Adjacent Bubbles Research Articles

Effects of nucleation site arrangement and spacing on bubble coalescence characteristics

The effects of the nucleation site spacing on the bubble coalescence characteristics were experimentally studied with a microheater array. Five nucleation sites with different separation distances were arranged in a two-site pattern and a triangular three-site pattern. Each nucleation site was approximately 300μm×300μm. The dimensionless spacings were 0.49, 0.80, 0.98 and 0.94. The nucleation sites were all kept at 100°C by the control circuits. Bubble images were captured while the heat fluxes from each heater were simultaneously measured. The results show that different nucleation site arrangements and spacings greatly affect the bubble coalescence type, bubble departure diameter, departure frequency and heat flux distributions on the surface. Nucleation sites that are too close result in single bubbles without coalescence, while large separation distances result in horizontal coalescence between adjacent bubbles. Moderate spacing results in sequential coalescence with higher bubble departure frequencies. The bubble coalescence frequency is much higher with the triangular three-site pattern than with the two-site arrangement, with the bubble departure frequency and then the average heat flux greatly enhanced by the higher coalescence frequency. However, the departure diameters are slightly smaller with the triangular pattern due to the shorter growth times.

Wall effect on rising foam drainage and its application to foam separation

A novel and structurally simple foam separation column with the cross internal was developed for investigating the effect of the column wall on the drainage of rising foam, using sodium dodecyl sulfate (SDS) as a foam agent. The cross internal enhances significantly wall effect due to an increase of wall contact area in the foam phase. Furthermore, the performances of foam separation column with the cross internal were assessed using crystal violet (CV) as the model system and SDS as the collector. The insertion of the cross internal significantly decreased the liquid holdup of the foam out of the column and increased the bubble size. The results showed that an increase in wall contact area in the foam phase could enhance the drainage of rising foam due to the higher drainage velocity between the bubbles and the column wall than that between the adjacent bubbles. Under the suitable operations conditions, the enrichment ratio of CV was 16.5 by using the column with the cross internal and it was nearly four times higher than the column without the cross internal. The better performances indicated that an increase of wall contact area in the foam phase is a simple and effective method to enhance foam drainage and improve enrichment of foam separation.

Effect of electrolytes on interphase mass transfer in microbubble-sparged airlift reactors

Better understanding of the impact that electrolytes have on the hydrodynamic performance of airlift reactors, ALR, is critical to improving the design and operational performance of these units which are increasingly being used as chemical and biochemical reactors. Such knowledge is particularly important to the saline environment encountered in water and wastewater treatment, algae production, treatment of produced water, aquaculture applications, and the management of marine environment.The effect of salt concentration on the mass transfer characteristics of ALR was investigated using a 200l pilot scale internally circulated unit equipped with a dual-fluid transonic sparger capable of generating microbubbles. Saline solutions containing 0–58,500ppm were tested.A critical analysis of the literature suggests that the liquid side mass transfer coefficient, kL, is significantly reduced in the presence of electrolytes. This adverse effect was however compensated for by the achievement of large gas holdups and the formation of very fine bubbles in the slowly-coalescent saline systems. The combined effect of these factors resulted in the achievement of kLa values as high as 0.028s−1 at a superficial velocity of 0.03m/s while maintaining the benefits of the homogeneous flow regime. The values achieved are much higher than most ALR data reported for the rapidly-coalescent air/water system and are about 5-fold larger than those obtained using conventional spargers in the case of the slowly-coalescent saline solutions. This superior performance is amplified by the coalescence retardation effect of electrolytes; hence, the mass transfer performance data depicted a transition/critical concentration at about 0.1M, a value that is slightly lower than that obtained in bubble columns but somewhat higher than that obtained using the lab-scale method for testing the coalescence of two adjacent bubbles

Bubble circling phenomena in subcooled nucleate pool boiling on microwires

The interaction between adjacent bubbles plays an important role in boiling heat transfer on a micro heat source. In this paper, the visualizing bubble behaviors during subcooled pool boiling on microwires were revisited experimentally. Bubble circling phenomena, the circling of small bubble around a larger one for subcooled boiling on platinum microwires submerged in ultrapure water or self-rewetting fluid, were investigated. Different types of bubble circling phenomena, including small bubble sweeping, departing, rotating, returning or chasing at a larger bubble interface, were described in detail. The roles of Marangoni effect and viscous force were analyzed in the process of bubble circling to understand the mechanism of bubble behaviors on micro heated surface. The investigation on the bubble circling phenomenon can help verify the crucial factors that influence the bubble dynamics, bubble interaction and boiling heat transfer. The bubble dynamics in this phenomenon also makes it an interesting choice for many microscale applications.

Large‐scale quasiperiodic plasma bubbles: C/NOFS observations and causal mechanism

Large‐scale periodic plasma bubbles are often observed by ionospheric radars and satellites. The seeding effect of atmospheric gravity waves has been widely used to explain the generation of periodic plasma bubbles. However, it has not been well understood where the seeding process occurs and how a series of plasma bubbles is triggered. In this study, we present the observations of equatorial plasma bubbles by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. We show examples of quasiperiodic plasma bubbles in the post‐midnight sector, with nearly equal distance of 800–1000 km between adjacent bubbles, in 2008 under deep solar minimum conditions. The bubble chain covered a longitudinal range of ~7000 km between 00:00 and 04:00 LT. Quasiperiodic plasma bubbles were also measured by C/NOFS in the evening sector in 2011 during the ascending phase of the solar activity, and the longitudinal distance between adjacent bubbles was ~500 km. We propose a causal mechanism to explain the generation of quasiperiodic plasma bubbles. In this scenario, atmospheric gravity waves are generated near the sunset terminator and initiate the Rayleigh‐Taylor instability there. The spatial (longitudinal) periodicity of plasma bubbles is determined by the temporal periodicity of the seeding gravity waves. A period of 15–30 min of the seeding gravity waves corresponds to a longitudinal separation of 500–1000 km between adjacent bubbles. This mechanism provides a reasonable explanation of the observed quasiperiodic plasma bubbles.

Molecular-Level Modeling of Interfacial Phenomena in Boiling Processes

The thermophysics of liquid–vapor interfaces has long been recognized as a critical element in the physical mechanisms of boiling processes. This article will describe results of recent molecular dynamics simulation studies that explore the structure and stability of liquid–vapor interfacial regions using a hybrid analysis scheme that combines new formulations of capillarity theory with molecular dynamics simulations that use similar interaction potentials. Two forms of this type of hybrid scheme have been developed: one for non-polar fluids based on a Lennard–Jones interaction potential and a second specifically for water using a modified treatment of the extended simple point charge interaction potential that accounts for water dipole interactions. The hybrid model has the advantage that the capillarity theory provides theoretical relationships among parameters that govern interfacial region structure and thermophysical behavior, while the companion molecular dynamics simulations allow more detailed molecular level exploration of the interfacial region thermophysics. Predictions of interfacial region structure indicated by this kind of hybrid modeling will be described for pure non-polar and water liquid–vapor interfaces and for water with dissolved ionic solutes (i.e., salts). Extension of the methodology to thin liquid films will also be described. Rupture of a free liquid film dictates merging of adjacent bubbles, which is particularly important in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. To understand the mechanisms of bubble merging in nanostructured boiling surfaces and in nanotubes, it is useful to explore film stability and the onset of rupture at the molecular level. Results obtained with the hybrid model indicate that wave instability predominates as an onset of rupture mechanism for free liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (e.g., in nanostructured boiling surfaces), lack of film core stability is more likely to be the mechanism. Predictions of the hybrid models will be compared to results of experimental studies of the effects of ionic solutes on interfacial tension and bubble merging. The implications of the molecular dynamics model predictions for boiling processes in micro-channels and boiling in nanostructured surfaces are also discussed.

Monolayer microbubbles fabricated by microfluidic device for keratocytes observation

In order to construct 3D culture system for tracking cell migration, microfluidic techniques were applied to generate micro bubbles composed of gelatin, and then the bubbles were fabricated into monolayer in the micro-channel which is cell-traceable under phase microscope. The keratocytes were harvested through primary culture from Hpsophrys nicaraguensis (a fish). Live cell imaging was used to record the migration trajectory and the morphology changes. Moreover, galvanotaxis, the tendency of cells directed by an electrical field was studied. Without the electrical field, random pattern was observed in the 2D micro-channel. When the electrical field was applied, the keratocytes in 3D gelatin bubbles migrated along the rim of bubbles and went across the interconnecting pore between the adjacent bubbles, causing the tortuous trajectory of migration patterns. However, in the 2D micro-channel, the keratocytes' migration paths are closer to straight lines. The speed and velocity of keratocytes for both 2D and 3D environment were also compared under the condition with electric field. We believe that the understanding of the keratocytes' migration behavior in 2D and 3D condition under electric field will provide valuable information in succeeding the model system of wound repairs and electrophysiological environment in vivo.

Local Nusselt number enhancement during gas–liquid Taylor bubble flow in a square mini-channel: An experimental study

Taylor bubble flow takes place when two immiscible fluids (liquid–liquid or gas–liquid) flow inside a tube of capillary dimensions within specific range of volume flow ratios. In the slug flows where gas and liquid are two different phases, liquid slugs are separated by elongated Taylor bubbles. This singular flow pattern is observed in many engineering mini-/micro-scale devices like pulsating heat pipes, gas–liquid–solid monolithic reactors, micro-two-phase heat exchangers, digital micro-fluidics, micro-scale mass transfer process, fuel cells, etc. The unique and complex flow characteristics require understanding on local, as well as global, spatio-temporal scales. In the present work, the axial streamwise profile of the fluid and wall temperature for air–water (i) isolated single Taylor bubble and, (ii) a train of Taylor bubbles, in a horizontal square channel of size 3.3 mm × 3.3 mm × 350 mm, heated from the bottom (heated length = 175 mm), with the other three sides kept insulated, are reported at different gas volume flow ratios. The primary aim is to study the enhancement of heat transfer due to the Taylor bubble train flow, in comparison with thermally developing single-phase flows. Intrusion of a bubble in the liquid flow drastically changes the local temperature profiles. The axial distribution of time-averaged local Nusselt number (Nu¯z) shows that Taylor bubble train regime increases the transport of heat up to 1.2–1.6 times more as compared with laminar single-phase liquid flow. In addition, for a given liquid flow Reynolds number, the heat transfer enhancement is a function of the geometrical parameters of the unit cell, i.e., the length of adjacent gas bubble and water plug.

Interaction of two oscillating bubbles near a rigid boundary

Oscillating bubbles or non-equilibrium bubbles are known to induce the development of liquid jets in the presence of a nearby rigid boundary. The liquid jets can be useful in many applications such as surface cleaning and drug delivery into biological cells. Single bubble oscillation near a rigid boundary has been studied in detail in the literature. In this paper, we extend the research in this area to a two-bubble system near a rigid boundary using high speed photography. Various measurements on the direction of the (bubble-collapse induced) water jets were performed. We identified the parameter sets where the influence of the adjacent bubble and the rigid boundary is important. The experimental results are summarized in a graph that enables easy prediction of the direction of the water jets induced by bubble collapse near another bubble and a rigid boundary.

Bubble jet flow interaction during pool boiling of subcooled water on horizontal thin wires

Both visual experiments and numerical analyses were conducted to investigate the interaction between bubble jet flows during pool boiling of subcooled water on horizontal thin wires. The bubble jet flows nearby attracted each other, and they can combine into one jet flow under strong interaction. As the adjacent bubble departs, the bubble jet flow would experience an unsteady evolution process with the jet flow interaction weakening. Since the unsymmetrical thermocapillary force at the bubble interface was induced by the adjacent bubble as a cold source, the bubble jet flow would trend to the adjacent bubble, and the mechanism based on thermocapillary force and cold source can explain the bubble jet flow interaction very well. The steady bubble jet flow interaction phenomena were further simulated by laminar model, and the calculated jet flow interaction phenomena were in a good agreement with the experimental results.

Study of a new multi channel foam and emulsion generator

ABSTRACTThis work describes the design and operation of two microfluidic emulsion and foam generators. The difference between the two devices consists in the geometry of the output channel - in one case, we used a straight microchannel, whereas in the second case we implemented a fractal channel output geometry. While both devices can produce highly monodisperse foams and emulsions at high throughput (coefficient of variation CV≈1.5% and production frequency of up to 2.5kHz per channel with a total of 256 micro channels capable, in theory, of operating in parallel), the actual CV at the exit of the device was significantly higher due to interactions between adjacent droplets or bubbles. Using a combination of high-viscosity continuous phase liquid and low-solubility gas phase resulted in improved monodispersity. In these conditions, the fractal output channel geometry resulted in the lowest overall CV (≈3%) at the exit of the device for both foam and emulsion production, due to an inhibition of aging and coalescence phenomena as compared to the straight channel.

Thermophysics of the Interfacial Region and Its Impact on Instability and Rupture of Thin Free Liquid Films

Merging of adjacent bubbles is a particularly important process in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. The merging process is ultimately associated with the rupture of the liquid film separating two bubbles, and the stability of the liquid film dictates whether coalescence is likely to occur. This paper examines two methods of exploring how the molecular-level features of the thin film and its interfaces affect the stability of the film and its subsequent impact on bubble merging. One method is an extended molecular capillarity model that has been developed for a free liquid film bounded by two liquid–vapor interfacial regions. This model predicts that as film thickness diminishes, a point is reached at which the mean centerline density in the film is between the spinodal limits for the fluid, implying that the film core lacks intrinsic stability. This suggests that loss of intrinsic stability may be the mechanism for film rupture in some cases. The second method examined here is use of molecular dynamics simulations to examine film structure and stability. Simulations using a Lennard–Jones interaction potential for non-polar fluids and the SPC/E potential for water and salt water are examined. Results of these explorations indicate that for water–NaCl solution films, surface tension increases with increasing NaCl concentration. These predictions are shown to be consistent with measured surface-tension data for such solutions. The results also indicate that for pure water and NaCl solutions, below a threshold thickness, the surface tension decreases with film thickness. Wave interface stability theory implies that this tends to make the film less stable. The implications of the results of these investigations for bubble merging in two-phase flow and boiling processes are also examined.

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm3) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.

On the effects of thermocapillary driven oscillations on bubble growth during boiling of FC-72 on a thin wire

Despite being a familiar and well-studied topic, the physical nature of boiling is still not well understood. There is a complex coupling of mass, momentum and energy transport that occurs between the solid surface, the wetting liquid and the vapour produced to generate bubbles. Our work investigated pool boiling of FC-72, a highly-wetting dielectric liquid, on a submerged electrically-heated platinum wire. The bulk fluid temperatures ranged from ambient to saturation allowing a range of subcooled and saturated boiling scenarios to be investigated. The boiling chamber could be pressurised and boiling of FC-72 was studied for pressures from 1–3 bar. High-speed digital video was used to record boiling phenomena and these were subsequently analysed. Thermocapillary convection was clearly visible and more noticeable as the degree of subcooling increased. Once vapour bubbles were initialised at a nucleation site, their growth and detachment frequency were recorded. Frequency was correlated with degree of subcooling and a power-law dependence established. After detachment, not all bubbles immediately rose to the surface. There was bubble slippage along the wire, driven by the distortions in the local temperature field. Slippage was also influenced by the presence of adjacent nucleation sites and bubbles (which themselves also influenced the local temperature field). The observed thermocapillary “tails” of the bubbles were affected by bubble motion. The slip velocity was plotted for the case of pure translational motion and also for periodic motion between nucleation sites. Oscillating bubbles are found to grow faster than stationary bubbles. This is attributed to the contribution of the superheated layer, which was measured and discussed. A form of the law for the growth of oscillating bubbles was proposed. Horizontal coalescence of two bubbles drawn towards each other is also presented and analysed.

Cold fronts and multi-temperature structures in the core of Abell 2052

The physics of the coolest phases in the hot Intra-Cluster Medium (ICM) of clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the central Active Galactic Nucleus (AGN) contain enough energy to heat the surrounding gas and stop cooling, but locally blobs or filaments of gas appear to be able to cool to low temperatures of 10^4 K. In X-rays, however, gas with temperatures lower than 0.5 keV is not observed. Using a deep XMM-Newton observation of the cluster of galaxies Abell 2052, we derive 2D maps of the temperature, entropy, and iron abundance in the core region. About 130 kpc South-West of the central galaxy, we discover a discontinuity in the surface brightness of the hot gas which is consistent with a cold front. Interestingly, the iron abundance jumps from ~0.75 to ~0.5 across the front. In a smaller region to the North-West of the central galaxy we find a relatively high contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below that temperature. However, the region appears to be associated with much cooler H-alpha filaments in the optical waveband. The elliptical shape of the cold front in the SW of the cluster suggests that the front is caused by sloshing of the hot gas in the clusters gravitational potential. This effect is probably an important mechanism to transport metals from the core region to the outer parts of the cluster. The smooth temperature profile across the sharp jump in the metalicity indicates the presence of heat conduction and the lack of mixing across the discontinuity. The cool blob of gas NW of the central galaxy was probably pushed away from the core and squeezed by the adjacent bubble, where it can cool efficiently and relatively undisturbed by the AGN. Shock induced mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively and explain our non-detection of gas below 0.5 keV.

Foam mechanics: spontaneous rupture of thinning liquid films with Plateau borders

Spontaneous film rupture from van der Waals instability is investigated in two dimensions. The focus is on pure liquids with clean interfaces. This case is applicable to metallic foams for which surfactants are not available. There are important implications in aqueous foams as well, but the main differences are noted. A thin liquid film between adjacent bubbles in a foam has finite length, curved boundaries (Plateau borders) and a drainage flow from capillary suction that causes it to thin. A full linear stability analysis of this thinning film shows that rupture occurs once the film has thinned to ‘tens’ of nanometres, whereas for a quiescent film with a constant and uniform thickness, rupture occurs when the thickness is ‘hundreds’ of nanometres. Plateau borders and flow are both found to contribute to the stabilization. The drainage flow leads to several distinct qualitative features as well. In particular, unstable disturbances are advected by the flow to the edges of the thin film. As a result, the edges of the film close to the Plateau borders appear more susceptible to rupture than the centre of the film.

Hybrid modeling of interfacial region thermophysics and intrinsic stability of thin free liquid films

The film rupture process that dictates merging of adjacent bubbles is particularly important in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. To understand the mechanisms of bubble merging in nanostructured boiling surfaces and in nanotubes, it is useful to explore film stability and onset of rupture at the molecular level. This paper reports the results of such an investigation using a hybrid analysis scheme that combines a new formulation of capillarity theory for free liquid films with molecular dynamics (MD) simulations that use similar interaction potentials. Two forms of our molecular film capillarity theory are developed here: one for non-polar fluids based on a Lennard-Jones interaction potential, and a second specifically for water using a modified treatment of the SPC/E interaction potential that accounts for water dipole interactions. The hybrid model has the advantage that the capillarity theory provides theoretical relationships among parameters that govern film structure and thermophysical behavior, while the companion MD simulations allow more detailed molecular level exploration of the film thermophysics. Results obtained with the hybrid model indicate that wave instability predominates as an onset of rupture mechanism for liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (in, for example, nanostructured boiling surfaces), lack of core stability is more likely to be the mechanism. The implications of these predictions for film rupture and bubble merging in nanostructured surfaces and nanotubes are examined in detail.

Sonoporation by Single-Shot Pulsed Ultrasound with Microbubbles Adjacent to Cells

In this article, membrane perforation of endothelial cells with attached microbubbles caused by exposure to single-shot short pulsed ultrasound is described, and the mechanisms of membrane damage and repair are discussed. Real-time optical observations of cell-bubble interaction during sonoporation and successive scanning electron microscope observations of the membrane damage with knowledge of bubble locations revealed production of micron-sized membrane perforations at the bubble locations. High-speed observations of the microbubbles visualized production of liquid microjets during nonuniform contraction of bubbles, indicating that the jets are responsible for cell membrane damage. The resealing process of sonoporated cells visualized using fluorescence microscopy suggested that Ca 2+-independent and Ca 2+-triggered resealing mechanisms were involved in the rapid resealing process. In an experimental condition in which almost all cells have one adjacent bubble, 25.4% of the cells were damaged by exposure to single-shot pulsed ultrasound, and 15.9% (∼60% of the damaged cells) were resealed within 5 s. These results demonstrate that single-shot pulsed ultrasound is sufficient to achieve sonoporation when microbubbles are attached to cells.

Effect of inhomogeneities on streamer propagation: II. Streamer dynamics in high pressure humid air with bubbles

The branching of electric discharge streamers in atmospheric pressure air, dense gases and liquids is a common occurrence whose origins are likely found with many causes, both deterministic and stochastic. One mechanism for streamer branching may be inhomogeneities in the path of a streamer which either divert the streamer (typically a region of lower ionization) or produce a new branch (a region of higher ionization). The propagation and branching of streamers in liquids is likely aided by low density inhomogeneities, bubbles; however, modeling of streamers in liquids is made difficult by the lack of transport coefficients. As a first step towards understanding the propagation and branching of streamers in liquids, we investigated the consequences of random inhomogeneities in the form of low pressure bubbles on the propagation of streamers in high pressure humid air. By virtue of their lower density, bubbles have larger E/N (electric field/gas number density) than the ambient gas with larger rates of ionization. The intersection of a streamer with a bubble will focus the plasma into the bubble by virtue of that higher rate of ionization but the details of the interaction depend on the relative sizes of the bubble and streamer. When a streamer intersects a field of bubbles, the large E/N in the bubble avalanches seed electrons produced by photoionization from the streamer. Each bubble then launches both a negative and positive going streamer that may link with those from adjacent bubbles or the original streamer. The total process then appears as streamer branching.

Study on pool boiling heat transfer in microgravity

摘要: 对利用中国返回式卫星搭载开展的两次微重力池沸腾空间实验及地基常重力和落塔短时微重力实验的结果进行了评述. 研究发现微重力时丝状加热器沸腾传热会略有强化, 而平板加热器则在高热流条件下明显恶化. 微重力时, 气泡脱落前存在沿加热面的横向运动, 加剧了相邻气泡间的合并, 合并气泡会在其表面振荡作用下从加热面脱落. Marangoni 效应对于微重力气泡行为有重要影响. 关键词: 微重力 / 池沸腾 / 传热 / 气泡行为 / 空间实验