Trapped Gas Bubbles Research Articles

Trapping of Bubbles in Oil Sands Tailing Ponds

Oil sands tailings ponds are significant emitters of greenhouse gases (GHGs) in Canada. To move beyond making surface or atmospheric measurements of GHG release, it is necessary to understand the physical mechanisms by which gas is generated, bubbles form and then are either released or remain trapped in the pond. We present a review of the physical description of tailings ponds, relevant to gas release models. In particular, we target rheological variations within a pond and how these directly affect the distribution of trapped gas bubbles with depth. Within the limits of the available data, we show how gas content may vary significantly across ponds, and develop data-driven one-dimensional models of gas distribution and rheology.

Impact of gas bubbles on bacterial adhesion on super-hydrophobic aluminum surfaces

Super-hydrophobic surface (SHS), which traps micro/nano-scale gas bubbles on solid walls, has been reported to greatly reduce bacterial adhesion and biofouling. However, it is unclear whether and how the trapped gas bubbles reduce the bacterial adhesion. Here, we examine the role of the trapped gas on the bacterial adhesion by measuring the spatial distributions of attached bacteria on SHS using scanning electron microscopy (SEM). Two SHSs, one with regular micro-grooves, and the other with randomly roughed textures at micro/nano-scales, both created on aluminum substrates, were tested. We found that for a SHS where gas bubbles are trapped between roughness elements, bacteria only attach to the surface areas that are not covered by gas (e.g., top of roughness elements), while the areas covered by gas bubbles - gaps between roughness elements - are free of bacteria. In contrast, for a rough surface with the same texture geometry as that of SHS but no hydrophobic chemistry, bacteria are distributed evenly to the top and gap of roughness elements. Our results suggest that the SHS may reduce biofouling by providing a physical barrier of gas-liquid interface which separates bacteria from the solid walls. Furthermore, we showed that nano-scale roughness on SHS is insufficient to prevent bacterial adhesion. Our results should provide useful guidance for the future engineering design of efficient anti-biofouling materials.

Flows in fluid-walled conduits driven by Laplace pressure

In conventional microfluidic devices, fluids are often confined behind solid plastic walls that restrict access and trap gas bubbles; in open microfluidics some solid walls are replaced by fluid ones (i.e. interfaces with immiscible fluids). In both cases, flows are usually driven by external pumps or gravity. An innovative open technology has been developed in which two-dimensional patterns of cell-culture medium in standard Petri dishes are confined by fluid walls made of an immiscible and bio-inert fluorocarbon (FC40). To provide refreshing media flows to cells in such circuits, an established pumping system that exploits differences in Laplace pressure across open interfaces has been applied to drive flow without using external pumps: a source drop autonomously empties through a straight conduit into the rest of the dish (the sink). Whereas conduits with solid walls have unchanging boundaries and flows within them are well understood, the challenge is to predict flows in circuits where fluid walls morph as pressures change. Numerical and semi-analytical equations enabling the prediction of changing flows are developed, and predictions validated experimentally.

Atomization of metal and alloy powders: Processes, parameters, and properties

AbstractAdditive manufacturing (AM) has been intensively studied over this decade due to the increasing demand of functional and precise materials in various industries. AM process leverages on the concept of layer by layer deposition or joining of a material at micro‐ or nano‐scale to produce the final products. As such, AM technology is widely used to manufacture products of complex geometries with high precision in a short time. As the quality and the performance of AM manufactured functional products depend on the quality and the cost of the atomized metal or alloy powder used, the use of powders of high size uniformity, high purity, high sphericity, high flowability, low cost, and no trapped gas bubble porosity is necessary. To this end, various metal production technologies were investigated and one of the popular technologies is atomization. This article reviewed the working principles of the various atomization technologies for the production of fine metal and alloy powders. The key atomization conditions discussed in this article include atomization type, material feed rate and temperature, atomizing fluid flow rate, type and temperature, cooling rate, and atomization pressure. In addition, several outlooks on the prospect and the optimization of atomization technologies were also discussed in this article.

Retention of PFOS and PFOA Mixtures by Trapped Gas Bubbles in Porous Media.

The transport of per- and polyfluoroalkyl substances (PFAS) in soil and groundwater is important for site investigation, risk characterization, and remediation planning. The adsorption of PFAS at air-water interfaces has been shown to significantly contribute to PFAS retention, with subsequent effects on concentrations and the time scales of transport. In this study, column experiments were conducted to investigate the transport of perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and 6:2 fluorotelomer sulfonate (6:2 FTS) individually and in binary mixtures in the presence of a trapped gas phase, using clean sands to isolate adsorption to air-water interfaces. Consistent with previous studies, the transport of PFOS, PFOA, and 6:2 FTS was retarded by adsorption at the air-water interface, with greater retention of PFOS due to its higher affinity for the air-water interface. Chromatographic separation occurred in the experiments using binary mixtures of PFOS and PFOA, with greater retention at lower influent concentrations. The mixture experiments also showed enhanced breakthrough of PFOA in the presence of PFOS, where effluent concentrations of PFOA were temporarily greater than the influent concentration prior to the breakthrough of PFOS. This enhanced breakthrough was attributed to competition between PFOS and PFOA for adsorption to the air-water interface.

A level set approach to Ostwald ripening of trapped gas bubbles in porous media

Ostwald ripening of gas bubbles is a thermodynamic process for mass transfer, which is important for both foam enhanced oil recovery and geological CO2 storage. We present a methodology for simulating Ostwald ripening of gas ganglia surrounded by liquid in arbitrary pore geometries. The method couples a conservative level set model for capillary-controlled displacement and a ghost-bubble technique that calculates mass transfer based on difference in chemical potentials. The methodology is implemented in a software framework for parallel computations. As a validation of the model, we show that simulations of bubble ripening in a pore throat connecting two pore bodies are consistent with previously reported trends in similar geometries. Then we investigate the impact of gas type, compressibility factor, and local capillary pressure on gas-bubble ripening in various water-wet pore geometries. The results confirm that gas solubility and compressibility factor are proportional to the rate of mass transfer. Our simulations suggest that Ostwald ripening has largest impact in heterogeneous or fractured porous structures where differences in gas-bubble potentials are high. However, if the liquid separating the gas bubbles is also a disconnected phase, which can happen in intermediate-wet porous media, the resulting local capillary pressure can limit the coarsening and stabilise smaller bubbles. Finally, we simulated Ostwald ripening on a 3-D pore-space image of sandstone containing a residual gas/water configuration after imbibition. Characterization of gas-bubble morphology during the coarsening shows that large ganglia get more ramified at the expense of small spherical ganglia that cease to exist.

Synthetic Strategies to Enhance the Electrocatalytic Properties of Branched Metal Nanoparticles.

Branched metal nanoparticles have unique catalytic properties because of their high surface area with multiple branches arranged in an open 3D structure that can interact with reacting species and tailorable branch surfaces that can maximize the exposure of desired catalytically active crystal facets. These exceptional properties have led to the exploration of the roles of branch structural features ranging from the number and dimensions of branches at the larger scales to the atomic-scale arrangement of atoms on precise crystal facets. The fundamental significance of how larger-scale branch structural features and atomic-scale surface faceting influence and control the catalytic properties has been at the forefront of the design of branched nanoparticles for catalysis. Current synthetic advances have enabled the formation of branched nanoparticles with an unprecedented degree of control over structural features down to the atomic scale, which have unlocked opportunities to make improved nanoparticle catalysts. These catalysts have high surface areas with controlled size and surface facets for achieving exceedingly high activity and stability. The synthetic advancement has recently led to the use of branched nanoparticles as ideal substrates that can be decorated with a second active metal in the form of islands and single atoms. These decorated branched nanoparticles have new and highly effective catalytic active sites where both branch metal and decorating metal play essential roles during catalysis.In the opening half of this Account, we critically assess the important structural features of branched nanoparticles that control catalytic properties. We first discuss the role of branch dimensions and the number of branches that can improve the surface area but can also trap gas bubbles. We then investigate the atomic-scale structural features of exposed surface facets, which are critical for enhancing catalytic activity and stability. Well-defined branched nanoparticles have led to a fundamental understanding of how the branch structural features influence the catalytic activity and stability, which we highlight for the oxygen evolution reaction (OER) and biomass oxidation. In discussing recent breakthroughs for branched nanoparticles, we explore the opportunities created by decorated branched nanoparticles and the unique bifunctional active sites that are exposed on the branched nanoparticle surfaces. This class of catalysts is of rapidly growing importance for reactions including the hydrogen evolution reaction (HER) and methanol oxidation reaction (MOR), where two exposed metals are required for efficient catalysis. In the second half of this Account, we explore recent advances in the synthesis of branched nanoparticles and highlight the cubic-core hexagonal-branch growth mechanism that has achieved excellent control of all of the important structural features, including branch dimensions, number of branches, and surface facets. We discuss the slow precursor reduction as an effective strategy for decorating metal islands with controlled loadings on the branched nanoparticle surfaces and the spread of these metal islands to form single-atom active sites. We envisage that the key synthetic and structural advances identified in this Account will guide the development of the next-generation electrocatalysts.

Ganglia dynamics during imbibition and drainage processes in nanoporous systems

We study the gas–water transient imbibition and drainage processes in two-dimensional nanoporous media using our recently developed lattice Boltzmann model. To describe the microscopic molecular interactions, the model employs a pseudopotential that correlates the local density and interaction strength to perform simulation at a mesoscopic scale. The primary interest is ganglia dynamics in the nanoporous media affected by fluid and geometrical properties of the porous structure. We performed sensitivity analyses on the fluid and rock characteristics such as the Euler number, gas–water interfacial area, water film area, capillary pressure, pore size distribution, specific surface area, and wettability. The simulation results revealed the fingering nature of the nonwetting phase. In the imbibition process, the flow pathway of water results in isolated and trapped gas bubble clusters because of the strong attraction between water and solid surfaces. In the drainage process, the pressure difference between the gas phase and the water phase depends on both the capillary pressure and the disjoining pressure due to the presence of water film. Pore topography and specific surface area control the continuity of the fluid phases in the imbibition process. In nonwet systems, the water phase starts fingering in the nanoporous system. The present work elucidates the microscopic ganglia dynamics of gas–water two-phase flow in nanoporous media. The microscopic scale details will help establish the macroscopic flow equation to accurately predict two-phase flow in shale gas, tight oil, and caprock seals.

Asymmetric Microelectrodes for Nanoliter Bubble Generation via Electrolysis

Microfabricated devices for gas bubble generation via water electrolysis typically employ two electrodes of identical size (i.e. interdigitated electrodes or microelectrode pairs). Contrary to this practice, asymmetrically sized electrode pairs offer distinct advantages in the formation of single, trapped nanoliter-volume gas bubbles in microfluidic devices. Although such principles may apply generally to MEMS or micro-scale water electrolysis devices, our model system consisted of a Parylene C microfluidic structure to contain the bubble and embedded thin-film Pt electrodes immersed in 1x phosphate buffered saline solution. The geometric area of one microelectrode was constant (working electrode) while that of the second electrode (counter electrode) was varied over five orders of magnitude. Although increased counter electrode size increases overall device size, it provides improvements in various performance metrics. The largest tested counter electrode size resulted in 41% energy savings per bubble, improved bubble volume uniformity, enabled successful discrimination between O₂ versus H₂ gas generation, and achieved bubble stability exceeding 500 minutes (a 25x improvement over previous work). The underlying theory motivating the use of asymmetric electrodes is detailed and followed by experimental demonstration. Finally, a discussion on tradeoffs to assist in the design of MEMS-based electrolytic bubble generation devices is provided. [2021-0172]

Multiple mechanisms of coseismic water level changes at the Rongchang well in a seismically active area in China

We examined water level data at the Rongchang (RC) well in the Rongchang natural gas field in China's Sichuan Basin from November 2007 through 2019, where injection-induced seismicity with a magnitude of up to 5.2 has been reported. Coseismic water level changes caused by earthquakes from 3 km to 3500 km away, as well as changes in water temperature, provide a good opportunity to systematically study the mechanisms underlying the response of water level to earthquakes. Among 41 ML ≥ 3.0 local earthquakes, 20 events exceeded the magnitude-distance threshold for the water level to change. Eleven events caused water levels to rise, and only one earthquake led to a decrease in water level. For the 22 distant earthquakes with a seismic energy density greater than the threshold of 0.0017 J/m3, ten events caused rises in the water level and only the MW 7.9 Wenchuan Earthquake caused a temporary, but large, decrease in water level. In total, eight local events and 11 distant events, which were expected to cause coseismic water level changes, actually resulted in no changes. The findings imply that an earlier sizable earthquake may decrease the sensitivity of the well for a certain period of time (approximately two months). The dominant mechanisms of coseismic water level changes in response to near-, mid-, and far-field earthquakes are different. For near-field earthquakes, the drop in coseismic water level is governed by the combined effects of static strain changes and the release of trapped gas bubbles due to the strong vibration of seismic waves in the aquifer; however, only static strain changes are dominant after mid-field earthquakes. For far-field earthquakes, the dominant mechanism is aquifer softening due to growth of gas bubbles in the groundwater that is associated with weak and low frequency seismic shaking of passing surface waves.

Microbial-Induced Desaturation in Stratified Soil Conditions

A feasibility study was performed to assess the potential of microbially induced desaturation and precipitation (MIDP) through denitrification to reduce the risk of earthquake-induced liquefaction and improve the resilience of embankments along the lower Fraser River in Richmond, British Columbia, Canada. The denitrification process produces nitrogen gas, which gradually desaturates the soil, and dissolved inorganic carbon, which in presence of dissolved calcium results in precipitation of calcium carbonate minerals, cementing the soil particles. The trapped gas bubbles dampens pore pressure build-up during cyclic loading. Consequently, microbially induced desaturation (MID) the first phase of the MIDP process, has great potential as soil improvement technique, especially for liquefaction mitigation, independently from the precipitation phase of the process. Numerous studies have demonstrated the effect of biogenic gas formation on the mechanical response for treated sand samples at lab scale. However, there is still limited knowledge on the impact of partial desaturation on the hydraulic properties of the treated sediments and the durability of the entrapped gas phase, particularly in naturally stratified and heterogenous formations. This is important because changes in porosity and hydraulic conductivity may affect the injection flow patterns and distribution of substrates and products in the subsurface. The objective of this study was to develop a test procedure to evaluate the applicability and performance of the MID technique in stratified subsurface conditions. The study involved a two-dimensional model (2D) tank set-up, in which substrates were laterally injected and extracted. The results demonstrated the effectiveness of treatment and showed how gas formation, migration and entrapment and resulting degree of desaturation and hydraulic conductivity are affected by stratifications in a natural soil.

Modelling of gas evolution during powder rehydration for Broadband Acoustic Resonance Dissolution Spectroscopy (BARDS)

As a powder wets and dissolves in a liquid, trapped gas bubbles are released and the measurement of the evolution of gas volume versus time can be used to quantify the ease with which powder wets and dissolves in the liquid. This phenomenon forms the basis of the BARDS analytical measurement technique. To complement this experimental approach, a model of the evolution of gas bubbles from a powder into a liquid during powder dispersion and solubilisation is presented. The model is validated against experimental data from the rehydration of Milk Protein Isolate (MPI) powder; both in its original state, and in the agglomerated state and is shown to match the experimental data with good accuracy. Use of the model facilitates a fuller interpretation of the experimental BARDS data and the values of dispersion and solubilisation rate constants can be applied for quantitatively comparing the behaviour of powders. It also permits development of robust rehydration parameters from the experimental BARDS signal.

Effect of capillary number on morphological characterizations of trapped gas bubbles: Study by using micro-tomography

A series of capillary trapping experiments were conducted in unconsolidated porous media using high-resolution micro-tomography. Morphological characterizations of trapped gas bubbles, including residual gas saturation, bubble size distribution and interfacial area, were determined from reconstructed three-dimensional images. Different flooding flow rates, corresponding to capillary number ranges from 1.0 × 10−7 to 3.8 × 10−4, were performed to investigate the impact of flow rate on these characterizations during imbibition processes. At low capillary number, where capillary forces are dominant, the residual gas saturation is independent of capillary number and the bubble size distribution shows a universal power-law behavior predicted from percolation theory. When the capillary number increases above a critical value, the residual saturation decreases sharply, and the bubble size distribution no longer exhibits power-law behavior. As capillary number increases, the volume fraction of single-pore bubbles increases from about 16% to more than 80%. Furthermore, we explored the interfacial area of trapped bubbles, including gas–solid and gas–water (meniscus) interfacial areas, which directly affect the mass transfer process inside porous media. The specific interfacial area exhibits a linear relation with residual saturation, and the effect of particle size can be normalized by multiplying the median particle diameter. Moreover, we observed that the meniscus/total interfacial area ratio increases from 0.61 to 0.78 with an increase in capillary number, because the smaller singlet has a higher interfacial contact with the wetting phase.

Capillary stop valve actuation by thermo-pneumatic- pressure for lab-on-chip systems

A novel method is presented for triggering a robust capillary stop valve fabricated in silicon using the thermal expansion of trapped air bubble (with a footprint of just 300 μm × 320 μm) as the actuation element. A heating element on the backside of a bubble trap chamber is utilized for thermal expansion of the air bubble. A voltage pulse of around 6 V, the capillary barrier, around 1400 Pa was easily breached. A non-dimensionalized model has been developed using equivalent circuit model to describe the complex thermal/hydraulic behavior of the system. The trapped gas bubble temperature is input as a function of time in the model. A thermal finite element-based simulation is conducted to determine the gas temperature from the experimentally measured heater temperature. The model results are validated against experiments to aid in characterizing the dynamics of the problem.

Microstreaming induced by acoustically trapped, non-spherically oscillating microbubbles

While numerous theoretical studies exist on microstreaming around acoustically excited, trapped gas bubbles, experimental approaches have mainly been conducted for bubbles attached to a solid boundary. One of the main difficulties lies in the positional stability of the microbubble. In the present work we trigger surface modes by bubble coalescence, with the advantage of limiting translational instabilities and controlling the orientation of the axisymmetric deformation. Furthermore, streaming is visualised by fluorescent tracer particles. In this way, bubble dynamics and streaming patterns can be studied together. Different types of streaming patterns are observed and correlated to the respective mode number. Besides the mode number, the bubble size and the phase difference between modal components are identified as important parameters in the definition of the pattern type.

Experimental study of the dynamic mechanism on gas bubbles migration, fragment, coalescence and trapping in a porous media

Immiscible gas injection recovery is one of the most important methods used for EOR. However, it is very difficult to directly observe the flow of gas bubbles in the reservoir stratum. In our experimental work, we investigated visually the morphological configurations of gas bubbles migration, fragment, coalescence and trapping in different liquid-simulated porous media packs of glass beads. Also, the volume fraction of the trapped gas bubbles in the drainage process were measured. A fast video recording technique was used for the visual observations of the morphological characteristics of the rising gas bubbles in the saturated porous media. The obtained experimental images recorded reveal three types of the rising migration bubbles namely: free migration in the pore throat, attached wall migration on the beads and deformation migration. Additionally, coalescence and fragment have an interdependency for a rising gas bubble. The adsorption and capillary trappings are both two kinds of important trapping mechanisms, which determine the volume fraction of the trapped bubbles. The volume fraction of the trapped bubbles increases with an increment in the gas injection time and rates with the trapped bubbles fluctuating upward and downward due to the more coalescence and fragment after a long gas injection time/period. Lastly, the volume fraction of the trapped bubbles increases with a decrease in the inclination angles and porosities, which influences the areal sweep efficiency for enhanced liquid recovery in a porous media.

Underwater spiders inspire bubble study that could help industry trap waste gas

Hydrophobic fibers that spiders employ to trap air so they can breathe underwater could help trap and remove gas bubbles in liquids in industrial situations.

High-speed imaging of ultrasound driven cavitation bubbles in blind and through holes

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5 kHz) cavitation bubbles inside the blind and through holes of PMMA plates by using high-speed imaging. Various diameters of blind (150, 200, 250 and 1000 µm) and through holes (200 and 1000 µm) were investigated. Gas bubbles are usually trapped in the holes during substrate immersion in the liquid thus preventing their complete wetting. We demonstrate that trapped gas can be successfully removed from the holes under ultrasound agitation. Besides the primary Bjerknes force and acoustic streaming, the shape oscillations of the trapped gas bubble seem to be a driving force for bubble removal out of the holes. We further discuss the bubble dynamics inside microholes for water and Cu2+ salt solution. It is found that the hole diameter and partly the type of liquid media influences the number, size and dynamics of the cavitation bubbles. The experiments also showed that a large amount of the liquid volume inside the holes can be displaced within one acoustic cycle by the expansion of the cavitation bubbles. This confirmed that ultrasound is a very effective tool to intensify liquid exchange processes, and it might significantly improve micro mixing in small structures. The investigation of the effect of ultrasound power on the bubble density distribution revealed the possibility to control the cavitation bubble distribution inside the microholes. At a high ultrasound power (31.5 W) we observed the highest bubble density at the hole entrances, while reducing the ultrasound power by a factor of ten shifted the bubble locations to the inner end of the blind holes or to the middle of the through holes.

Drag reduction induced by superhydrophobic surfaces in turbulent pipe flow

The drag reduction induced by superhydrophobic surfaces is investigated in a turbulent pipe flow. Wetted superhydrophobic surfaces are shown to trap gas bubbles in their asperities. This stops the liquid from coming in direct contact with the wall in that location, allowing the flow to slip over the air bubbles. We consider a well-defined texture with streamwise grooves at the walls in which the gas is expected to be entrapped. This configuration is modeled with alternating no-slip and shear-free boundary conditions at the wall. With respect to the classical turbulent pipe flow, a substantial drag reduction is observed which strongly depends on the grooves’ dimension and on the solid fraction, i.e., the ratio between the solid wall surface and the total surface of the pipe’s circumference. The drag reduction is due to the mean slip velocity at the wall which increases the flow rate at a fixed pressure drop. The enforced boundary conditions also produce peculiar turbulent structures which on the contrary decrease the flow rate. The two concurrent effects provide an overall flow rate increase as demonstrated by means of the mean axial momentum balance. This equation provides the balance between the mean pressure gradient, the Reynolds stress, the mean flow rate, and the mean slip velocity contributions.

Application of boundary element method to Stokes flows over a striped superhydrophobic surface with trapped gas bubbles

A slow steady flow of a viscous fluid over a superhydrophobic surface with a periodic striped system of 2D rectangular microcavities is considered. The microcavities contain small gas bubbles on the curved surface of which the shear stress vanishes. The general case is analyzed when the bubble occupies only a part of the cavity, and the flow velocity far from the surface is directed at an arbitrary angle to the cavity edge. Due to the linearity of the Stokes flow problem, the solution is split into two parts, corresponding to the flows perpendicular and along the cavities. Two variants of a boundary element method are developed and used to construct numerical solutions on the scale of a single cavity with periodic boundary conditions. By averaging these solutions, the average slip velocity and the slip length tensor components are calculated over a wide range of variation of governing parameters for the cases of a shear-driven flow and a pressure-driven channel flow. For a sufficiently high pressure drop in a microchannel of finite length, the variation of the bubble surface shift into the cavities induced by the streamwise pressure variation is estimated from numerical calculations.