Collapse Of Nanobubbles Research Articles

Effective cracking of heavy crude oil by using shock-induced nanobubble collapse: A molecular dynamics study

We investigate molecular mechanisms of hydrocarbon cracking in heavy crude oil promoted by nanobubble collapse due to ultrasound-originated shock wave. Our millions-atom molecular dynamics simulations reveal rapid flow of molecules, causing nanobubble collapse and formation of energetic nanojet with a mushroom-like shape. The nanojet apex exhibits extremely high temperature before and just after the bubble collapse, which makes a favorable condition for effective cracking of paraffin molecules. The cracking rate and portion of nanobubble-containing systems are larger than those of perfect system before the bubble collapse, and larger size of nanobubble is preferable for the cracking. The cracking efficiency is improved for the paraffin molecules with longer carbon chain.

Electrostatic-Field-Induced Collapse of Nanobubbles in Nanochannels.

The adsorbed nanobubbles inside the nanochannels can cause fluid transport blockages, which will obviously degrade the nanodevice performance and reduce the lifetime. However, due to small-scale effects, the removal of nanobubbles is a huge challenge at the nanoscale. Herein, molecular dynamics simulations are carried out to study the effect of the electrostatic field on underwater nitrogen nanobubbles confined in nanochannels. It is found that the nanobubbles will collapse under an appropriate electrostatic field, thereby unblocking the transport of water in the nanochannels. The formation of ordered water structures induced by electrostatic fields plays an important role in the removal of nanobubbles from the nanochannels. Our findings provide a convenient, controllable, and remote way to address the blockage problem of nanobubbles in nanochannels, which may have potential applications in improving the performance of fuel cells.

Molecular dynamics simulations of a nanobubble’s collapse-induced erosion on nickel boundary and porous nickel foam boundary

Nickel (Ni) alloys and their foam composites are extensively acknowledged materials utilized in the construction of propellers and hulls for the shipbuilding industry. Erosion caused by bubble collapse, a well-documented form of damage on ship rudders, propellers and hulls, can alter the physical properties of Ni and its foam, consequently constraining their potential applications. The present study aims to investigate the process of nanobubble collapse and its impact on the mechanical and thermal properties of Ni boundary and porous Ni foam boundary. To achieve this, a molecular dynamics (MD) simulations was used to study nanobubble collapse dynamics near the boundaries. Results indicated that the nanobubble collapses due to the contrasting pressure and velocity at the interface between the shock wave and the nanobubble surface. As the nanobubble collapses, its volume decreases, resulting in the formation of a water nano-hammer with a pressure and temperature of approximately 30 GPa and 5000 K. The high impact of the nano-hammer leads to erosion in the case of the Ni boundary and compression in the porous Ni foam boundary. The mechanical stability of an eroded Ni boundary decreases due to the creation of dislocations under the impact of the nano-hammer. However, the thermal stability of the Ni boundary remains relatively unaffected by the erosion process. The compressed Ni foam boundary exhibits a lower specific surface area, smaller cell size, and shorter struts compared to the uncompressed Ni foam boundary, resulting in increasing of the mechanical and thermal stability.

Analysis of nanobubble collapse process by molecular simulation method

This study employs molecular dynamics simulations to investigate the process of nanobubble gradual indentation and eventual collapse. The research primarily focuses on the mechanisms by which impact velocity and bubble size influence the dynamic characteristics of nanobubble collapse. The results indicate that nanobubble collapse generally proceeds through three stages. Initially, there is a compression phase of water molecules surrounding the bubble, followed by a phase where the shock wave disrupts the stable structure of the liquid film, and finally, the complete collapse of the bubble. At higher impact velocities, smaller bubbles collapse more rapidly due to stronger shock effects. Post-collapse, a high-speed jet forms a protrusion on the right end of the velocity contour. The degree of protrusion increases with bubble size and impact velocity. Water molecules converge towards the bubble center, forming vortex structures above and below the bubble, effectively enhancing internal mass transfer. As bubble size and impact velocity increase, the density around the bubble gradually rises, reaching approximately 1.5 g/cm³ in localized areas upon complete collapse. When the bubble system decays to half its original size, a water hammer effect occurs. This effect becomes more pronounced with increasing bubble size and impact velocity. For a nanobubble structure with <i>u</i><sub>p</sub> = 3.0 km/s and <i>D</i> = 10 nm, the local pressure formed by the water hammer impact of the jet after collapse can reach 30 GPa.

Resonance and relaxation effects governing the sorption of O2 and CO2 in deionized water under dynamic conditions at 21 ± 1 °C

The behavior of water samples (deionized, deaerated and washed with He) was investigated in dynamics by physicochemical and electrochemical methods. The susceptibility of water samples to sorption of O2 and CO2 from air depends on their pretreatment. Sorption of O2 and CO2 accompanied by the desorption of dissolved helium is a multivariable nonlinear process, presumably, based on resonance and relaxation phenomena within the surface layer. Sorption of O2 and CO2 in water is considered as an interaction between gas molecules and ensembles of H2O molecules, which is assumed to be based on resonance effects. Sorption of gases leads to a, decrease in the surface tension at the water/gas interface. The effect is most pronounced in case of He-washed samples. The decrease in the surface energy upon CO2 absorption and the increase in the relaxation properties of the water surface are probably factors that facilitate sorption of gases, including O2. A fraction of sorbed O2 are contained in nanobubbles formed via water surface relaxation. A change in the viscosity of water samples during the sorption of gases may serve as a relaxation factor favorably affecting the formation and stability of nanobubbles in the surface layer and in bulk water. Irradiation of water with a laser light at λ = 633 nm can generate nano-cavities in the laser beam. In case of very low dissolved O2 concentrations, nanobubbles have no noticeable effect on the oxygen reduction reaction. Water samples washed with helium demonstrate a higher physical and chemical activity compared to other water samples, the activity being manifested in sorption properties. Electrochemical properties of water samples - deionized, deaerated and washed with He - are somewhat different. This is primarily due to the different content of dissolved O2 in the samples. Nonempirical simulations helped in clarifying the experimental data. The key factor that predetermines stability and/or collapse of nanobubbles is the organization of the hydrogen-bond network of water that strongly depends on the presence of water ions and foreign particles. In the case of both deionized water and water washed with He, HCO3-anion cannot participate in stabilization of nanobubbles. On the other hand, OH– anions can stabilize nanobubbles.

Molecular dynamics simulation of ultrasound cavitation occurring in copper–water nanofluid

It is necessary to reveal the impact of nanoparticles on ultrasonic cavitation phenomena in nanofluids, which is conducive to the heterogeneous nucleation applications of ultrasonic cavitation. In this work, the ultrasonic cavitation processes in pure water and nanofluids were simulated by molecular dynamics. Then, the effect of nanoparticles on ultrasonic cavitation was investigated by adding alternate positive and negative pressure waves. After that, the formation of critical bubbles in cavitation and the collapse of nanobubbles by shock waves were studied by using Voronoi mosaic method and rigid body model, respectively. Finally, the regenerated nanobubbles were analyzed after the collapse of nanobubbles. The results show that the nanoparticles could promote the formation of nanobubbles, and consequently, the nano-jets and nanoparticles movement occur during the collapse of nano-bubbles. Additionally, more tiny cavities generated after the collapsing of nanobubbles and the number of nanobubbles during second cycle will be larger than that of the first cycle. As a result, some of these cavities promote the generation of multiple ultrasonic cavitation bubbles in the subsequent ultrasonic cycle. This leads to the chain reaction effect of ultrasonic cavitation phenomenon occurring in nanofluids.

Nanobubble Collapse Induced Erosion near Flexible and Rigid Boundaries: A Molecular Dynamics Study

In this work, we performed molecular dynamics simulations to study the dynamics of a shock wave-induced single nanobubble collapsing near one flexible and two rigid boundaries. The flexible boundary consisted of polyethylene, and the rigid boundaries were made of aluminum and iron. The shock waves impinging on the nanobubble inside a molecular system were generated using a momentum mirror approach. For two relative wall distances, we studied the dynamics of the shock-induced single nanobubble and its collapse near the flexible and the rigid boundaries. The atomic velocity contours surrounding the single nanobubble and the collapse-induced damage on the boundaries were analyzed. We obtained this collapse-induced damage from ten collapsing nanobubbles. Results showed that the relative wall distance affected the single nanobubble’s collapse dynamics near the boundaries. A generated nanojet was directed on the surfaces during the collapse process. From the collapse-induced damage point of view, the depth damage of the polyethylene, iron, and aluminum boundaries for the relative wall distance of γ = 1.3 were 6.0, 0.47 and 0.63 nm, respectively. It was observed that the extensive collapse-induced damage occurred only on the polyethylene boundary.

Reactive-dynamic characteristics of a nanobubble collapse near a solid boundary using molecular dynamic simulation

In the current research, we studied the collapse mechanism of the nanobubble under mirror and real wall protocols using molecular dynamics simulation. Moreover, we analyzed reactive properties of the real wall during the collapse process. Towards this aim, an aluminum (Al) slab has been considered as a real wall, and its behavior after the formation of a nanojet has been investigated. The obtained results indicated that the dynamics of nanobubble collapse under the mirror and real protocols are similar. The collision between the shock wave and the nanobubble leads to the collapse of nanobubble, and the nanojet was formed during this collapse process. However, the nanobubble can collapse sooner when the Al slab is used as a real wall. Moreover, the surface roughness of the Al slab during the shock propagation and nanobubble collapse was increased due to the formation of the chemical reaction between Al and water under the real wall protocol, while the mirror wall has a roughness-free surface from the beginning to the end. The chemical reaction 2Al + H2O→ AlOH + AlH creates the amorphous layer containing the AlOH and AlH species on the surface of the Al slab. This layer grows semi-smoothly during bubble shrinkage and collapse process, while the growth type was changed to an island shape after the complete collapsing. The island shape on the Al slab was formed behind the nanojet due to the water vortices that are formed after the nanobubble collapse.

Oxygen-Rich Graphene/ZnO2-Ag nanoframeworks with pH-Switchable Catalase/Peroxidase activity as O2 Nanobubble-Self generator for bacterial inactivation

The oxygen-rich organic/inorganic (reduced graphene oxide (rGO)/ZnO2-Ag) nanoframeworks as suppliers of O2 nanobubbles (NBs) with dual pH-and-temperature-sensitive behavior were developed to suppress bacterial growth. It was demonstrated that not only the rate but also the final product of oxygen-rich ZnO2 decomposition (to an intermediate product of H2O2) rate was dramatically controlled by pH adjustment. Furthermore, in the presence of Ag nanoparticles, ̇OH radical generation switched to O2 NBs evolution by shifting the pH from acidic to basic/neutral conditions, demonstrating an adjustable nanozyme function-ability between catalase and peroxidase-like activity, respectively. Antibacterial properties of the in-situ generated O2 NBs substantially enhanced against bacterial models including methicillin-resistant Staphylococcus aureus in the presence of rGO. In fact, deflecting the electrons from their main respiratory chain to an oxygen-rich bypath through rGO significantly stimulated reactive oxygen species (ROS) generation, combating bacteria more efficiently. Moreover, NIR laser irradiation-induced temperature rise (due to the inherent photothermal properties of rGO) facilitated ZnO2 decomposition and accelerated growth and collapse of NBs. The simultaneous microscale thermal and mechanical destructions induced stronger antibacterial behavior. These results hold great promises for designing simple organic/inorganic nanoframeworks as solid sources of NBs with tunable enzyme-like ability in response to environmental conditions suitable for forthcoming graphene-based bio-applications.

Degradation Mechanism of Micro-Nanobubble Technology for Organic Pollutants in Aqueous Solutions.

Micro-nanobubbles (MNBs) technology has emerged as an effective means of sewage treatment, while the molecular mechanism for its pollutant degradation is still unknown. In this paper, the reactive molecular dynamics simulation technique is used to study the degradation mechanism of pollutants caused by shock-induced nanobubble collapse. We first demonstrate that the propagating shock wave can induce nanobubble collapse, and the collapsing nanobubble has the ability to focus mechanical energy via the converging motion of liquid in the interior of the bubble, leading to the formation of a high-speed jet with a much higher energy density. We also unveil the mechanical nature of long-chain pollutant degradation and the mechanism of free radical generation. Due to the impacting jet, the high-gradient flow has the ability to stretch the long-chain molecule and cause mechanical scission of the molecule in a homolytic manner. Finally, our simulation results reveal that adding ozone molecules to the collapsing bubble would introduce an additional dehydrogenation mechanism.

Dynamic Processes of Nanobubbles: Growth, Collapse, and Coalescence

Abstract Nanobubbles are typically classified as gas/vapor phase cavities in an aqueous solution with a characteristic length of approximately 100 nanometers (nm). The theoretical lifetime of these nanobubbles has been estimated to be less than ∼1 μs at a diameter of 100 nm based upon the Young-Laplace pressure, but experimental observations have been reported that indicate that they may exist for many hours, or even days. These nanobubbles can be generated by a number of different methods, such as solvent exchange, pressure and/or temperature variations, chemical reactions, or through the electron beam radiolysis of water. The imaging methods utilized to observe these nanobubbles have evolved from low temporal resolution/high spatial resolution, using atomic force microscopy (AFM); or low spatial resolution/high temporal resolution, using optical microscopy (X-rays); or finally, high spatial/high temporal resolution using more recent electron microscopy techniques. A review of the various methods utilized in the nucleation of nanobubbles and the different imaging technologies utilized, along with a summary of the most recent experimental and theoretical investigations of the dynamic behavior and processes of these nanobubbles, including nanobubble growth, nanobubble collapse, and nanobubble coalescence, are presented, discussed and summarized.

Molecular dynamics analysis of shock wave-induced nanobubble collapse in water

The nanobubbles contained in the human body are induced to collapse by the shock wave, and thus produce a strong impact and high-speed nanojet, resulting in trauma to human tissues. The collapse of nanobubbles in water caused by shock waves is investigated by molecular dynamics. Nanobubbles are divided into three types: vacuum nanobubble, carbon dioxide nanobubble, and oxygen nanobubble. The influence of factors such as the number of gas molecules, the diameter of the nanobubbles, and the impulse of the shock wave on the bubble collapse are considered separately. The results show that the addition of gas molecules to vacuum nanobubbles does not affect the propagation of shock waves. However, before the nanobubbles are completely collapsed, the maximum velocity of the nanojet formed by the collapse of nanobubbles containing 718 carbon dioxide molecules (or 733 oxygen molecules) is larger than that of vacuum and nanobubbles containing 1368 carbon dioxide molecules (or 1409 oxygen molecules). After the nanobubbles are completely collapsed, the gas molecules cause the velocity of the nanojet to decay, and finally the maximum velocity of the nanojet containing gas molecules is less than that of the vacuum nanojet. In addition, it is also found that the collapse time of nanobubbles is short at high impulse, and the density and pressure when the shock wave passes at the same time are both greater. After the bubble collapses, the maximum velocity of the nanojet is larger, and the impact force is much stronger than that at a small impulse. Larger diameter nanobubble has a longer collapse time, and the density and pressure when the shock wave passes at the same time are both smaller, and the shock wave propagation is slower, but the maximum speed of the nanojet is larger. The impact is stronger. The greater the maximum velocity of the nanojet, the greater the distance that is dispersed by the gas molecules of the gas-containing nanobubbles in the impact direction will be and the deeper the depression.

Shock-induced collapse of surface nanobubbles.

The collapse of cavitation bubbles often releases high-speed liquid jets capable of surface damage, with applications in drug delivery, cancer treatment, and surface cleaning. Spherical cap-shaped surface nanobubbles have previously been found to exist on immersed substrates. Despite being known nucleation sites for cavitation, their collapsing dynamics are currently unexplored. Here, we use molecular dynamics simulations to model the shock-induced collapse of different surface nanobubble sizes and contact angles. Comparisons are made with additional collapsing spherical nanobubble simulations near a substrate, to investigate the differences in their jet formation and resulting substrate pitting damage. Our main finding is that the pitting damage in the surface nanobubble simulations is greatly reduced, when compared to the spherical nanobubbles, which is primarily caused by the weaker jets formed during their collapse. Furthermore, the pit depths for surface nanobubble collapse do not depend on bubble size, unlike in the spherical nanobubble cases, but instead depend only on their contact angle. We also find a linear scaling relationship for all bubble cases between the final substrate damage and the peak pressure impulse at the impact centre, which can now be exploited to assess the relative damage in other computational studies of collapsing bubbles. We anticipate the more controlled surface-damage features produced by surface nanobubble cavitation jets will open up new applications in advanced manufacturing, medicine, and precision cleaning.

Oxygen nanobubbles enhanced photodegradation of oxytetracycline under visible light: Synergistic effect and mechanism

Nanobubbles have gained much attention due to their unusual properties such as large specific surface area, surface energy and internal energy, thereby, increase the surface reaction and improve the mass transfer efficiency. The long term stability of nanobubbles and generation of reactive oxygen species enables them to address various environmental issues for water purification without secondary pollution. However, there are few studies concerning the application of oxygen nanobubbles in the advanced oxidation processes (AOPs). In this study, first time, oxygen nanobubbles were introduced into the photoreaction system under visible light irradiation for enhanced photodegradation of oxytetracycline (OTC). Results indicated that oxygen nanobubbles significantly improved the photodegradation of OTC through high mass transfer and generation of reactive radicals during the nanobubble collapse. The oxygen nanobubble stability was confirmed to be pH dependent. The nanobubbles concentration increased from 0.76 to 3.78 × 108 particles/mL, while the mean size decreased from 205 nm to 138 nm as the pH value increased (3.2–11.0). The photodegradation efficiency of OTC was improved from 45% to 98% with the increase of pH (4.0–11.0). Oxygen nanobubbles hold promise to improve the AOPs, such as reducing catalysts and chemicals usage and costs. Quenching experiments demonstrated that OH was the predominant active species induced the photodegradation of OTC. With the development of nanobubbles technology, it is anticipated to realize totally chemicals free green technology. This study provides a novel nanobubble/AOPs technology, which will be more efficient and environmental friendly and will have broad application in waste water treatment and drinking water purification.

Numerical evaluations of membrane poration by shockwave induced multiple nanobubble collapse in presence of electric fields for transport through cells

Synergistic applications of an electric field combined with nanojet-based mechanical pressure, have recently been shown to help create larger pores and provide control of the aspect ratio in biological membranes. The nanojets are formed by the collapse of nanobubbles in the vicinity of biomembranes upon being subjected to external shockwaves. Here we analyze the effects produced by the collapse of multiple nanobubbles in the presence of an electric field. Our simulations, based on molecular dynamics, show that not only would multiple nanobubbles make it possible to create larger pores, but also increase the pore density on the surface of biological cells. Both aspects could aid in the transport of drugs and genes for bio-medical applications.

Nanoscale cavitation in perforation of cellular membrane by shock-wave induced nanobubble collapse

The collapse of the bubble induced by the shock wave leads to nano-jet, which is able to perforate cellular membranes. This phenomenon is investigated by Martini coarse-grained molecular dynamic (CG-MD) simulations in the present study. It is found that the occurrence of cavitation nucleation at the nanoscale can be observed during the perforation process. The cavitation locates near the puncture of the cell membrane and its ultimate evolutionary form presents a ring-like structure. The volume of the cavitation is calculated for different initial bubble sizes, and it is found that the maximum volume of the cavitation area has a correlation with the initial bubble size. To understand the underlying physics of the cavitation phenomenon, the classical nucleation theory based on the Rayleigh-Plesset equation is applied to the non-equilibrium nanoscale system after the pressure field is obtained by using the Irving-Kirkwood-Noll procedure. The consistence between the results of CG-MD and the theory reveals that the average pressure of the local environment plays a crucial role in cavitation occurrence on the non-equilibrium system subjected to strong inertia, e.g., shock wave and nano-jet.

Effects of nanobubble collapse on cell membrane integrity.

Recent studies have shown that ultrasound is used to open drug-carrying liposomes to release their payloads; however, a shockwave energetic enough to rupture lipid membranes can cause collateral damage to surrounding cells. Similarly, a destructive shockwave, which may be used to rupture a cell membrane in order to lyse the cell (e.g., as in cancer treatments) may also impair or destroy nearby healthy tissue. To address this problem, we use dissipative particle dynamic (DPD) simulation to investigate the addition of a cavitation bubble between the shockwave and the model cell membrane to alter the shockwave front, allowing low-velocity shockwaves to specifically damage an intended target. We focus specifically on a spherical lipid bilayer model, and note the effect of shockwave velocity, bubble size, and orientation on the damage to the model cell. We show that a cavitation bubble greatly decreases the necessary shockwave velocity required to damage the lipid bilayer and rupture the model cell. The cavitation bubble focuses the kinetic energy of the shockwave front into a smaller area, inducing penetration at the edge of the model cell. With this work, we provide a comprehensive approach to the intricacies of model cell destruction via shockwave impact, and hope to offer a guideline for initiating targeted cellular destruction using induced cavitation bubbles and low-velocity shockwaves.

Ultrafast Near-Infrared Light-triggered Intracellular Uncaging to Probe Cell Signaling.

The possibility of regulating cell signaling with high spatial and temporal resolution within individual cells and complex cellular networks has important implications in biomedicine. In this report, we demonstrate a general strategy that uses near-infrared tissue-penetrating laser pulses to uncage biomolecules from plasmonic gold-coated liposomes, i.e. plasmonic liposomes, to activate cell signaling in a non-thermal, ultrafast and highly controllable fashion. Near-infrared picosecond laser pulse induces transient nanobubbles around plasmonic liposomes. The mechanical force generated from the collapse of nanobubbles rapidly ejects encapsulated compound within 0.1 ms. We showed that single pulse irradiation triggers the rapid intracellular uncaging of calcein from plasmonic liposomes inside endo-lysosomes. The uncaged calcein then evenly distributes over the entire cytosol and nucleus. Furthermore, we demonstrated the ability to trigger calcium signaling in both an immortalized cell line and primary dorsal root ganglion (DRG) neurons by intracellular uncaging of inositol triphosphate (IP3), an endogenous cell calcium signaling second messenger. Compared with other uncaging techniques, this ultrafast near-infrared light-driven molecular uncaging method is easily adaptable to deliver a wide range of bioactive molecules with an ultrafast optical switch, enabling new possibilities to investigate signaling pathways within individual cells and cellular networks.

Molecular modeling of transmembrane delivery of paclitaxel by shock waves with nanobubbles

The development of advanced delivery strategies for anticancer drugs that can permeate through cellular membranes is urgently required for biomedical applications. In this work, we investigated the dynamic transmembrane behavior of paclitaxel (PTX), a powerful anticancer drug, under the combined impact of shock waves and nanobubbles, by using atomistic molecular dynamics simulations. Our simulations show that the PTX molecule experiences complicated motion modes during the action process with the membrane, as a consequence of its interplay with the lipid bilayer and water, under the joint effect of the shock wave and nanobubble. Moreover, it was found that the transmembrane movement of PTX is closely associated with the conformation changes of PTX, as well as the structural changes of the membrane (e.g., compression and poration in membrane). The nanobubble collapse induced by the shock wave, the proper PTX location with respect to the nanobubble, and a suitable nanobubble size and shock impulse are all necessary for the delivery of PTX into the cell. This work provides a molecular understanding of the interaction mechanism between drug molecules and cell membranes under the influence of shock waves and nanobubbles, and paves the way for exploiting targeted drug delivery systems that combine nanobubbles and ultrasound.

Stress wave emission from plasmonic nanobubbles

Stress wave emission from the collapse of cavitation nanobubbles, generated after irradiation of single-spherical gold nanoparticles with laser pulses, was investigated numerically. The significant parameters of this study are the nanoparticle radius, laser pulse duration, and laser fluence. For conditions comparable to those existing during plasmonic photothermal therapy, a purely compressive pressure wave is emitted during nanobubble collapse, not a shock. In the initial stage of its propagation, the stress wave amplitude is proportional to the inverse of the stress wave radius. The maximum amplitude and the duration of the stress wave decreases with the laser fluence, laser pulse duration, and gold nanoparticle radius. The full width at half maximum duration of the stress wave is almost constant up to a distance of 50 µm from the emission center. The stress wave amplitude is smaller than 5 MPa, while the stress wave duration is smaller than 35 ns. The stress wave propagation results in minor mechanical effects on biological tissue that are restricted to very small dimensions on a cellular or sub-cellular level. The stress wave is, however, able to produce breaching of the human cell membrane and bacterial wall even at distances as large as 50 µm from the emission centre. The experimentally observed melting of gold nanoparticles comes from the large temperature reached inside the nanoparticles during laser irradiation and not from the propagation of the stress wave into the surrounding liquid during nanobubble rebound.