Cavitation inception of water with solid nanoparticles: A molecular dynamics study

Near-Wall Cavitation Effect: A Molecular Dynamics Study

Cavitation has been the subject of abundant studies, but the internal mechanism of cavitation is less well known. In this article, a microlevel near-wall model was established by using LAMMPS to present the process of cavitation effect. The results of molecular dynamics simulation revealed the fluctuation process of the liquid near the wall with the change in pressure. Molecular dynamics was also used to evaluate the void volume fraction and density distribution of the system. The results exhibited that the cavitation process can be divided into two stages: the initial cavitation stage and the rapid growth stage. Based on these results, the effects of wettability and initial system temperature on the near-wall cavitation effect were demonstrated. The results indicated that the hydrophobic near-wall forms a gas layer to weaken the density fluctuation, while the hydrophilic wall is opposite. Increasing the temperature could positively affect molecular motion and cavitation. This work provides a theoretical basis for further exploration of the cavitation effect.

CFD-assisted modeling of the hydrodynamic cavitation reactors for wastewater treatment - A review

Hydrodynamic cavitation has been a promising method and technology in wastewater treatment, while the principles based on the design of cavitational reactors to optimize cavitation yield and performance remains lacking. Computational fluid dynamics (CFD), a supplementation of experimental optimization, has become an essential tool for this issue, owing to the merits of low investment and operating costs. Nevertheless, researchers with a non-engineering background or few CFD fundamentals used straightforward numerical strategies to treat cavitating flows, and this might result in many misinterpretations and consequently poor computations. This review paper presents the rationale behind hydrodynamic cavitation and application of cavitation modeling specific to the reactors in wastewater treatment. In particular, the mathematical models of multiphase flow simulation, including turbulence closures and cavitation models, are comprehensively described, whilst the advantages and shortcomings of each model are also identified and discussed. Examples and methods of the coupling of CFD technology, with experimental observations to investigate into the hydrodynamic behavior of cavitating devices that feature linear and swirling flows, are also critically summarized. Modeling issues, which remain unaddressed, i.e., the implementation strategies of numerical models, and the definition of cavitation numbers are identified and discussed. Finally, the advantages of CFD modeling are discussed and the future of CFD applications in this research area is also outlined. It is expected that the present paper would provide decision-making support for CFD beginners to efficiently perform CFD modeling and promote the advancement of cavitation simulation of reactors in the field of wastewater treatment.

Flexible CuS-embedded human serum albumin hollow nanocapsules with peroxidase-like activity for synergistic sonodynamic and photothermal cancer therapy

Nanoparticle flexibility is an important parameter in determining cell uptake and tumor accumulation, thus modulating therapeutic efficiency in cancer treatment. Herein, we successfully prepared CuS-embedded human serum albumin hollow nanocapsules (denoted CuS/HSA) by a hard-core-assisted layer-by-layer coating approach. This approach afforded CuS/HSA hollow nanocapsules with controllable shell thickness, tunable flexibility, uniform size (272.9 nm), a large hollow cavity, peroxidase-like activity, excellent photothermal conversion ability, and a high tetra-(4-aminophenyl) porphyrin (TAPP) loading capacity (27.3 wt%). The peroxidase-like activity of the CuS nanoparticles enabled them to overcome tumor hypoxia and augment the sonodynamic therapeutic (SDT) effects and photothermal conversion ability for photothermal therapy (PTT). In vitro experiments showed that the CuS/HSA-TAPP hollow nanocapsules efficiently induced cancer cell apoptosis under US irradiation and cancer cell ablation under laser irradiation, thus facilitating synergistic SDT and PTT. Importantly, the flexibility of the CuS/HSA hollow nanocapsules resulted in significantly enhanced cellular internalization and a longer mean residence time (131.3 h) than their solid counterparts (21.0 h). In a breast tumor model, the flexible CuS/HSA hollow nanocapsules exhibited high tumor accumulation of up to 27.1%. In vivo experiments demonstrated that the flexible CuS/HSA-TAPP hollow nanocapsules effectively eliminated breast tumors via the synergistic effect of SDT and PTT.

Numerical modelling of acoustic cavitation threshold in water with non-condensable bubble nuclei

Numerical modelling of acoustic cavitation threshold in water is presented taking into account non-condensable bubble nuclei, which are composed of water vapor and non-condensable air. The cavitation bubble growth and collapse dynamics are modeled by solving the Rayleigh-Plesset or Keller-Miksis equation, which is combined with the energy equations for both the bubble and liquid domains, and directly evaluating the phase-change rate from the liquid and bubble side temperature gradients. The present work focuses on elucidating acoustic cavitation in water with a wide range of cavitation thresholds (0.02-30 MPa) reported in the literature. Computations for different nucleus sizes and acoustic frequencies are performed to investigate their effects on bubble growth and cavitation threshold. The numerical predictions are observed to be comparable to the experimental data in the previous works and show that the cavitation threshold in water has a wide range depending on the bubble nucleus size.

Clarification of regimes determining sonochemical reactions in solid particle suspensions

Although there has been extensive research on the factors that influence sonochemical reactions in solid particle suspensions, the role that solid particles play in the process remains unclear. Herein, the effect of monodisperse silica particles (10-100 μm, 0.05-10 vol%) on the sonochemical activity (20 kHz) was investigated using triiodide formation monitoring and luminol tests. The results demonstrate that, in the particle size range considered, the sonochemical yields were enhanced in dilute suspensions (0.05-1 vol%), while further particle addition in semi-dilute suspensions (1-10 vol%) decreased the yields. Two regimes, namely the site-increasing regime and sound-damping regime, are identified in respect of the enhancing and inhibiting effects of the particles, respectively, and their dependence on particle characteristics is analyzed. Both regimes are confirmed based on the cavitation erosion test results or cavitation noise analysis. The clarification of the two regimes provides a better understanding of the dominant factors controlling sonochemistry in the presence of solid particles, as well as a guide for sonochemical efficiency prediction.

Ballistic Brownian motion of supercavitating nanoparticles

We show that the Brownian motion of a nanoparticle (NP) can reach a ballistic limit when intensely heated to form supercavitation. As the NP temperature increases, its Brownian motion displays a sharp transition from normal to ballistic diffusion upon the formation of a vapor bubble to encapsulate the NP. Intense heating allows the NP to instantaneously extend the bubble boundary via evaporation, so the NP moves in a low-friction gaseous environment. We find the dynamics of the supercavitating NP is largely determined by the near field effect, i.e., highly localized vapor phase property in the vicinity of the NP.

Nanopumping of water via rotation of graphene nanoribbons

In this paper, we perform molecular dynamics simulations to propose a novel bio-inspired nanopumping mechanism that is achieved through the rotation of graphene nanoribbons. Due to the rotation and interaction with water, the graphene nanoribbons undergo morphological transformation. It is shown that with appropriate geometrical and spatial parameters, the resulting morphology is twisted ribbon, which is efficient in pumping of water through a channel. This mimics the propulsive behavior of bacterial flagella through continual rotation at the base and causing morphology of the geometry into twisted ribbons, thus driving flow. It was observed that the maximum flux rate decreases upon reaching the optimal configuration even with increasing rotational speed and graphene width. This is due to the development of cavitation near the region of the nanoribbon with tip velocities approaching the speed of sound in water. The simulation shows promising results where the flux rate of the driven flow outperforms various nanopump configurations that have been reported in recent literature by more than one order.

Nanoparticle-Mediated Cavitation via CO2 Laser Impacting on Water: Concentration Effect, Temperature Visualization, and Core-Shell Structures

By taking advantage of seeded polymer nanoparticles and strong photo energy absorption, we report CO₂ laser impacting on water to produce cavitation at the air/water interface. Using a high-speed camera, three regimes (no cavitation, cavitation, and pseudo-cavitation) are identified within a broad range of nanoparticles concentration and size. The underlying correlation among cavitation, nanoparticles and temperature is revealed by the direct observation of spatiotemporal evolution of temperature using a thermal cameral. These findings indicate that nanoparticles not only act as preexisted nuclei to promote nucleation for cavitation, but also likely affect temperature to change the nucleation rate as well. Moreover, by exploiting a compound hexane/water interface, a novel core-shell cavitation is demonstrated. This approach might be utilized to attain and control cavitations by choosing nanoparticles and designing interfaces while operating at a lower laser intensity, for versatile technological applications in material science and medical surgery.

Enhanced Cavitation and Hydration Crossover of Stretched Water in the Presence of C60

We performed molecular dynamics simulations on systems containing stretched water and a C₆₀ buckyball molecule. Our goals were to understand how the presence of the hydrophobic impurity influences the rate of cavitation in stretched water and how the change in pressure (an increase in the value of negative pressure) affects the nature of hydrophobic hydration. Our simulations show that the presence of a buckyball increases the rate of cavitation in water under negative pressure. When studying the influence of the degree of stretching on hydration, we observed that at pressures above -100 MPa the mechanism of hydrophobic hydration is the one that characterizes hydration of a small particle. At some pressure below -100 MPa, there is a crossover in the mechanism of hydration where dewetting occurs by forming cavities next to the surface of the buckyball, and this is characteristic of hydrophobic hydration of large particles.

Construction of molecularly imprinted nanoparticles by employing ultrasound waves for selective determination of doxepin from human plasma samples: Modeling and optimization

In this work, molecularly imprinted nanoparticles (MINPs) were applied as selective adsorbent for ultrasound-assisted micro-solid-phase extraction (UAMSPE) of doxepin (DP) from human plasma samples, which was then cleaned up, pre-concentrated and subjected to HPLC. The MINPs were synthesized based on a non-covalent approach by precipitation polymerization utilizing methacrylic acid and styrene as functional monomers, DP as template, ethylene glycol dimethacrylate as cross-linker and 2,2-azobisisobutyronitrile (AIBN) as initiator. The obtained MINPs were characterized by Fourier transform-infrared and field emission scanning electron microscopy. Factors influencing the efficiency of UAMSPE such as sonication time, volume of eluent solvent and amount of sorbent were investigated using a central composite design and the optimal points were identified as 4 min of sonication time, 380 μL of eluent solvent and 30 mg of sorbent. Under optimized conditions, the proposed method has linear responses in the range of 0.2-2000 ng mL^-1 , with a satisfactory limit of detection of 0.04 ng mL^-1 and limit of quantification of 0.11 ng mL^-1 .