Larger Bubble Sizes Research Articles

Thin Liquid Film View and Shear Stress During the Sliding of Air Bubbles on Tilted Plates.

The challenge when studying the impact and sliding of free-rising air bubbles on tilted surfaces is an experimental limitation in obtaining the film thickness of thin liquid film (TLF) during the bubbles' sliding on tilted surfaces. In this work, spatiotemporal evolution in the film thickness of the moving TLF between a sliding air bubble and a tilted plate was monitored by using a two-wavelength synchronized reflection interferometry microscopy (SRIM) technique. The evolution of the film thickness was directly determined from a timed series of monochromatic interference fringes recorded simultaneously at two different wavelengths. From the film thickness profile, a shear stress map at a given time was determined at different bubble sizes and inclination angles. Results showed that the film thickness of TLFs during the bubbles' sliding on tilted surfaces was in the range of 300-1200 nm, depending on bubble size and tilting angles. Sliding of air bubbles on tilted plates over a thin gap with a few hundred nanometers thickness yielded shear stress in the order of 10-50 Pa. Both the larger bubble size and higher tilting angles yielded a higher shear stress. Experimental results were quantitatively compared to numerical results obtained using the Reynolds lubrication theory. A good match between the two results was achieved. Numerical results suggested that a maximum shear stress exerted on a tilted plate occurred at a 25° tilting angle. This is the first time that the spatiotemporal evolution of TLF during bubbles' sliding on tilted surfaces has been achieved, and the shear stress exerted on the tilted surface has been directly determined.

Evaluation of foaming performance for polymer modified and virgin asphalt binders

The pavement suffers rapid deterioration as a result of the weak bonding strength exhibited by the foamed virgin asphalt in the mixture. Consequently, it is crucial to develop a modified asphalt suitable for foaming. The expansion height and bubble size of foamed asphalt can be continuously and simultaneously monitored by binocular ranging equipment. By analyzing these dynamic responses, the evolution of the spatial domain expansion characteristics and the geometric features of the surface bubbles of foamed thermoplastic resin modified asphalt (TRA) was analyzed. Meanwhile, multi-stress creep recovery (MSCR) and bending beam rheometer (BBR) tests were conducted to analyze its rheological properties. Subsequently, the dispersibility of foamed TRA in the mixture was explored. The results showed that thermoplastic resin significantly improved the physical performance stability of foamed asphalt, as evidenced by an increase in its half-life by 2–3 times compared to the virgin asphalt when the dosage of thermoplastic resin reached 8 %. Foamed modified asphalt has fewer bubbles, but it has larger bubble sizes and more uniform distribution of bubbles. Meanwhile, the TRA exhibited excellent high-temperature deformation resistance and elastic recovery ability. The dispersion uniformity of foamed TRA in the mixture was superior to that of virgin asphalt. This study serves as a valuable reference for the exploration of foaming behavior in modified asphalt and the utilization of foamed modified asphalt in the mixture.

JWST observations of galaxy-damping wings during reionization interpreted with cosmological simulations

ABSTRACT Spectra of the highest redshift galaxies taken with JWST are now allowing us to see into the heart of the reionization epoch. Many of these observed galaxies exhibit strong damping wing absorption redward of their Lyman-$\alpha$ emission. These observations have been used to measure the redshift evolution of the neutral fraction of the intergalactic medium and sizes of ionized bubbles. However, these estimates have been made using a simple analytic model for the intergalactic damping wing. We explore the recent observations with models of inhomogeneous reionization from the Sherwood–Relics simulation suite. We carry out a comparison between the damping wings calculated from the simulations and from the analytic model. We find that although the agreement is good on the red side of the Lyman-$\alpha$ emission, there is a discrepancy on the blue side due to residual neutral hydrogen present in the simulations, which saturates the intergalactic absorption. For this reason, we find that it is difficult to reproduce the claimed observations of large bubble sizes at $z \sim 7$, which are driven by a detection of transmitted flux blueward of the Lyman-$\alpha$ emission. We suggest instead that the observations can be explained by a model with smaller ionized bubbles and larger intrinsic Lyman-$\alpha$ emission from the host galaxy.

One-Step Preparation of Magnetic Lipid Bubbles: Magnetothermal Effect Induces the Simultaneous Formation of Gas Nuclei and Self-Assembly of Phospholipids.

In recent years, enveloped micro-nanobubbles have garnered significant attention in research due to their commendable stability, biocompatibility, and other notable properties. Currently, the preparation methods of enveloped micro-nanobubbles have limitations such as complicated preparation process, large bubble size, wide distribution range, low yield, etc. There exists an urgent demand to devise a simple and efficient method for the preparation of enveloped micro-nanobubbles, ensuring both high concentration and a uniform particle size distribution. Magnetic lipid bubbles (MLBs) are a multifunctional type of enveloped micro-nanobubble combining magnetic nanoparticles with lipid-coated bubbles. In this study, MLBs are prepared simply and efficiently by a magneto internal heat bubble generation process based on the interfacial self-assembly of iron oxide nanoparticles induced by the thermogenic effect in an alternating magnetic field. The mean hydrodynamic diameter of the MLBs obtained was 384.9 ± 8.5 nm, with a polydispersity index (PDI) of 0.248 ± 0.021, a zeta potential of -30.5 ± 1.0 mV, and a concentration of (7.92 ± 0.46) × 109 bubbles/mL. Electron microscopy results show that the MLBs have a regular spherical stable core-shell structure. The superparamagnetic iron oxide nanoparticles (SPIONs) and phospholipid layers adsorbed around the spherical gas nuclei of the MLBs, leading the particles to demonstrate commendable superparamagnetic and magnetic properties. In addition, the effects of process parameters on the morphology of MLBs, including phospholipid concentration, phospholipid proportiona, current intensity, magnetothermal time, and SPION concentration, were investigated and discussed to achieve controlled preparation of MLBs. In vitro imaging results reveal that the higher the concentration of MLBs loaded with iron oxide nanoparticles, the better the in vitro ultrasound (US) imaging and magnetic resonance imaging (MRI) results. This study proves that the magneto internal heat bubble generation process is a simple and efficient technique for preparing MLBs with high concentration, regular structure, and commendable properties. These findings lay a robust foundation for the mass production and application of enveloped micro-nanobubbles, particularly in biomedical fields and other related domains.

Effect of foaming characteristics of air entrained agents on mechanical properties and pore structures of cement-based materials in plateau mountainous regions

Concrete freeze-thaw damage is common in high-altitude regions. Adding an appropriate amount of air-entraining agents (AEA) can improve the flowability of fresh concrete and the frost resistance of hardened concrete, without significantly affecting concrete strength. However, the existing studies of the influence and mechanism of AEAs on concrete mainly focus on conventional environmental tests and simulated conditions, with less consideration to the various environmental factors caused by real altitude changes. In this study, five types of AEAs are utilized according to bubble shaking tests in two different altitude environments: Chengdu, Sichuan (altitude: 300 m; air pressure: 96.7 kPa), and Aba Prefecture, Sichuan (altitude: 3700 m; air pressure: 66.7 kPa). Physical and mechanical properties as well as pore structure tests are also conducted on AEA paste and concrete. The results show that the relationship between air bubble sizes and surface tension of AEA solutions is the closest, with larger bubble sizes, smaller bubble volumes, and poorer foaming properties observed at high-altitude regions. Adding an appropriate amount of AEA can improve concrete workability. In high-altitude and low-temperature regions, AEA concrete exhibits better slump retention. The addition of triterpenoid saponin AEA increases the proportion of small pores (0.5 μm–200 μm) in paste and concrete, with a lower growth rate observed at high altitudes compared to lower altitudes. Additionally, the average pore diameter and bubble spacing coefficient of hardened paste decrease, optimizing the pore structure. Triterpenoid saponin AEA is recommended for improving concrete performance in high-altitude environments, with a recommended dosage of 0.01 %.

Study of Fluidization Behavior Transition from Geldart B to A Induced by High Temperature Using Electrical Capacitance Tomography

The transition of the fluidization behavior of Geldart B particles to that of A particles induced by temperature change was investigated by a developed high-temperature electrical capacitance tomography (ECT) sensor. Silica particles with a Sauter mean diameter of 237 μm and density of 2650 kg/m3, typically Geldart B particles under ambient conditions, were fluidized in a column of 5.5 cm from 20 to 600 °C. With the increase in temperature, ECT measurements showed a decrease in minimum bubbling velocity (Umb), no bed expansion characteristic in the homogeneous fluidization regime, an absence of multiple-bubbles regime, and a larger bubble size. The pressure drop against superficial gas velocity curves at elevated temperatures confirmed homogeneous fluidization between the minimum fluidization velocity (Umf) and Umb. Our analysis demonstrates that cohesive interparticle forces, which increase linearly with temperature, are responsible for the fluidization behavior transition of the silica particles.

Flattening of a hollow droplet impacting a solid surface

The interaction of a hollow droplet impacting a solid surface occurs in several applications, including controllable biomedicine and thermal spray coating. Understanding the physics of the hollow droplet spreading is the key to maintaining the mass transfer process in all relevant applications. In this work, a comprehensive experimental, numerical and theoretical study is performed on water hollow droplets impacting a rigid surface to better understand the flattening process of a hollow droplet. In the numerical part, compressible Navier–Stokes equations are solved using the volume of fluid (VOF) method in a two-dimensional (2-D)-axisymmetric model. The comparison of simulation results with the experimental photographs shows that the numerical solution can correctly predict the hollow droplet shape evolution. The results show that the spreading diameter and height of the counter-jet formed after the hollow droplet impact grow with impact velocity. Investigating the size and location of the entrapped bubble shows an optimum bubble size that facilitates the hollow droplet flattening. It is also shown that the ripples on splats produced by the hollow droplets with a larger bubble size are higher than those of small bubbles. In the end, a theoretical model is developed to analyse the maximum spreading diameter of the hollow droplet impact analytically. Its prediction is in good agreement with the experimental and numerical results.

A microfluidic study of bubble formation and coalescence tuned by dynamic adsorption of SDS and proteins

During foaming, bubbles are formed at (sub)millisecond time scales, and emulsifier adsorption determines bubble formation and stabilisation against coalescence. Here we introduce a microfluidic device, in which bubbles are formed in two distinct pressure regimes at low versus high pressures, and emulsifier adsorption can be inferred from the easily-monitored dynamics of bubble formation and coalescence, as well as the bubble properties. We studied dynamic adsorption for various types and concentrations of emulsifiers, namely sodium dodecyl sulfate (SDS; 0.05–3% wt.) and various proteins (5% wt.) including whey protein isolate (WPI), β-lactoglobulin (β-lac), and bovine serum albumin (BSA). The results show that as SDS concentration increases up to 3% wt., the bubble formation time decreases in the low-pressure regime due to the enhanced adsorption of SDS, yet increases in the high-pressure regime due to the promoted ‘dripping-jetting’ transition; in both pressure regimes the neck thinning process slows down, leading to longer bubble growth time and thus larger bubble size. To suppress coalescence of bubbles formed at time scales down to 10 μs, SDS is the most efficient, followed by BSA, WPI, and finally, β-lac; this is ascribed to the dynamics of emulsifier adsorption, e.g., the resulting dynamic surface tension. To conclude, our microfluidic study highlights the dynamic effects in the presence of SDS and proteins during bubble formation, even at high concentrations. This means that to explain the properties of a freshly-formed foam product, it is crucial to understand (and thereafter manipulate) emulsifier adsorption at time scales relevant to bubble formation and coalescence.

Experimental observations and modelling of sub-Hinze bubble production by turbulent bubble break-up

We present experiments on large air cavities spanning a wide range of sizes relative to the Hinze scale$d_{H}$, the scale at which turbulent stresses are balanced by surface tension, disintegrating in turbulence. For cavities with initial sizes$d_0$much larger than$d_{H}$(probing up to$d_0/d_{H} = 8.3$), the size distribution of bubbles smaller than$d_{H}$follows$N(d) \propto d^{-3/2}$, with$d$the bubble diameter. The capillary instability of ligaments involved in the deformation of the large bubbles is shown visually to be responsible for the creation of the small bubbles. Turning to dynamical, three-dimensional measurements of individual break-up events, we describe the break-up child size distribution and the number of child bubbles formed as a function of$d_0/d_{H}$. Then, to model the evolution of a population of bubbles produced by turbulent bubble break-up, we propose a population balance framework in which break-up involves two physical processes: an inertial deformation to the parent bubble that sets the size of large child bubbles, and a capillary instability that sets the size of small child bubbles. A Monte Carlo approach is used to construct the child size distribution, with simulated stochastic break-ups constrained by our experimental measurements and the understanding of the role of capillarity in small bubble production. This approach reproduces the experimental time evolution of the bubble size distribution during the disintegration of large air cavities in turbulence.

Air-in-liquid compound drop impact onto a pool

We investigate numerically the dynamics of a drop containing a bubble impacting onto a pool of the same liquid. We show that the bubble can be engulfed into the pool after impact only for a limited range of impact velocities and bubble sizes. Below a critical Weber number, the compound drop bounces from the surface. By contrast, above a second threshold in Weber number, the bubble bursts during impact. Depending on the bubble size, we identify two different mechanisms responsible for this higher impact velocity threshold, with central bursting at lower bubble sizes, or dimple bursting at larger bubble sizes. We then characterize and model the dynamics of the cavity, to finally provide an overview of the mechanisms affecting the bubble stability in the liquid.

Wash water addition on protein foam for removal of soluble impurities in foam fractionation process

This work evaluates the addition of wash water to a foam fractionation cell in order to remove the soluble impurities from a protein foam. Due to the irreversible adsorption of the proteins at the air interface, the addition of wash water to the foam can displace the entrained substances downward together with the liquid, while the adsorbed proteins are not desorbed from the foam surface. Here, we performed experiments on bovine serum albumin (BSA), as a model protein and NaCl salt, as a model of soluble impurities. The experiments were conducted in a glass foam fractionation cell, where the liquid level was kept constant. The wash water was added on the foam top with different flow rates and BSA and NaCl concentrations were measured at the outlets for further analysis. The influence of initial bubble size and the wash water rate on purification efficiency were investigated. The results show that wash water displaces the entrained liquid in foam and reduces the salt content of the foam. The process shows higher salt removal for higher wash water rates as well as for foams with larger bubble sizes, where up to 93 % of the salt was removed from the main solution. A lower air flow rate can further enhance the washing efficiency. However, it intensifies the foam collapse and hence diminishes the foam outflow.

Gas phase hydrodynamics in a surface‐aerated tank with a long‐short blades agitator

AbstractThis work aims to study the gas phase hydrodynamics in a stirred tank with a surface‐aerated long‐short blades agitator by the Eulerian–Eulerian approach coupled with population balance model. Predicted local gas holdup and bubble size distribution agree well with those measured by a conductivity probe technique. The predictions demonstrate that the pressure depression in the center is the main driving force for gas suction and the downward flow carries the bubbles down to redistribute in the whole tank. The gas phase has higher gas holdup with large bubble size in the upper part and lower gas holdup but with small bubble size in the lower part of the tank. The predicted liquid‐phase mass transfer coefficients agree well with our previous experimental results and just depend on the power consumption per unit volume when the aspect ratio of the liquid height to the tank diameter varies from 1.1 to 2.0.

Numerical and experimental investigations into the rheological behaviour of wet foam flowing under a fence

Experimental and numerical investigations were carried out to describe the flow of a wet foam under a fence placed in the middle of a horizontal duct with a square crosssection. The aim was to characterize the foam’s flow behaviour and model it using computational fluid dynamics in complex geometrical situations such as those encountered in industrial systems as for the cleaning of food equipment. As fences affect foam structure, organization and flow dynamics, these were characterized for three different flow rate conditions. Measurements using pressure sensors, particle image velocimetry and a conductimetry technique show a reorganization of the foam downstream, with a thicker liquid film at the bottom, larger bubble sizes at the top, as well as a larger void fraction increasing from the bottom to the top. The foam revealed a viscoelastic character as would be the case for a non-Newtonian fluid when passing through the fence. Herschel–Bulkley and/or Bingham laws relating to the presence of an underlying liquid film at the bottom of the channel were selected as being appropriate for this description. After adjusting these parameters from experimental data, computational fluid dynamics analysis was carried out. Results shown a close agreement with experiments using the Herschel–Bulkley model for two very different flow regimes.

One-equation turbulence models applied to practical scramjet inlet

Abstract Reynolds-averaged Navier–Stokes equations are used to simulate a practical scramjet inlet geometry using the shock-unsteadiness modified Spalart–Allmaras (SA) turbulence model. The geometry consists of fore-body ramps, expansion corners, and inlet ducts. The focus is to study the impingement of the cowl shock on the opposite wall boundary-layer. The resulting separation bubble can lead to blockage and inlet unstarts. The shock-unsteadiness correction is employed and is found to improve the computational fluid dynamics (CFD) prediction of flow separation in shock/boundary-layer interactions. The shock-unsteadiness parameter is calibrated against available experimental data of canonical flows, and the predicted flow-field is analyzed in detail. A large separation bubble size normalized to the upstream boundary-layer thickness of 4.6 is observed in the interaction region. Across the reattachment region in the interaction region, a peak value of wall pressure is observed. The inlet performance parameters are also calculated. The total pressure losses of 62% are observed across different shock waves, with an additional loss of 15% due to viscous boundary-layer effects.

One-equation turbulence models applied to practical scramjet inlet

Abstract Reynolds-averaged Navier–Stokes equations are used to simulate a practical scramjet inlet geometry using the shock-unsteadiness modified Spalart–Allmaras (SA) turbulence model. The geometry consists of fore-body ramps, expansion corners, and inlet ducts. The focus is to study the impingement of the cowl shock on the opposite wall boundary-layer. The resulting separation bubble can lead to blockage and inlet unstarts. The shock-unsteadiness correction is employed and is found to improve the computational fluid dynamics (CFD) prediction of flow separation in shock/boundary-layer interactions. The shock-unsteadiness parameter is calibrated against available experimental data of canonical flows, and the predicted flow-field is analyzed in detail. A large separation bubble size normalized to the upstream boundary-layer thickness of 4.6 is observed in the interaction region. Across the reattachment region in the interaction region, a peak value of wall pressure is observed. The inlet performance parameters are also calculated. The total pressure losses of 62% are observed across different shock waves, with an additional loss of 15% due to viscous boundary-layer effects.

Interfacial area transport for bubbly-to-slug transition flows in small diameter pipes

This study aims to experimentally investigate the two-group interfacial area transport in small diameter pipes. Experimental data focusing on the bubbly to slug transition regime, namely one-group to two-group transport region, are collected in a 12.7 mm vertical pipe under adiabatic, air-water conditions. The result shows the intergroup transfer in the small diameter pipe can be drastic, especially under low superficial liquid velocities. The cause of this phenomenon is mainly due to the large relative bubble size comparing to the pipe cross-sectional area. The wake entrainment effect could be enhanced by the small spherical bubbles that are acting like cap or slug bubbles in a medium-size pipe. Based on the experimental observation, a throughout analysis of the dependence of the drastic intergroup transfer is provided in this study. The models predicting the initiation of drastic intergroup transfer in small diameter pipes in terms of the bubble diameter and the void fraction are developed. These models are compared with the two-phase data among the different pipe sizes and the results show a good agreement. These newly developed models are applied to the IATE wake entrainment model by developing a transition function analogous to the sigmoid function. With the transition function, the revised IATE model is given the new ability on predicting the drastic intergroup transfer phenomenon.

Hydrodynamic study of bubble characteristics and bubble rise velocities in batch electrocoagulation with vibration-induced electrode plates using the PIV technique

In most of electrocoagulation processes, the efficiency in removing pollutant is not just dependent on anode oxidation but is also determined by the action of gas bubbles produced during the treatment. The bubbles formation and bubbles rise velocities are important especially in electrochemical treatment for which generated gas bubbles improved the ionic transfer and dispersion of particles. This study used the Particle Image Velocimetry (PIV) technique to investigate the hydrodynamics of bubbles in terms of size, distribution and rise velocity in batch electrocoagulation with vibration-induced electrode plates in comparison with stationary plates. An experimental set up consisting of an electrocoagulation reactor and electrode plates equipped with a high-speed camera recorder and a computer embedded with PIVlab software was employed to determine the bubble sizes and the rise velocities in varying conditions, such as current intensity, initial pH, and vibration intensity. The results showed that, for both stationary and vibration-induced plates, smaller bubble sizes were obtained in acidic conditions (pH 5) compared with alkaline conditions (pH 10). The larger bubble sizes were found resulted from the coalescence between smaller and larger bubbles before detachment from the plates. However, due to the vibration of the plates, the bubbles were able to completely detach from the plate surfaces starting at a vibration intensity of 1.8 V. With stationary electrode plates, the bubble rise velocities were found to be higher at initial pH 5 with velocities ranging from 4.7 to 15.3 cm/s, while the velocities measured at initial pH 10 ranged from only 2 to 4.7 cm/s. The flow trajectory of the stationary plates showed that the bubbles flowed only in a single direction parallel to the plate (y-direction). On the other hand, increasing the vibration intensity of the vibration-induced plates from 0.8 to 3.3 V during the treatment resulted in a decrease in the bubble rise velocities at both initial pH 5 and pH 10. The gas bubbles flow was governed by both the x- and y-direction of the electrode plates, with the flow in the x-direction observable at a vibration intensity of 2.8 V or 80 Hz, which is the resonant frequency of the vibrating plates.

Effects of polycarboxylate superplasticisers with various functional groups on the pore structure of cement mortar

The air-entraining capabilities of polycarboxylate superplasticisers (PCEs) with different functional groups were systematically analysed. Using different molecular PCEs, the evolution process from bubbles in fresh cement mortar to pores in hardened cement mortar was investigated, considering their effects on mortar workability and mechanical performance. The results showed that PCEs with various functional groups have different air-entraining capabilities. PCEs with ester groups decreased the surface tension and significantly promoted the air-entraining capability, resulting in a notable increase of air content in fresh cement mortar. PCEs with sulfonic acid groups increased the surface tension and remarkably decreased the air content in the fresh cement mortar. A higher air-entraining capability benefits the dispersing properties of PCEs and therefore the PCEs with ester groups performed better in dispersibility in comparison with the PCEs with sulfonic acid groups. The stability of bubbles entrained in fresh cement mortar leads to differences in the pore structure of hardened cement mortar. The PCEs with ester groups diminished the size of large bubbles, leading to significantly decreased porosity of the hardened mortar. PCEs with sulfonic acid groups in the backbone barely optimised the pore structure, with a large amount of large pores remaining in the hardened mortar. Decreased porosity and pore size hence led to higher compressive strength of the hardened cement mortar.

Detecting circular polarisation in the stochastic gravitational-wave background from a first-order cosmological phase transition

We discuss the observability of circular polarisation of the stochastic gravitational-wave background (SGWB) generated by helical turbulence following a first-order cosmological phase transition, using a model that incorporates the effects of both direct and inverse energy cascades. We explore the strength of the gravitational-wave signal and the dependence of its polarisation on the helicity fraction, ζ*, the strength of the transition, α, the bubble size, R*, and the temperature, T*, at which the transition finishes. We calculate the prospective signal-to-noise ratios of the SGWB strength and polarisation signals in the LISA experiment, exploring the parameter space in a way that is minimally sensitive to the underlying particle physics model. We find that discovery of SGWB polarisation is generally more challenging than measuring the total SGWB signal, but would be possible for appropriately strong transitions with large bubble sizes and a substantial polarisation fraction.

Formation and detachment of the enclosing water film as a bubble passes through the water-oil interface

The present study aims to reveal characteristics of a bubble as it passes through the interface between two immiscible liquids, namely deionized water and mineral oil. Air was injected into the deionized water to produce bubbles. Initial bubble sizes were varied through using three nozzles with the outlet diameters of 2.0, 3.0 and 4.0 mm respectively. Instantaneous bubble behaviors as the bubble penetrates the water-oil interface were observed using the technique of high-speed photography. A computer code was developed to process the captured images and to extract the bubble shape and trajectory. A comprehensive comparison was implemented among bubbles with different sizes. The results indicate the bubble evolution in the mineral oil is considerably stabilized relative to that in the deionized water. In the mineral oil, the bubble size is small and the bubble shape is regular. As a bubble passes through the water-oil interface, a water film coating the bubble is gradually formed. Larger bubbles entrain more water as it migrates from the deionized water to the mineral oil. With the drainage and detachment of the water film from the bubble, a waterjet is identified inside the bubble, as is observed by all three nozzles. The intensity of the waterjet varies with the bubble size. Large bubble size is responsible for an early detachment of the water film and the lateral disturbance to the bubble motion is suppressed. For the small bubble, it undergoes relatively high resistance of the water-oil interface and a long time span is covered before it passes though the water-oil interface.