Underwater Bubble Research Articles

Dependence of EHD instability of plasma/liquid interface on liquid conductivity

Besides numerous experimental reports revealing the characteristic dependence of depression on liquid surfaces induced by impinging plasma beams, and despite its scientific and practical importance, the physical mechanism responsible for this dependency has been missed. In this study, based on the border electrons' role, the water/plasma interfacial dynamics relevant to electrohydrodynamic instability (EHD) have been theoretically modeled, focusing on the characteristic dependence of the EHD growth rate driven by charge mobility mechanisms. The predictions of the growth rates dependent on water conductivity from theory agree well with our observations of faster plasma-filled underwater bubble explosions under lower conductivity conditions, indicating that the model contains the essence of the underlying physics of liquid surface deformation in the presence of plasma.

Damage effect of underwater explosion bubble on concrete gravity dam

The underwater explosion bubble accounts for half of the total explosive energy and has been found to induce serious structural damage to ship structures. However, the existing research has commonly ignored the damage effect of an underwater explosion bubble on a concrete gravity dam. The current study aims to narrow this gap through high-fidelity numerical simulations and scale model tests. A fully-coupled numerical model is established first and is validated by small-scale centrifugal model tests. The bubble dynamics of underwater explosions near the perpendicular dam structure have been explored. The main focus is then on the sequential damage effects of the underwater explosion shock wave, bubble pulse, bubble jet, and bubble collapse on a concrete gravity dam. It is found that the shock wave causes a first attack, inducing tensile cracks in the vulnerable part of the dam. Subsequently, the bubble pulse generates a secondary attack, facilitating the penetration of the tensile crack throughout the entire dam structure. Additionally, the bubble jet driving the high-speed fluid flow and the bubble collapse causing the uplifted free water surface, united with the upstream reservoir water with huge hydrostatic pressure, largely accelerate the fracture of the dam, then facilitate the fractured dam to fall downstream leaving a breach with certain widths in the dam’s upper part, and finally speed up the discharge of the upstream reservoir water through the breach. It also finds that the underwater explosion bubble can generate no new cracks in the gravity dam, even in the pre-damaged area as long as there are no obvious pre-cracks by the shock wave. That is, the shock wave is the key trigger for dam failure, without which the dam failure will not happen, while the bubble, united with the upstream reservoir water, accelerates the process of dam failure.

On the bubble-bubbleless ocean continuum and its meaning for the LiDAR equation: LiDAR measurement of underwater bubble properties during storm conditions

This paper presents the NRL shipboard LiDAR and the first LiDAR dataset of underwater bubbles. The meaning of these LiDAR observations, the algorithms used and their current limitations are discussed. The derivation of the LiDAR multiple scattering regime is derived from the LiDAR observations and theory. The detection of the underwater bubble presence and their depth is straightforward to estimate from the depolarized laser return. This dataset strongly suggest that the whitecaps term in the LiDAR equation formalism needs to be revisited. The retrieval of the fraction of air volume within a given volume of water (void fraction) is possible and the algorithm is stable with a simple ocean backscatter LiDAR system. The accuracy of the void fraction retrieval will increase significantly with future developments.

Adjusting droplet adhesion of superhydrophobic coating via surface embedding of microparticles with mixed shapes

Designing intelligent superhydrophobic surfaces with versatile surface properties has garnered scholarly attention in field of heterogeneous catalysis, lab-on-chip, water harvesting, etc. However, the precise modulation of fluid adhesion on superhydrophobic surfaces, particularly when being applied as coatings, remains a challenge. Here we employ a “glue + powder” method for the fabrication of superhydrophobic surfaces with tunable droplet adhesion by embedding microparticles with mixed shapes. By adjusting the mixing ratio of rod-like and spherical particles, the sliding angle of superhydrophobic coating can be facilely controlled as well as the underwater bubble holding ability. During the dewetting process, the difference in the contact line lengths between micro-rod and micro-sphere contributes to the varying droplet adhesive force from 43.7 μN to 109.4 μN, which corresponds to the rolling-off angle change of a 10 μL droplet from 9.6° to 45.7°. Furthermore, by mixing fumed silica with branched structure, the lowest range of the rolling-off angle adjustment can be extended down to 0.5°, showing a precise and universal strategy for tuning the surface adhesion. On the basis of the meticulous manipulation of surface adhesion, controlling processes of single droplet and droplets accumulation can be fulfilled, with implications for enhancing the functionality of current superhydrophobic materials. Taking the advantage of the convenient and reliable fabrication method, a series of demonstration including manual droplet holders, multi-tiered droplet capturing interface, droplet sieving platforms, and gravity-driven droplet reactors has been realized. We envision that the precise control of droplet adhesion on superhydrophobic materials should further provides substantial opportunities for innovating fluid manipulation systems possessing heightened operability and multifunctionality.

Biomimetic multilayer flexible sensors for multifunctional underwater sensing

The development of multifunctional flexible sensors capable of monitoring different physiological signals of the body underwater holds great practical significance in safeguarding the well-being of divers during their subaquatic activities. Here, we prepared a multilayer multifunctional sensor that mimics the structure of human skin by sequentially constructing a sensing layer formed by multi-walled carbon nanotubes and a hydrophobic protective layer formed by organosilicon dioxide (o-SiO2) on the elastomer surface. The o-SiO2 layer is superhydrophobic and breathable, also allowing the sensing layer to be only partially wetted. The sensor can monitor human respiration by sensing humidity due to breathability of protective layer. Additionally, the superhydrophobic surface ensures stable strain monitoring even underwater. The partially wetted sensing layer allows the sensor to respond to its immersion and surfacing processes, as well as underwater bubbles. These multifunctional sensors are capable of real-time monitoring of divers' physiological signals, including breathing patterns and motions, throughout underwater activities.

Experimental research into the dynamics of underwater explosion bubbles near mutually perpendicular walls

It is of great significance to characterize the dynamics of underwater explosive bubbles in close proximity to mutually perpendicular walls for ensuring the safety of important underwater structures. In this paper, a dynamic experiment on underwater explosion bubbles was carried out near constructed mutually perpendicular walls. High-speed cameras were utilized to capture high-resolution images, while pressure sensors recorded pressure–time history curves. The main focus was on studying the evolution process of bubble morphology and pulse characteristics. When the position of the charge's center relative to the horizontal wall remained fixed, decreasing the distance between the charge's center and the vertical wall resulted in a reduction in the equivalent maximum radius of bubbles and an increase in its pulsation period. Additionally, the asymmetric collapse of bubbles on a single wall transformed into asynchronous collapse on two walls, with most collapsed bubbles tending to migrate and expand toward the corner formed by mutually perpendicular walls. The resulting jet from the collapse of bubbles exhibited deflection toward the vertical wall, with an inclination angle increasing approximately proportionally with dimensionless distance ratio γh/γv. Moreover, it became more difficult for achieving effective focusing of bubble energy as the jet approached the corners formed by mutually perpendicular walls. The experiments also implied that reducing the dead weight of the vertical wall weakened its contact with the horizontal wall, causing an increase in the equivalent maximum radius of bubbles and jet inclination, as well as a decrease in the bubble pulsation period, under the same dimensionless distance γv.

Effects of computational domain in underwater explosion numerical simulation on bubble pulsation

To clarify the influence of the computational domain size on the underwater explosion bubble pulsation, a one-dimensional axisymmetric model of the underwater explosion was established. The computational model was verified using empirical formulas. Further research was conducted on the variation patterns of the bubble pulsation period and maximum radius with changes in the proportional computational domain radius. The results indicate that as the proportional computational domain radius increases, the maximum radius of the bubble and the bubble pulsation period gradually increase and stabilize. The proportional computational domain radius required to achieve stability is related to the static water pressure of the computational domain. Increasing static water pressure makes the required computational domain size for achieving high numerical accuracy smaller. When the proportional computational domain radius reaches around 1600, the calculation results are highly accurate under different static water pressures at different depths of explosion.

Hollow NiMo-based nitride heterojunction with super-hydrophilic/aerophobic surface for efficient urea-assisted hydrogen production

Hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) are key reactions of the water-cycling associated catalytic process/device. The design of catalysts with a super-hydrophilic/aerophobic structure and optimized electron distribution holds great promise. Here, we have designed a three-dimensional (3D) hollow Ni/NiMoN hierarchical structure with arrayed-sheet surface based on a one-pot hydrothermal route for efficient urea-assisted HER based on a simple hydrothermal process. The Ni/NiMoN catalyst exhibits super-hydrophilic/aerophobic properties with a small droplet contact angle of 6.07° and an underwater bubble contact angle of 155.7°, thus facilitating an escape of bubbles from the electrodes. Density functional theory calculations and X-ray photoelectron spectroscopy results indicate the optimized electronic structure at the interface of Ni and NiMoN, which can promote the adsorption/desorption of reactants and intermediates. The virtues combining with a large specific surface area endow Ni/NiMoN with efficient catalytic activity of low potentials of 25 mV for HER and 1.33 V for UOR at 10 mA cm−2. The coupled HER and UOR system demonstrates a low cell voltage of 1.42 V at 10 mA cm−2, which is approximately 209 mV lower than water electrolysis.

Breakdown dynamics and instability of underwater metallic aerosol bubble atomized by electrical explosion

This study delves into the electrophysical processes and intricate fluid dynamics of an electrical-explosion-induced bubble in water. A fine copper wire is heated up and exploded to dense metallic aerosol (vapor–drop mixture) via a μs-timescale 10 kA current pulse, crossing a wide range of the density–temperature parametric space. High-speed photography along with discharge diagnostics reveals two modes for plasma development (restrike) inside explosion products: gas discharge and volume ionization. Experimental results indicate the metal–insulator transition of metal can easily throttle down circuit current at a moderate degree of vaporization, resulting in a free-expanding metallic aerosol in the presence of a quasi-direct current axial electric field of kV/cm level. After dozens of μs, an anode-directed, “ionization wave” is observed inside the aerosol bubble, propagating with a speed of 3–10 km/s. Remarkably, adjustments in the electric field permit the observation of cathode-directed discharge development. Increasing the charging voltage or wire diameter promotes the overheating degree, accompanied by partial ionized striation of electro-thermal instability. With sufficient high overheating of the wire (ξ > 1), the gas discharge disappears and restrike is dominated by volume ionization.

Robust Free-Space Optical Communication Utilizing Polarization for the Advancement of Quantum Communication.

Free-space optical (FSO) communication can be subject to various types of distortion and loss as the signal propagates through non-uniform media. In experiment and simulation, we demonstrate that the state of polarization and degree of polarization of light passed though underwater bubbles, causing turbulence, is preserved. Our experimental setup serves as an efficient, low cost alternative approach to long distance atmospheric or underwater testing. We compare our experimental results with those of simulations, in which we model underwater bubbles, and separately, atmospheric turbulence. Our findings suggest potential improvements in polarization based FSO communication schemes.

Underwater Bubble Manipulation on Surfaces with Patterned Regions with Infused Lubricants.

The flexible manipulation of underwater gas bubbles on solid substrates has attracted considerable research interest from scientists in the fields of water electrolysis, bubble microreactions, drug delivery, and heat transfer. Inspired by the oxygen-binding mechanisms of aquatic organisms, scientists have designed a series of interfacial materials for use in collecting gases, detecting and grading bubbles, and conducting microbubble reactions. Aerophilic surfaces are commonly used in underwater bubble manipulation platforms due to their excellent gas-trapping properties. However, during bubble transport, some of the bubbles are retained in the rough structure of the aerophilic surface and cause gas loss, which in the long run reduces the gas transport function. In addition, the aerophilic surface is prone to failure in high-humidity and high-pressure underwater environments. The lubricant-infused surface features an oil layer that remains stable on a rough substrate and is immiscible with water. Additionally, the bubbles are transported over the oil layer without causing losses other than those dissolved in water. These attributes make it more favorable than the aerophilic surface. Inspired by the unique properties of Nepenthes and cactus spines, we developed a patterned slippery surface [patterned lubricant-infused surface (PLIS)] through laser etching and ammonia etching that facilitates the coexistence of superaerophobic and aerophilic surfaces. The PLIS executes bubble capture utilizing a difference in wettability measuring 78°, transports bubbles through Laplace force and buoyancy, and regulates bubble release by restricting the contact area on the PLIS. The PLIS can be prepared rapidly and affordably in just about an hour, and its potential for large-scale production is high. Following tests for shear, acid and alkali resistance, and corrosion resistance, the PLIS exhibited impressive weathering resistance and appears to have potential for application in some extreme environments.

Reactive species and mechanisms of perfluorooctanoic acid (PFOA) degradation in water by cold plasma: The role of HV waveform, reactor design, water matrix and plasma gas

The scope of this study was to optimize dielectric barrier discharge (DBD) plasma for the degradation of perfluorooctanoic acid (PFOA) in water by comparing several critical aspects such as pulsed-plasma waveform (nanosecond and microsecond high voltage (HV) pulses), reactor configuration (gas–liquid treatment and underwater plasma bubbles), water matrix and plasma gas. The physicochemical properties and species concentration of plasma-treated water, under the aforementioned conditions, were also determined and correlated with PFOA degradation. PFOA was more efficiently degraded by air–liquid DBD compared to DBD-based underwater air-plasma bubbles consistent with the surfactant nature of PFOA and the more intense plasma-liquid interaction and species formation prevailing during gas–liquid treatment compared to plasma bubbles treatment. After 30 min of gas–liquid DBD treatment, the degradation of PFOA in ultrapure water was >99.9, 98.8, 95.1 and 63% under air-, N2-, Ar- and O2-plasma respectively, whereas its degradation was almost equally effective in the whole range of its initial concentration (10 to 0.1 mg/L). Air was almost equally effective at degrading PFOA in ultrapure and tap water, while argon was much more effective at degrading PFOA in tap water (>99.9% after 20 min) possibly due to the prevailing alkaline conditions (pH ∼8.7) favoring its degradation. Faster PFOA degradation was achieved under HV micropulses compared to HV nanopulses. Nevertheless, in terms of energy requirements, HV nanopulses were superior to HV micropulses, with the lowest electrical energy per order achieved under nanopulsed-argon (∼23.6 kWh/m3). The results of this study could contribute important knowledge to the development of plasma-based treatment of PFOA-contaminated water.

Bubble Unidirectional Transportation on Multipath Aerophilic Surfaces by Adjusting the Surface Microstructure.

Comprehending and controlling the behavior of bubbles on solid surfaces is of significant importance in various fields including catalysis and drag reduction, both industrially and scientifically. Herein, Inspired by the superaerophilic properties of the lotus leaf surface, a series of asymmetrically patterned aerophilic surfaces were prepared by utilizing a facile mask-spraying method for directional transport of underwater bubbles. The ability of bubbles to undergo self-driven transportation in an asymmetric pattern is attributed to the natural tendency of bubbles to move toward regions with lower surface energy. In this work, the microstructure of the aerophilic surface is demonstrated as a critical element that influences the self-driven transport of bubbles toward regions of lower surface energy. The microstructure characteristic affects the energy barrier of forming a continuous gas film on the final regions. We classify three distinct bubble behaviors on the aerophilic surface, which align with three different underwater gas film evolution states: Model I, Model II, and Model III. Furthermore, utilizing the energy difference between the energy barrier that forms a continuous gas film and the gas-gas merging, gas-liquid microreaction in a specific destination on the multiple paths can be easily realized by preinjecting a bubble in the final region. This work provides a new view of the microevolutionary process for the diffusion, transport, and merging behavior of bubbles upon contact with an aerophilic pattern surface.

Comparative analysis of blast load transfer and structural damage in multi-cabin structures during air and underwater explosions

This study presents comprehensive experimental and numerical findings regarding the impact of the fluid medium on blast load transfer and structural damage mechanisms in multi-cabin structures subjected to air and underwater explosions. The experiments demonstrated that the structure primarily underwent a comprehensive dynamic response during an air explosion. In contrast, underwater explosions induced severe localized damage, with fluid properties significantly influencing the shockwave intensity. The observed plastic deformation, shear tearing of the structure, and shock wave characteristics in the experiment were successfully predicted through numerical simulations. Additionally, the numerical simulation unveiled distinct mechanisms of pressure changes in the cabin, attributing air and underwater explosions to transmitted pressure waves and compression shocks of the outer plate, respectively. The high-intensity shock waves generated by underwater explosions and bubble loads sequentially impacted the structure, resulting in more pronounced structural damage compared to the weaker shock wave effect of an air explosion. This study provides valuable insights for comprehending and predicting the response of multi-cabin structures in diverse explosion scenarios.

Measurement of pressure and tension waves from underwater bubble collapse using a novel pressure bar sensor

In order to obtain the stress waveforms of the bubble collapse impact, we chose the pressure bar method with a high-sensitivity laser transducer to record the stress wave on the rod. The pressure bar device has been improved to measure multiple consecutive microsecond-scale impact, while retaining its ability to measure tensile effects. The drop-weight calibration method was adjusted to make the stress pulse width and amplitude of the calibration impact on the same order of magnitude as the test signal. A more practical method for measuring underwater impact, especially tension, was established, compared to multi-layer structure sensors. The collapse processes of spark-generated bubbles at different stand-off distances were discussed. The mechanism of tension waves was analyzed by the comparation of stress waveforms and secondary cavitation phenomena, which would make the cavitation impact mechanism analysis no longer limited to pressure.

Numerical Simulation of Icebreaking by Underwater-Explosion Bubbles and Compressed-Gas Bubbles Based on the ALE Method

Icebreaking by using underwater explosion bubbles and compressed high-pressure gas bubbles has gradually become an effective icebreaking method. In order to compare the damaging effect of these two methods on the ice body, a fluid–structure coupling model was established based on the arbitrary Lagrangian–Eulerian (ALE) method and a series of calculations were carried out. The morphological changes of underwater explosion bubbles and compressed gas bubbles at the same energy under the free surface; the changes of flow load near the rigid wall; and the damage caused to the ice plate were studied and compared. The damage effect of the ice plate was analyzed by detecting the number of failure elements of the ice plate, and the optimum standoff distance was found. For an ice plate with a radius of 0.19 m and a thickness of 0.15 m, the optimum standoff distance of the compressed gas bubbles with 120 J is 0.03 m, and the optimum standoff distance of the TNT with 120 J is 0.02875 m. The similarities and differences of the two sources of bubbles on ice plate damage were summarized.

Dynamic behaviors of cavitation bubbles near biomimetic surfaces: A numerical study

This work is devoted to numerical studies of dynamic behaviors of underwater cavitation bubbles and the associated fluid flow phenomena such as high-speed liquid jet and pressure loads characteristics near biomimetic shark skin surfaces. Potential damages to the surface caused by cavitation bubbles are numerically quantified by the pressure peak, pressure duration and pressure impulse at the surface center. This work utilizes a compressible VOF multiphase model in conjunction with user defined functions (UDFs). The model has been validated against published experimental data in terms of bubble morphological evolution and the time-dependent liquid film thickness. The results reveal that the shark skin-inspired surfaces have important influence on bubble dynamics, water jets and shock wave pressures under various stand-off parameters. Violent deformation and splashing of the bubbles with weak liquid jet impact and pressure impulse can be observed on the surface covered with triangular shaped structures, accompanied by a certain degree of attenuation on the impacting strength and range of the shock waves. Additionally, the migrating and coalescing behaviors of bubble pairs are reduced with the increase of the initial interbubble distance. The large bubble–wall distance can considerably alter the direction of the microjet flow away from the shark skin-inspired surface. These findings shed the light for designing and controlling of multiple cavitation bubbles collapse near biomimetic surfaces of high-speed marine applications.

Experimental Study on the Underwater Explosion Bubble Near deformable boundary

Studying near-field underwater explosions is important for the research of submersible and underwater explosive weapons. In this study, we conducted experiments to investigate the coupling of near-field underwater explosion bubbles with titanium alloy plates and steel plates. Our findings show that the boundary of a titanium alloy plate causes the first pulsation period of a bubble to be longer than in the free field, while the boundary of a steel plate causes the first pulsation period of a bubble to be shorter than in the free field. Furthermore, we simulated the process of the explosion and found that changes in the period may be caused by ventilation.

Controllable penetration of underwater bubble via an integrated Janus mesh

Underwater bubble dynamics and manipulation have emerged as an important research hotspot for boiling heat transfer, as well as bubble transport and collection. However, achieving switchable and controllable penetration of bubbles remains a challenge. In this work, by using laser etching and post-modification, Janus meshes with four pore sizes ranging from micron to millimeter sizes were prepared for unidirectional penetration of bubbles. Although typical unidirectional penetration was observed in these Janus meshes of four pore sizes, they all exhibit easy passage of bubbles without the desired controllable and switchable properties for penetration behaviors. The Janus mesh (with a pore size of ∼930 μm) is respectively superimposed with hydrophilic meshes of different pore sizes as integrated meshes. Interestingly, the integrated meshes take a penetration behavior transition of the bubbles from prohibited to a critical state, and easy permeation by changing the pore size and thickness of the hydrophilic mesh. The controllable permeability of the integrated mesh is attributed to different levels of resistance (i.e., the gas pressure at the upper interface, Pup) before acquiring a Laplace pressure. The integrated mesh can function as a bubble switch, offering a flexible and effective strategy for controllable penetration of underwater bubbles.

A comprehensive insight on plasma-catalytic degradation of organic pollutants in water: Comparison between ZnO and TiO2

A novel system combining underwater plasma bubbles and high voltage nanopulses was combined for the first time with ZnO and TiO2 for the degradation of organic pollutants in water. The effect of catalyst loading, discharge power and plasma gas on pollutant degradation was investigated whereas the plasma-catalytic mechanism was explored through the quantification of plasma species, COD/TOC measurements and scavenging experiments in the presence and absence of catalysts. The increased efficiency in the presence of either ZnO or TiO2, especially under plasma gases (air and oxygen) able to produce UV radiation in the range of wavelengths absorbed by both catalysts, lies on the increased concentration of the critical reactive species (e.g. ·O2−, ·ΟΗ, H2O2). Compared to plasma alone process, H2O2 was significantly enhanced in the presence of TiO2 and decreased in the presence of ZnO, whereas ·OH concentration was higher in the plasma-ZnO but lower in the plasma-TiO2 system which supports the overall superior performance of ZnO compared to TiO2. The synergy of plasma-ZnO process compared to that of plasma-TiO2 was ∼2.4 and ∼1.5 times higher for Orange II (OII) and Methylene Blue (MB), respectively, exhibiting a very low electrical energy per order (1.4 kWh m−3 for OII and 0.31 kWh m−3 or MB). The present effort contributes on providing fundamental insights and further expand of plasma-catalysis for water treatment.