bubbles from electrode surfaces is crucial to realize efficient and stable energy conversion devices based on electrochemical gas evolution reactions. Conventional studies on bubble removal have limited applicability and scalability due to their reliance on complex and energy/time-intensive processes. In this study, a simple and versatile method is reported to fabricate large-area superaerophobic electrodes (up to 100 cm²) for diverse gas evolution reactions using the gel-like aerophobic surface system (GLASS). GLASS electrodes are readily and uniformly fabricated by simple spin-coating and cross-linking of polyallylamine on virtually any kinds of electrodes within 5 min under ambient conditions. Intrinsically hydrophilic gel overlayers with interconnected open pores allow the physical separation of bubble adhesion and catalytic active sites, reducing bubble adhesion strength, and promoting the removal of gas bubbles. As a result, GLASS electrodes exhibit greatly enhanced efficiency and stability for diverse gas evolution reactions, such as hydrogen evolution, hydrazine oxidation, and oxygen evolution reactions. This study provides deeper insights into understanding the effect of the hydrophilic microenvironment on gas evolution reactions and designing practical electrochemical devices.     

Bubble behavior characteristics based on virtual binocular stereo vision

The three-dimensional (3D) behavior characteristics of bubble rising in gas-liquid two-phase flow are of great importance to study bubbly flow mechanism and guide engineering practice. Based on the dual-perspective imaging of virtual binocular stereo vision, the 3D behavior characteristics of bubbles in gas-liquid two-phase flow are studied in detail, which effectively increases the projection information of bubbles to acquire more accurate behavior features. In this paper, the variations of bubble equivalent diameter, volume, velocity and trajectory in the rising process are estimated, and the factors affecting bubble behavior characteristics are analyzed. It is shown that the method is real-time and valid, the equivalent diameter of the rising bubble in the stagnant water is periodically changed, and the crests and troughs in the equivalent diameter curve appear alternately. The bubble behavior characteristics as well as the spiral amplitude are affected by the orifice diameter and the gas volume flow.     

Anatomy and flow in normal and ischemic microvasculature based on a novel temporal fractal dimension analysis algorithm using contrast enhanced ultrasound

Strategies for improvement of blood flow by promoting new vessel growth in ischemic tissue are being developed. Recently, contrast-enhanced ultrasound (CEU) imaging has been used to assess tissue perfusion in models of ischemia-related angiogenesis, growth-factor mediated angiogenesis, and tumor angiogenesis. In these studies, microvascular flow is measured in order to assess the total impact of adaptations at different vascular levels. High-resolution methods for imaging larger vessels have been developed in order to derive "angiograms" of arteries, veins, and medium to large microvessels. We describe a novel method of vascular bed (microvessel and arterial) characterization of vessel anatomy and flow simultaneously, using serial measurement of the fractal dimension (FD) of a temporal sequence of CEU images. This method is proposed as an experimental methodology to distinguish ischemic from nonischemic tissue. Moreover, an improved approach for extracting the FD unique to this application is introduced.     

Spontaneous and Directional Transportation of Gas Bubbles on Superhydrophobic Cones

Understanding the behavior of gas bubbles in aqueous media and realizing their spontaneous and directional manipulation are of vital importance in both scientific research and industrial applications, owing to their significant influences on many processes, such as waste water treatment, gas evolution reactions, and the recovery of valuable minerals. However, the behaviors of gas bubbles in aqueous media are mainly dominated by the buoyant force, which greatly impedes gas bubble transportation to any other direction except upward. Consequently, the spontaneous and directional transportation of gas bubbles in aqueous media is still identified as a big issue. Here, superhydrophobic copper cones have been successfully fabricated by integrating low‐surface‐tension chemical coatings with conical morphology. The generated superhydrophobic copper cones are capable of transporting gas bubbles from their tip to the base spontaneously and directionally underwater, even when they are vertically fixed with tips pointing up. The present study will inspire people to develop novel strategies to achieve efficient manipulation of gas bubbles in practical applications.     

Time-scale detection of microemboli in flowing blood with Dopplerultrasound

Small formed elements and gas bubbles in flowing blood, called microemboli, can be detected using Doppler ultrasound. In this application, a pulsed constant-frequency ultrasound signal insonates a volume of blood in the middle cerebral artery, and microemboli moving through its sample volume produce a Doppler-shifted transient reflection. Current detection methods include searching for these transients in a short-time Fourier transform (STFT) of the reflected signal. However, since the embolus transit time through the Doppler sample volume is inversely proportional to the embolus velocity (Doppler-shift frequency), a matched-filter detector should in principle use a wavelet transform, rather than a short-time Fourier transform, for optimal results. Closer examination of the Doppler shift signals usually shows a chirping behavior apparently due to acceleration or deceleration of the emboli during their transit through the Doppler sample volume. These variations imply that a linear wavelet detector is not optimal. We apply linear and quadratic time-frequency and time-scale detectors to a set of noise-corrupted embolus data. Our results show improvements of about 1 dB using the time-scale detectors versus an STFT-based detector signifying that embolus detection is best approached as a time-scale problem. A time-scale-chirp detector is also applied and is found to have the overall best performance by about 0.5-0.7 dB while coming fairly close (about 0.75 dB) to a theoretical upper bound.     

Annealing behavior of electrodeposited gold containing entrapments

An experimental study of electrodeposited gold shows that the presence of entrapments, mostly in the form of gas bubbles containing hydrogen, can drastically affect its physical and mechanical properties. These property changes are expected to manifest themselves as unique problems in many electronic packaging applications. It has been shown that bubbles, ranging from less than 50 to 2000 angstroms in diameter, can be present in densities as high as 10¹⁷/cm³. Annealing studies in conjunction with changes in bubble size and density, electrical resistivity, outgassing, and hardness are specifically discussed.     

Aerophilic Electrode with Cone Shape for Continuous Generation and Efficient Collection of H2 Bubbles

Hydrogen as a sustainable and clean energy source has attracted great attention with the increasing global energy crisis. However, sufficient production of hydrogen is seriously impeded by the adhesion of hydrogen bubble to electrodes. Efficient removal of hydrogen bubbles attached to the electrode can improve the efficiency of the hydrogen evolution reaction. Following this concept, numerous approaches to shorten the adhesion time of hydrogen bubbles on electrodes have been presented, such as ultrasonic treatment and electrode surface micro/nano‐modification. Almost all of the existing solutions are based on the instant and direct release of generated hydrogen bubbles into the electrolyte, which can be identified as “Releasing strategy” accordingly. In this contribution, an aerophilic electrode with cone shape is fabricated, from which the generated hydrogen bubbles can be timely removed through efficient and directional transportation (from tip to the base). Correspondingly, this approach is defined as “Transporting strategy”. Furthermore, integrating the base of electrode with a superaerophilic sponge, which possesses excellent properties of efficiently absorbing and releasing gas bubbles, can realize the collection of generated hydrogen. It is believed that the present approach can contribute to promising applications in water electrolysis and will offer inspiration for fabricating novel hydrogen collector.     

The effect of geometrical spectral broadening on the estimation ofmean blood velocity using wide and narrow ultrasound beams

If an ultrasound beam uniformly insonates the cross section of a blood vessel then the Doppler signal can be analyzed to give a frequency proportional to the spatial mean blood velocity. This is also possible if the beam can be thought of as negligibly thin compared to the blood vessel radius, centrally placed, and the blood velocity profile is axisymmetric and monotonic, although the analysis takes a different form. The immunity of these mean velocity estimators to broadening of the ideal frequency spectrum is studied. If the broadening of a frequency component is such that its mean frequency, weighted by intensity, is unchanged then the analysis with a uniformly insonating beam still leads to the correct mean velocity. In contrast, for any such broadening, the analysis if the beam is negligibly thin produces an underestimate of the mean velocity. Error expressions are derived for idealized cases and some practical results given     

An Interferometric Blood Flow Measurement Technique?A Brief Analysis

An ultrasonic interferometric technique of measuring blood flow with an extravascular probe is presented and analyzed. The difference in phase between pulses of ultrasonic energy transmitted simultaneously from a pair of barium titanate crystals diagonally through the vessel and received by the same pair of crystals is employed as the basic measurement of blood flow velocity. It is shown that the sensitivity is influenced mainly by variability in the internal diameter of the vessel and the zero-flow baseline voltage position is influenced principally by variations in the level of energy reflected from the vessel surfaces. Baseline stability is enhanced by the use of an asymmetrical probe which minimizes the variable reflection components. This measurement technique is currently being employed in long term telemetry implants in which flow measurements have been satisfactorily performed for periods in excess of three months duration.     

Accuracy of carotid strain estimates from ultrasonic wall tracking: a study based on multiphysics simulations and in vivo data

We used a multiphysics model to assess the accuracy of carotid strain estimates derived from a 1-D ultrasonic wall tracking algorithm. The presented tool integrates fluid-structure interaction (FSI) simulations with an ultrasound simulator (Field II), which allows comparison of the ultrasound (US) images with a ground truth. Field II represents tissue as random points on which US waves reflect and whose position can be updated based on the flow field and vessel wall deformation from FSI. We simulated the RF-signal of a patient-specific carotid bifurcation, including the blood pool as well as the vessel wall and surrounding tissue. Distension estimates were obtained from a wall tracking algorithm using tracking points at various depths within the wall, and further processed to assess radial and circumferential strain. The simulated data demonstrated that circumferential strain can be estimated with reasonable accuracy (especially for the common carotid artery and at the lumen-intima and media-adventitia interface), but the technique does not allow to reliably assess intra-arterial radial strain. These findings were supported by in vivo data of 10 healthy adults, showing similar circumferential and radial strain profiles throughout the arterial wall. We concluded that these deviations are present due to the complex 3-D vessel wall deformation, the presence of specular reflections and, to a lesser extent, the spatially varying beam profile, with the error depending on the phase in the cardiac cycle and the scanning location.