Bubble Stability Research Articles

Effect of emulsifier-fat interactions and interfacial competitive adsorption of emulsifiers with proteins on fat crystallization and stability of whipped-frozen emulsions

Fat crystals and adsorbed interfacial layer are very important in controlling the stability of partly crystalline fat globules. In this study, the effects of competitive adsorption of emulsifiers, including glyceryl monooleate (GMO) and sucrose ester (SE) with milk proteins and emulsifier-fat (anhydrous milk fat, AMF; coconut oil, CO) interactions on the fat crystallization behavior and stability of whipped-frozen emulsions were investigated. Parameters such as zeta potential, surface protein coverage, rheological property, fat crystallization behavior, overrun, melting rate and microstructure of ice cream were determined. The results indicated both GMO and SE affected the crystallization behavior, changing the crystal growth, morphology and stiffness. Different mechanism was involved in fat partial coalescence in emulsion systems with different fat-emulsifier interactions. The promotion of partial coalescence in whipped-frozen emulsions with increasing concentration of GMO was mainly related to fat crystallization behavior combined with increased fat crystal growth dimension and protrusion length at the fat globule surface, since protein surface coverage was not affected by GMO concentration. However, in the case of emulsions with SE, enhancement of partial coalescence was mainly related to decreased protein surface coverage combined with less resistance of interfacial membranes to penetration by small fat crystals. Ice cream based on CO were distinguished by higher overruns and lower melting rates compared to ice cream based on AMF with the same levels of emulsifiers, especially strong fat crystal network in the presence of GMO accounts for the increased air bubble stability. These findings provides a theoretical basis for aerated food processing.

The numerical modeling of the vapor bubble growth on the silicon substrate inside the flat plate heat pipe

Inside the flat plate heat pipes for the microelectronics packages, the micro scale boiling is a common phenomenon under the suddenly high heat flux. The internal vast vapor bubble nucleation and growth become a challenge for the stable operations. Due to the lack of the direct observations, the vapor bubble growth in early stage is still unclear. In order to study the initial bubble growth, the molecular dynamics models of water-silicon interfaces which have different sizes of simulation boxes were examined. The vapor bubble diameter growth rates (0.18 ± 0.02 × 106 mm/s) were calculated among the different sizes of simulation boxes. Since the lack of extra energy input, the initial vapor bubble can only expand to 45 nm diameter large (under 100 W/cm2 heat flux at 350 K initial temperature). The growth phenomena of 0.1 μm to 0.3 mm scale bubbles could be further modeled by the Mixture models. The observed bubble (bubble diameter between 10 μm and 0.3 mm) had a diameter growth of 0.71 ± 0.19 mm/s that was close to the Mixture model simulated one (0.75 mm/s). In addition, the ratio of the bubble volume growth rate to the bubble covered bottom area was about 7 ± 0.2 mm/s. By calculating the ratio between the growth rate of observed bubbles and volume of the simulated stable nucleation bubble, the bubble generation rate (3 × 1014 bubble/s mm2) under certain conditions might be estimated. Therefore, the whole process of bubbles growth could be deduced.

Experiments on pool boiling regimes and bubble departure characteristics of single vapor bubble under subcooled bulk conditions

An experimental study for understanding the effects of subcooling of bulk fluid on the phenomenon of nucleate boiling heat transfer was carried out. The whole field experimental data was recorded in a purely non-intrusive manner using rainbow schlieren deflectometry as the optical diagnostic tool. Boiling experiments were conducted over a range of heat flux conditions (1⩽q′′⩽40 kW/m2) and for varying levels of subcooling (0 K ≤ ΔTsubcooling ≤ 10 K). The recorded images were first been interpreted to understand the effect of subcooling on various regimes of boiling phenomenon, which include convection, isolated nucleate boiling and vertical coalescence of the departing bubbles. Observations on convection regime showed an increasing influence of natural convection heat transfer. Based on the formation of vapor bubble and its departure characteristics, the isolated nucleate boiling regime of the classical boiling curve was further segmented into three sub-regimes namely, stable vapor bubbles, oscillating vapor bubbles and oscillating-departing vapor bubbles. A mechanism explaining the oscillations of vapor bubbles under subcooled condition was proposed that takes into account the relative dominance of condensing apex region and evaporating microlayer region. Various bubble dynamic parameters such as equivalent bubble diameter, departure frequency, aspect ratio and oscillation frequency were determined quantitatively to understand the effect of subcooling on the bubble departure characteristics. Based on the experimental results, an empirical correlation for predicting the bubble departure diameter and evaporative heat flux was developed and proposed.

Thermodynamic stability of droplets, bubbles and thick films in open and closed pores

A fluid in a pore can form diverse heterogeneous structures. We combine a capillary description with the cubic-plus-association equation of state to study the thermodynamic stability of droplets, bubbles and films of water at 358 K in a cylindrically symmetric pore. The equilibrium structure depends strongly on the size of the pore and whether the pore is closed (canonical ensemble) or connected to a particle reservoir (grand canonical ensemble). A new methodology is presented to analyze the thermodynamic stability of films, where the integral that describes the total energy of the system is approximated by a quadrature rule. We show that, for large pores, the thermodynamic stability limit of adsorbed droplets and bubbles in both open and closed pores is governed by their mechanical stability, which is closely linked to the pore shape. This is also the case for a film in a closed pore. In open pores, the film is chemically unstable except for very low film-phase contact angles and for a limited range in external pressure. This result emphasizes the need to invoke a complete thermodynamic stability analysis, and not restrict the discussion to mechanical stability. A common feature for most of the heterogeneous structures examined is the appearance of regions where the structure is metastable with respect to a pore filled with a homogeneous fluid. In the closed pores, these regions grow considerably in size when the pores become smaller. This can be understood from the larger energy cost of the interfaces relative to the energy gained from having two phases. Complete phase diagrams are presented that compare all the investigated structures. In open pores at equilibrium, the most stable structure is either the homogeneous phase or adsorbed droplets and bubbles, depending on the type of phase in the external reservoir. Smaller pores allow for droplets and bubbles to adsorb for a larger span in pressure. In closed pores, most of the investigated configurations can occur depending on the total density, the contact angle, the pore shape and the pore size. The analysis presented in this work is a step towards developing a thermodynamic framework to map the rich heterogeneous phase diagrams of porous media and other confined systems.

Effect of interparticle force on gas dynamics in a bubbling gas–solid fluidized bed: A CFD-DEM study

The influence of cohesive interparticle force (IPF) on gas dynamics in a bubbling gas–solid fluidized bed was numerically investigated while the bed hydrodynamics was described with the help of computational fluid dynamics and discrete element method (CFD-DEM). The model was validated by experimental results in terms of total bed pressure drop profile, probability density distribution of instantaneous local bed voidage signals, and Eulerian solid velocity field. The results showed that the model can satisfactorily predict the hydrodynamics of a bubbling gas–solid fluidized bed that is impacted by the presence of cohesive IPF. The validated CFD-DEM model was adopted to delineate the effects of IPF on the distribution of fluidizing gas between the bubble and emulsion phases and bubble characteristics, such as bubble stability and rise path, which could hardly be explored experimentally. Simulation results revealed that the presence of IPF in the bubbling bed alters the distribution of the fluidizing gas between the bubble and emulsion phases in favor of an increase in the propensity of gas to pass through the bed in the emulsion phase. The results also indicated that the bubble stability increases and the straight rise path of bubbles changes to a tortuous path when enhancing IPF.

Application of Microfluidics in the Production and Analysis of Food Foams.

Emulsifiers play a key role in the stabilization of foam bubbles. In food foams, biopolymers such as proteins are contributing to long-term stability through several effects such as increasing bulk viscosity and the formation of viscoelastic interfaces. Recent studies have identified promising new stabilizers for (food) foams and emulsions, for instance biological particles derived from water-soluble or water-insoluble proteins, (modified) starch as well as chitin. Microfluidic platforms could provide a valuable tool to study foam formation on the single-bubble level, yielding mechanistic insights into the formation and stabilization (as well as destabilization) of foams stabilized by these new stabilizers. Yet, the recent developments in microfluidic technology have mainly focused on emulsions rather than foams. Microfluidic devices have been up-scaled (to some extent) for large-scale emulsion production, and also designed as investigative tools to monitor interfaces at the (sub)millisecond time scale. In this review, we summarize the current state of the art in droplet microfluidics (and, where available, bubble microfluidics), and provide a perspective on the applications for (food) foams. Microfluidic investigations into foam formation and stability are expected to aid in optimization of stabilizer selection and production conditions for food foams, as well as provide a platform for (large-scale) production of monodisperse foams.

Nonlinear dynamics of self-organising bubble departures from twin nozzles

The nonlinear dynamics of self-organising bubble departures from twin nozzles in engine oils was analysed. Air bubbles were generated from twin brass nozzles with an inner diameter equal to 1 mm. The flow of bubbles in bubble chains was recorded using high-speed camera. The time series of air pressure oscillations and signal from laser–phototransistor identifying the presence of bubbles over the nozzles outlet were recorded simultaneously. The self-organising bubble departures were observed and their stability was analysed. It was found that self-organising bubble departures become unstable because of successive (during subsequent bubble departures) decrease of the mean air pressure in one of the nozzle air supply system. It was shown that instability of self-organising bubble departures leads to equalization of pressures in both nozzles air supply systems which causes that simultaneous bubble departures appear. In the present experiment, this process was repeated in a cyclic and chaotic way. It was shown that stable self-organising bubble departures are accompanied by periodic air pressure oscillations in one of the nozzles and chaotic air pressure oscillations in the second one.

Bubble Formation in a Finite Cone: More Pieces to the Puzzle.

We investigate the stability of bubble formation, starting with a convex or a concave meniscus, from a liquid solution (of water and a dissolved gas) inside a finite cone at constant temperature and constant liquid pressure (above the saturation pressure of the pure solvent). It is assumed that the dissolved gas (nitrogen) forms a dilute solution at equilibrium, which can be described by Henry's law. The number and nature of equilibrium states are determined with Gibbsian composite-system thermodynamics, both from the intersection of the equilibrium Kelvin radius with the geometry radius and from the extrema in the plot of free energy of the system versus size of the new phase. Bubble stability is studied along the whole growth path, as the bubble grows inside, gets pinned, and grows further outside the finite cone. The changes in the concentration of the liquid bulk phase and the vapor phase during the growth of the bubble are carefully incorporated in the equations. The effects of various parameters, including cone apex angle, cone half mouth radius, contact angle, total number of moles, and initial degree of saturation, on the stability of the bubble are also investigated. Stability of bubble formation from a liquid solution inside a confined geometry such as a finite cone is of interest in areas such as restoring underwater superhydrophobicity and adhesion of particles to the roughness of synthetic biomaterials.

Flow visualization of two-phase R-245fa at low mass flux in a plate heat exchanger near the micro-macroscale transition

Two-phase R-245fa flow in a plate heat exchanger (PHE) is experimentally investigated to understand the unique flow regimes found during adiabatic operation at low mass flux. A transparent PHE replica with 3.4 mm hydraulic diameter is 3D-printed for flow visualization using high-speed videography. Observed flow regimes support that the thermofluidic characteristics peculiar to PHE operation are due to the macro-microscale transitional two-phase flow from the coexistence of fluid inertial forces and surface tension effects, corresponding to the operating conditions. Maximum stable bubble diameter is bigger at low mass flux than at high mass flux, and the bubbles can become big enough to be fully confined in the millimeter-scale PHE channel to be deformed or elongated. This represents the main thermo-physical characteristics of two-phase flow in mini- and microchannels, which is different from turbulent mixing flow found at high-mass-flux operation or in channels of conventional macroscale. Flow morphology involving bubble coalescence and breakup dynamics is captured and analyzed in relation to the fluid properties and geometric obstructions provided by the heat exchanger channel. Current observations validate some previously published findings that suggest potential microscale boiling mechanisms in PHEs.

Manipulation of Chain Entanglement and Crystal Networks of Biodegradable Poly(butylene adipate-co-butylene terephthalate) During Film Blowing through the Addition of a Chain Extender: An In Situ Synchrotron Radiation X-ray Scattering Study

One prerequisite for the large-scale application of biodegradable polymers is the manipulation of macroscopic performances of commercially available biopolymers during processing according to different real service requirements. Herein, the microstructural evolution of poly(butylene adipate-co-butylene terephthalate) (PBAT) modified by chain extender during film blowing was investigated by in situ synchrotron radiation X-ray scattering to unveil the origin of different performances. The chain dynamics difference induced by the chain extender was first characterized by the rheological measurement and 1H Multiple Quantum (MQ) NMR. It shows that the terminal relaxation is significantly slowed down, while the locally segmental dynamics is not apparently changed. With the assistance of the custom-built film blowing apparatus, the microstructure right above the die exit (D = 13-165 mm) was in situ, simultaneously captured by small- and wide-angle scattering (SAXS/WAXS), where four distinct regimes can be defined. Only the PBAT melt signals are found in regime I, whereas the formation of the mesomorphic domains as shown by the SAXS streaks appearing in regime II. The crystal shows up in regime III, where the WAXS signal appears. A dramatic increment of the crystallinity is found in regime III, which contributes to the continuous increasing bubble modulus with the formation of the crystal-based network. Such a crystal-based network is filled with crystals in regime IV, where the diameter of the PBAT bubble remains constant. The addition of the chain extender is found to significantly influence the structural evolution within different regimes. These dynamics and structure information could supply general guidance for bubble stability improvement and modification of macroscopic performances of biodegradable polymer products.

Prediction of bubble terminal velocity in surfactant aqueous solutions

AbstractBubble terminal velocity has a significant effect on gas holdup, residence time, and efficiency of the interface transfer. Surfactant is often required to generate small and stable bubbles in gas‐liquid two‐phase devices. In this paper, bubble terminal velocities were obtained for different surfactant aqueous solutions using high‐speed CCD (charged couple device) system and digital image analysis technology. Experimental results showed that available correlations were not able to accurately predict terminal velocity of bubbles rising in surfactant aqueous solutions. Thus, a new correlation is proposed based on experimental data and it provides an accurate approximation of bubble terminal velocity. The average relative error for the proposed correlation is determined to be 7.2% in MIBC aqueous solutions, 4.5% in OP‐10 aqueous solutions, and 4.6% in 2‐octanol aqueous solutions. The proposed correlation agrees well with experiment data from literate within ranges of the Morton number, Mo, the bubble Reynolds number, Re, and the Eötvös number, Eo: 3.29 × 10−11<Mo<4.29, 0.08<Re<1062, 0.04<Eo<91.16.

Real estate bubbles in a bank-real estate loan network model integrating economic cycle and macro-prudential stress testing

The real estate industry plays a decisive role in the national economy. Once real estate bubbles burst, it will trigger the turbulence of the whole economic system and even lead to serious economic crisis. From the perspective of macro-prudential stress testing, this paper constructs a risk contagion model of bank-real estate loan network. With the help of the data of 136 banks that provided loans for the real estate industry from 2012 to 2017, a macro-prudential stress testing is conducted on the bank-real estate loan system and real estate bubbles. The main conclusions are as follows: (1) Asset depreciation coefficient, bank panic coefficient and bank leverage rate have little impact on the stability of the banking system and the state of real estate bubbles, but the scale of bank assets has a significant impact on them in the whole economic cycle. (2) During the recession period, when China’s banks that are “too big to fail” have not been subjected to excessive shocks, the entire banking system will remain relatively stable and real estate bubbles will not burst. (3) Compared with the recession period, the bank survival ratio is higher under the influence of asset depreciation coefficient, bank panic coefficient and bank leverage rate in the recovery period. And the probability of the system being completely robust increases under the influence of the scale of bank asset, while the probability of being extremely vulnerable decreases. (4) Compared with the recession and recovery period, the bank survival ratio is higher under the influence of asset depreciation coefficient in overheating period, and it is lower under the influence of bank panic coefficient and bank leverage rate. Moreover, under the influence of the scale of bank assets, the probability of the system being completely robust increases, while the probability of being extremely vulnerable decreases. (5) The banking system has significant hierarchical and functional differences in China. It reflects that China should focus on large state-owned banks and joint-stock banks when preventing real estate bubbles from breaking up.

Towards Gas Sensors for Aquatic Chemical Sensing Inspired By the Star-Nosed Mole

Aquatic chemical sensors, as any other surface submerged in water, are subject to biofouling, by bacteria and algae that quickly form biofilms on the submerged surface [1]. In the case of sensors, accumulated biofilms skew the chemical measurements until the sensors are cleaned and recalibrated. A number of mechanical and chemical solutions have been proposed to scrape off or kill the biofilms, but they can be sensor-specific, costly, or have adverse effects on the environment [2]. The process of biofouling happens over the span of days, making long-term chemical measurements labor-intensive and oftentimes unfeasible. The star-nosed mole, with its bizarre nose geometry, presents a way to circumvent the issue of biofouling. These semi-aquatic moles exhale and inhale bubbles rapidly underwater. When the bubbles contact a target of interest, molecules are collected on the bubble surface and brought back into the nose. In this fashion, its olfactory receptors never contact the water and the mole can use its terrestrial nose to track prey at the bottom of swamps [3]. The appendages that make up the star on their nose stabilize the bubbles, allowing them to consistently blow larger bubbles without the bubble pinching off and floating away [4]. We develop an underwater electronic nose, inspired by the star-nosed mole’s underwater sniffing. A motor pushes and pulls on a syringe to form and contract a 0.5 mL bubble underwater at 1 Hz. A 3D printed star, inspired by the mole’s nose geometry, stabilizes the bubble and prevents pinch-off [4]. The 6 fins of the star, separated by 30-degree angles, are slanted downward at a 35-degree angle to optimally stabilize the largest volume bubble. The air from the bubble is pushed and pulled past an array of metal oxide sensors. As a proof of concept, this underwater e-nose is used in an on-going experiment to detect the presence or absence of sparkler fragments at the bottom of a water sample. In the event of the bubble pinching off, inhaled water shorts out the sensor array. At the same time, if the volume of dead space between the star and the sensor array is greater than the volume of the bubble, tests show that the e-nose will be incapable of detecting changes in the underwater environment. Thus, stabilizing the bubble during the sniff is key in designing this underwater e-nose. This prototype e-nose marks the expansion of machine olfaction to underwater applications. Figure 1. Underwater bubble sniffing can allow gas sensors to be used underwater. A) The star-nosed mole sniffs bubbles to pick up scents underwater. Photo courtesy of K. Catania. B) Mimicking this behavior could allow gas sensors to detect chemicals without directly exposing the sensor to its environment. C) A motor and syringe sniff underwater across a metal oxide sensor to form an underwater e-nose. [1] G. Xu, W. Shen, and X. Wang, “Applications of Wireless Sensor Networks in Marine Environment Monitoring: A Survey,” Sensors, vol. 14, no. 9, pp. 16932–16954, Sep. 2014. [2] L. Delauney, C. Compère, and M. Lehaitre, “Biofouling protection for marine environmental sensors,” Ocean Sci, vol. 6, no. 2, pp. 503–511, May 2010. [3] K. C. Catania, “Olfaction: Underwater ‘sniffing’ by semi-aquatic mammals,” Nature, vol. 444, no. 7122, p. 1024, Dec. 2006. [4] A. B. Lee and D. L. Hu, “Bubble stabilization by the star-nosed mole,” Phys. Rev. Fluids, vol. 3, no. 12, 2018. Figure 1

A Novel Preparation Method of Electrically Conductive Adhesives by Powder Spraying Process.

In a conventional electrically conductive adhesive (ECA) preparation process, typical ECAs are made by adding an appropriate amount of electrically conductive fillers, such as silver, into a polymer matrix, such as epoxy resin, to form a uniformly dispersed mixture by mixing and stirring operations. However, during the preparation process, secondary pollution and mass loss are caused by the vigorous mixture process. At the same time, the stirring operation introduces many small and stable bubbles, which affect the electrical conductivity of the ECAs. In light of these problems with the conventional preparation of ECAs, we developed a novel ECA preparation method that employs a powder spraying process: silver flakes are sprayed into the epoxy resin with a certain mass fraction to form formulated pastes. The as-prepared ECAs exhibited excellent properties compared with those prepared by the conventional process. This proves that the powder spraying process is feasible and superior to the conventional process.

A pore-scale study on improving CTAB foam stability in heavy crude oil−water system using TiO2 nanoparticles

The main challenge of foam injection for enhanced oil recovery (EOR) is foam stability at harsh reservoir conditions. With regard to this matter, this research aims to investigate the effect of TiO2 nanoparticles (NPs) in improving the stability of foam bubbles in porous media. The conductivity of hexadecyltrimethylammonium bromide (CTAB) solutions was measured to determine critical micelle concentration (CMC). Thereafter, TiO2 NPs were added into the foam solution and pH, contact angle, and interfacial tension (IFT) in the presence and absence of CTAB were measured. The stability of foam bubbles was evaluated through bulk foam tests and absorbance measurements. Finally, different nanofluids (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.10 wt%), nano-CTAB solutions (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.1 wt% NPs at the CMC of CTAB) and foams (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.1 wt% NPs and at the CMC of CTAB) were injected into a heavy oil saturated heterogeneous porous medium. The results of the conductivity measurements showed that the highest adsorption of CTAB molecules onto the NPs surfaces occurs at 0.03 wt% NPs and 0.03 wt% CTAB. The results of micro- and macroscopic images from bulk foam tests revealed improved foam stability for this nano-CTAB foam. Moreover, contact angle and IFT results showed further improvement in interfacial properties for the mixture of NPs and CTAB solutions. Ultimately, in terms of oil recovery, the highest oil recovery factor (54%) was observed upon injection of foam with 0.03 wt% TiO2 NPs and 0.03 wt% CTAB, compared to the 22% recovery factor of DW injection. The visualization experiments showed that 32% enhancement in the recovery factor was mainly due to oil production from dead−end pores. This was because of different functions of NPs including stabilization of foam bubbles, wettability alteration, and improved foam generation mechanisms.

Application of the Diffused Double Layer Theory to Nanobubbles.

Nanobubbles have electrically charged interfaces; hence, the diffused double layer theory can be applied to explain the behavior of nanobubbles in different electrolytic solutions. In this research, oxygen nanobubbles were generated in NaCl solutions of different concentrations, and bubble size and ζ potentials were measured just after the generation and after 1 week. The measured data and diffused double layer theory were used to compute the surface charge density, the potential due to the surface charge, and the interaction energy between bubbles. With the increased NaCl concentration, bubble size, surface charge density, and the number of negative charges increased, while the magnitude of ζ potential/surface potential, double layer thickness, internal pressure, and the electrostatic repulsion force decreased. The same trend was observed after 1 week. The net total energy calculation for the 0.001 M NaCl solution showed that the bubble repulsion for an intermediate separation distance had a 6.99 × 10-20 J energy barrier, which prevented bubble coalescence. Hence, the 0.001 M NaCl solution produced stable nanobubbles. The calculation of internal pressure inside nanobubbles showed a reduction in the interfacial pressure difference with the increased NaCl concentration. The test results, as well as diffuse double layer and net total energy calculations, showed that the most stable bubbles were obtained with 0.001 M NaCl concentration and the least stability was recorded with the highest amount (0.1 M) of NaCl concentration.

Egg white protein microgels as aqueous Pickering foam stabilizers: Bubble stability and interfacial properties

The aim of this study was to design and characterize aqueous foams stabilized by egg white protein microgels (EWPM) and compare their stability with conventional foams stabilized by egg white protein (EWP). Sub-micron sized EWPM (hydrodynamic diameter = 359 ± 21 nm) were designed using a top-down approach involving the formation of a thermally-crosslinked egg white protein hydrogel (90 °C/30 min, pH 7.0) followed by controlled shearing using jet homogenization (300 bar, two passes). Microstructural evaluation at multiple length scales (confocal laser scanning microscopy and cryogenic scanning electron microscopy) indicated that the EWPM stabilized the aqueous foams via a Pickering-type mechanism. Foamability was higher in EWP-stabilized foams compared to EWPM-stabilized foams, irrespective of the protein concentrations tested (0.5–3.0 wt%). However, EWPM-stabilized foams exhibited higher stability to disproportionation over long periods (p < 0.05), even though the initial air bubble size was smaller than with EWP. Bubble coalescence experiments also confirmed that the fraction of the coalescence was much lower in EWPM systems as compared to the EWP counterparts. Changes of surface shear viscosities of EWP at the air-water interface indicated that EWP films were more brittle, exhibiting shear thinning during the measurements, whereas the viscosities of the EWPM films were independent of shear after 24 h of ageing. In summary, our study demonstrates for the first time that microgels of EWP have distinct advantages over EWP itself in terms of generating edible foams with ultra-high stability against disproportionation.

Tuning the pore structure of porous tin foam electrodes for enhanced electrochemical reduction of carbon dioxide to formate

Tin-based catalysts have been regarded as a promising candidate for electrochemical reduction of CO2 to produce value-added chemicals and reduce CO2 emissions. However, the catalysts still suffer from unsatisfied performance like large overpotential, rapid deactivation, and lower stability. In this work, the tunable pore structure of porous Sn foam electrodes were prepared using a simple acetic acid (bubble stabilizer)-assisted hydrogen bubble dynamic template method and evaluated as electrocatalysts for CO2 reduction. The Sn-0.15 (bubble stabilizer concentration of 0.15 M in precursor electrolyte), characterized by pore diameter of 50–60 µm and 3.2 times higher of pore number on surface layer of electrode, exhibited an excellent activity towards CO2-to-formate conversion with high Faradaic efficiency in a wide potential from −1.2 to −1.8 V, and acquired the maximum of 95.6% at −1.6 V vs. Ag/AgCl. With this porous electrode, formate started to produce at overpotentials as low as 473 mV and showed negligible degradation over electrolysis for 55 h. Electrochemical tests indicated the notable activity was attributed to the coordinative effect of enlarged reaction sites, weakened interaction strength of CO2− on the low-coordination sites, and alleviated reactant transfer resistance at reaction interfaces. These results demonstrate that tuning pore structure of tin catalyst is a convenient and promising approach for efficient CO2 electroreduction.

Ozone ultrafine bubble water induces the cellular signaling involved in oxidative stress responses in human periodontal ligament fibroblasts

ABSTRACT Periodontitis is a chronic inflammatory disease caused by oral microorganisms in the subgingival biofilm. Stable aqueous ozone ultrafine bubble water (OUFBW) has recently begun to be used as an antiseptic in the treatment of periodontitis. The effectiveness of OUFBW is thought to depend on the bactericidal actions of dissolved ozone exerted via its oxidizing effect. On the other hand, the effects of ozone on the periodontal tissues are largely unknown. In this paper we examined the cellular responses after OUFBW treatment. Human primary periodontal ligament fibroblasts (hPDLFs) or Ca9-22 human gingival epithelial cells were treated with OUFBW or UV-inactivated OUFBW. The production of reactive oxygen species (ROS), the activation of mitogen-activated protein kinase (MAPK) and the nuclear factor-kappa B (NF-κB) activation were analyzed. The transcript profiles of hPDLFs after OUFBW treatment were also analyzed by RNA sequencing (RNA-seq). Our results showed that OUFBW induces oxidative stress by generating ROS, which, in turn, activated the MAPK pathway. OUFBW triggered activation of c-Fos, a major component of the transcription factor activator protein 1 (AP-1), and also nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2), which possessed a high sensitivity to oxidative stress. The results of RNA-seq analysis revealed that the numerous genes involved in oxidative stress responses or MAPK signaling pathway were up-regulated after OUFBW treatment. Investigation of the signaling pathways activated by OUFBW highlights another aspect of the biological roles of OUFBW, in addition to its bactericidal activity, in the treatment of periodontitis.

Bubble-Population-Balance Modeling for Supercritical Carbon Dioxide Foam Enhanced-Oil-Recovery Processes: From Pore-Scale to Core-Scale and Field-Scale Events

Summary A bubble-population-balance foam-modeling technique is developed to investigate how carbon dioxide (CO2) foam behaves rheologically and propagates in a field-scale radial system. The modeling technique is based on pore-scale events and honors three different foam states (weak, strong, and intermediate) and two steady-state strong-foam-flow regimes (high- and low-quality) measured in core-scale experiments. The model parameters are first obtained from a fit to laboratory–coreflood experimental data, and then the mechanistic model is applied to different types of CO2 foams, ranging from gaseous to supercritical–CO2 foams, represented by various mobilization pressure gradients. The results from the fit to existing coreflood data show that a reasonable match can be made satisfying multiple constraints, such as hysteresis exerted by three foam states, non–Newtonian flow behavior caused by gas trapping and shear–thinning rheology, and bubble stability in different capillary pressure environments. When applied to field–scale scenarios, supercritical–CO2 foams requiring low mobilization pressure gradients propagate much farther than gaseous–CO2 foams, far enough to make use of promising supercritical–CO2 foams in the field. This study, for the first time, theoretically demonstrates why supercritical–CO2 foams should be preferred in the field compared with gaseous N2 or CO2 foams. The companion paper to extend this study to full–field–scale foam propagation in conjunction with gravity segregation is Izadi and Kam (2018).