Cavities In Liquids Research Articles

Mechanistic model to predict oscillating frequency of flow boiling in large length to diameter ratio micro-channel heat sinks

This study presents a new mechanistic model for density wave oscillation (DWO) of flow boiling in horizontal micro-channel heat sinks that enables prediction of oscillation frequency, utilizing both flow visualization results and transient experimental data. The proposed model is based on the hypothesis that the period of oscillation consists of two sub-periods, flow surge and reversal, and establishes a set of 1D transient conservation equations. The flow surge period is associated with liquid rapidly entering the channels, momentarily establishing high pressure gradient between inlet and outlet plenums before the incoming flow is decelerated by a combination of increased flow resistance and bubble expansion within the channels. The initial location of the bubble nucleation is estimated using a criterion for onset of nucleate boiling (ONB) that governs bubble formation from cavities in superheated near-wall liquid. The flow reversal period is associated with back flow into the inlet and estimated by time elapsed before the reversed flow returns to its original location where bubble expansion and onset of annular flow are initiated. Unlike prior models, the proposed model simulates a full cycle of flow oscillation. In this model, a new mass conservation criterion is introduced to ensure agreement between predicted and experimental mass outflows during one cycle. Also developed is a detailed spatial and temporal description of the high density wave front (HDWF) largely responsible for both pressure drop and mass velocity fluctuations during the oscillation period. Finally, a periodic flow boundary condition is imposed on flow velocities and locations to achieve same flow behavior at the start and end of cycle. The proposed model shows good agreement with measured oscillation frequencies.

Cavitation-Effect-Based Treatments and Extractions for Superior Fruit and Milk Valorisation.

Ultrasound generates cavities in liquids with high-energy behaviour due to large pressure variations, leading to (bio)chemical effects and material modification. Numerous cavity-based treatments in food processes have been reported, but the transition from research to industrial applications is hampered by specific engineering factors, such as the combination of several ultrasound sources, more powerful wave generators or tank geometry. The challenges and development of cavity-based treatments developed for the food industry are reviewed with examples limited to two representative raw materials (fruit and milk) with significantly different properties. Both active compound extraction and food processing techniques based on ultrasound are taken into consideration.

Dramatically reducing the critical velocity of air cavity generation via biomimetic microstructure effects.

Control over the generation of air cavities in liquids is vitally important in numerous underwater marine systems, with behaviors ranging from largely reducing the fluid resistance to minimizing the energy consumption. However, reducing the critical velocity of air cavity generation simultaneously on both original and hydrophobic materials can be mutually exclusive. In strong contrast to the view that the surface wettability is what determines the critical velocity of air cavity entrainment, we report a facile method to dramatically reduce the critical velocity of air cavity generation by etching a kind of biomimetic microstructure on the sphere surface. It is worth noting that the generation of air cavities induced by the microstructure effect is almost independent of surface wettability; even for the hydrophilic spheres, the critical velocity of air cavity generation is reduced by over 86.3%. The physical mechanism is mainly related to the pinning of the moving contact line and the regulable transition of the wetting state from the Wenzel model to the Cassie-Baxter model at the solid-liquid-air interface. We also investigated the stability of the microstructured spheres to induce underwater air cavities and found that a continuous and robust air cavity could still be produced even after 1200 repeated impacts. This research provides a simple, economical and energy-efficient method for minimizing the critical velocity for generating air cavity entrainment to reduce hydrodynamic drag.

Permanent cavities in ionic liquids created by metal–organic polyhedra

A new porous liquid system based on the coalition of MOPs and bulky IL was designed. The liquid materials exhibit permanent cavities, long-term durability, and good ability to separate CO2 from N2 and CH4 not only as an entity but also as a membrane.

Initiation of nanosecond-pulsed discharge in water: Electrostriction effect

Underwater nanosecond-pulsed discharges have been widely utilized in numerous industrial applications. The initial stage of nanosecond-pulsed discharge in water contains extremely abundant physical processes, however, it is still difficult to reveal the details of charge transportation and multiplicative process in liquid within several nanoseconds by currently existing experimental diagnostic techniques. Up to now, the initiation mechanism of underwater nanosecond discharge has been still a puzzle. In this paper, we develop a two-dimensional axially symmetric underwater discharge model of pin-to-plane, and numerically investigate the electrostriction process, cavitation process, and ionization process in water, induced by nanosecond-pulsed voltage. The negative pressure in water caused by tensile ponderomotive force is calculated. The creation of nanoscale cavities (so-called nanopores) in liquid due to negative pressure is modeled by classical nucleation theory with modified nucleation energy barrier. When estimating the temporal development of nanopore radius, a varying hydrostatic pressure is considered to restrain the unlimited expansion of nanopores. We estimate the electron generation rate by the product of the generation rate of incident electrons and the number density of nanopores. The simulation results show that cavitation occurs in liquid within several microns from pin electrode due to the electrostriction, which results in the formation of a large number of nanopores. The expansion of nanopore, caused by electrostrictive pressure on nanopore surface, provides a sufficient acceleration distance for electrons. The impact ionization of water molecules can be triggered by energetic electrons, leading the local liquid to be ionized rapidly. The effects of nanopores on rapid electron generation in water are discussed. Once nanopores are formed, the electrons can be generated in the following ways: 1) Field ionization of water molecules on the nanopore wall continuously provides seed electrons; 2) the seed electrons accelerated in nanopores enter into the liquid and collide with water molecules, resulting in the rapid increase of electrons. It can be inferred that the randomly scattered nanopores act as micro-sources of charges that contribute to the continuing ionization of liquid water in cavitation region near pin electrode. Electrostriction mechanism provides a new perspective for understanding the initiation of nanosecond-pulsed discharge in water.

The ping-pong ball water cannon

The course Phy Ex was created by Yves Couder in the Paris VII university to teach experimental physics through projects. In this article, we present this teaching method through a particular project that took place in the autumn semester 2019: the ping-pong ball water cannon. In this experiment, a glass containing water and a floating table tennis ball is dropped from some height to the ground. Following the impact, the ball is ejected vertically upwards at speeds that can be several times the impact speed. We report the student team's initial dimensional and order-of-magnitude analysis, and describe the successive experimental setups that showed (1) that free flight is essential for the phenomenon to occur, (2) that the order of magnitude of the ball ejection momentum is correctly predicted by a momentum balance based on integrating the pressure impulse during impact and (3) that making the ball surface more wettable, or stirring the liquid, drastically increases the momentum transfer. The proposed explanation, confirmed by direct high-speed video observations, is that the immersion depth of the ball increases during free fall due to capillary forces or vortex depression-in the absence of buoyancy-and that the enormous excess pressure on the bottom of the ball during impact drives the ball up towards its buoyancy equilibrium. The transfered momentum is sufficient to expel the ball at high velocity, very similar to the formation of liquid jets in collapsing cavities in liquids.

Insights into the local structures of water in 1-butyl-3-methylimidazolium iodide

Recently, the presence of cavities in ionic liquids (ILs), which affects the solubility of solute molecules such as CO2, has been demonstrated. With the aim of gaining more insight into the interactions between the IL and guest water molecules, the changes experienced upon addition of water molecules to an IL have been monitored by Raman spectroscopy. This study focuses on how the difference in the nanoheterogeneous structures of two ILs, 1-butyl-3-methylimidazolium iodide (abbreviated as [bmim][I]) and 1-butyl-3-methylimidazolium tetrafluoroborate (abbreviated as [bmim][BF4]) influences the local structure of water embedded in the ILs. The evolution of local water clusters formed in the ILs is compared. Then, the connections between microscopic and macroscopic properties were reviewed.

Shear-wave generation from cavitation in soft solids

The formation and dynamics of cavities in liquids lead to focusing of kinetic energy and emission of longitudinal stress waves during the cavity collapse. Here, we report that cavitation in elastic solids may additionally emit shear waves that could affect soft tissues in human bodies/brains. During the collapse of the cavity close to an air-solid boundary, the cavity moves away from the boundary and forms a directed jet flow, which confines shear stresses in a volume between the bubble and the free boundary. Elastographic imaging and high-speed imaging resolve this process and reveal the origin of a shear wave in this region. Additionally, the gelatin surface deforms and a conical crack evolves. We speculate that tissue fracture observed in medical therapy may be linked to the non-spherical cavitation bubble collapse.

Subsurface imaging of cavities in liquid by higher harmonic atomic force microscopy

Subsurface imaging capability of liquid-environment higher-harmonic atomic force microscopy (AFM) was investigated using a reference artifact. A series of cylindrical cavities with pre-known dimensions were fabricated on a silicon substrate via electron beam lithography and then covered by a set of highly oriented pyrolytic graphite (HOPG) pieces with different thicknesses. Experiments on these structures demonstrated that the higher-harmonic amplitude sensitivity to the local stiffness in liquids was at least an order of magnitude larger than that in ambient air under the same parameter settings. The harmonic AFM in liquids could detect the cavities beneath over a 200 nm thick HOPG cover. Theoretical analyses based on the cantilever dynamics and the membrane mechanical properties well interpreted the experimental results. Furthermore, it was verified that the momentary excitation of the non-driven higher eigenmode in a low-Q environment could play a critical role in the enhanced subsurface imaging capability of harmonic AFM in liquids.

Numerical Investigation of Cavitating Flows with Liquid Degassing

Cavitation is a phenomenon of formation of bubbles (cavities) in liquid as a result of pressure drop. Cavitation plays an important role in a wide range of applications. For example, cavitation is one of the key problems of design and manufacturing of pumps, hydraulic turbines, ship’s propellers, etc. Special attention is paid to cavitation erosion and to performance degradation of hydraulic devices (noise, fluctuation of the mass flow rate, etc.) caused by formation of a two phase system with an increased compressibility. One more important problem accompanying cavitation is liquid degassing due to diffusion of the dissolved gas into the cavities. Determination of the degassed air content is important for the performance forecast of devices. Mathematical models of the degassing in cavitating flows are not yet developed and presented in the literature satisfactorily. Therefore, development of a model for simultaneous cavitation and liquid degassing is an important fundamental and applied task. To validate the algorithm simulations of unsteady flow of a cavitating liquid in a channel have been conducted. The obtained results are in satisfactory agreement with the experimental data and demonstrate the efficiency and robustness of the formulated model and the algorithm.

Shapes of steady slender axisymmetric ventilated cavities in ponderable liquid

Shapes of steady slender axisymmetric ventilated cavities in ponderable liquid

Parameters of ultrasonic dispersion of polymer-composite solutions

Conditions allowing the formation of three types of characteristic cavities in liquid via ultrasonic cavitations have been considered. The effect of the ultrasonic-field parameters on the dispersion efficiency has been analyzed, and the choice of criteria for the evaluation of the quality of steering and dispersion of a polymer-composite material (PCM) for reconstruction of body parts of automotive engineering has been grounded. The conditions for effective dispersion of a PCM solution have been established on the basis of the sound-pressure amplitude upon ultrasonic treatment and based on the maximum permissible level of the solution in an ultrasonic bath.

Usable work of macro-scale cavities in liquids

It is shown that the generation of cavities in a liquid can produce usable work, which is illustrated by the stretching of a string. This work is done during the expansion of the cavity, and not with its collapse. Basic equations are presented for the movement of a device moved by the so called cavity events. A theoretical solution is also proposed, which uses polynomial functions relating the so called excess of pressure in the cavity and time. Evaluations of the force generated during the expansion of the cavity showed a mean peak value of about 58 N for the moving container, while measurements with the container fixed to a support showed a peak value of 476 N, considered somewhat overestimated, because high frequency oscillations seem to superpose the mean behavior. Simultaneous phenomena occurring during the cavity events are also described. Series of pictures of the experiments are presented.

Putting the squeeze on cavities in liquids: Quantifying pressure effects on solvation using simulations and scaled-particle theory

Extensive molecular simulations of the Lennard-Jones fluid are performed to examine the response of the excess chemical potential of cavitylike solutes to applied pressure. Solutes as large as ten times the solvent diameter are considered. The simulations are analyzed using the revised scaled-particle theory developed by Ashbaugh and Pratt to evaluate the thermodynamics of cavity solvation and curvature dependent interfacial properties well into the compressed liquid portion of the solvent phase diagram. The revised theory provides a quantitatively accurate description of the solvent-solute contact correlation function for all solutes and state points considered. The main structural effect of increasing pressure is to push the solvent molecules up against the solute surfaces, counteracting the dewetting that is observed at lower pressures along the solvent saturation curve. Decomposing the excess chemical potential of cavities into volume and surface-area contributions shows that pressure differentially affects the interfacial free energies of molecular versus macroscopic solutes. The interfacial free energy of surfaces of molecular dimension monotonically decreases with applied pressure, while that of surfaces larger than a small cluster of solvent molecules exhibit a maximum with increasing pressure, which may play a role in pressure-induced disaggregation of molecular assemblies. Moreover, since the pressure dependence of the interfacial free energy is thermodynamically linked to the excess adsorption of solvent on the solute surface, the former is potentially a measurable macroscopic indicator of microscopic wetting∕dewetting phenomena, implicated in hydrophobic interactions between macroscopic hydrophobic particles. Finally, some inferences about pressure-dependent solvation processes in water are made by using the revised theory to analyze previously published simulation data.

Last Decade of Research in Sonochemistry for Green Organic Synthesis

Ultrasounds, an efficient and virtually innocuous means of activation in synthetic chemistry, have been employed for decades with varied success. This high-energy input enhances not only mechanical effects in heterogeneous processes, but it is also known to induce new reactivities leading to the formation of unexpected chemical species. The remarkable phenomenon of cavitation makes sonochemistry unique. This patent review is aimed at summarizing the status of cavitational chemistry in organic synthesis and to highlight the more recent applications in this field. The use of ultrasound to promote chemical reactions is called sonochemistry. The effects of ultrasound observed during organic reactions are due to cavitation, a physical process that creates, enlarges, and implodes gaseous and vaporous cavities in an irradiated liquid. Cavitation induces very high local temperatures and pressures inside the bubbles (cavities), leading to turbulent flow of the liquid and enhanced mass transfer. Keywords: Ultrasounds, cavitation, heterogeneous catalysis, photocatalysis, phase transfer catalysis, aqueous media, ionic liquids, acid-catalyzed reactions, base-catalyzed reactions, C-C bond formation, oxidation reactions, reduction reactions

Critical Parameters of Gas Cavities in Dielectric Liquids Stressed with High Electric Fields

Electric field stimulates formation of gas-filled cavities on sharp electrodes in insulating liquids which may initiate the dielectric breakdown of these liquids. Using an analytical model based on electrical and thermodynamic properties of fluids, the critical dimensions of gas-filled bubbles in ester fluid and transformer oils stressed with an impulse electric field were calculated. The results obtained in this paper could be important for the optimisation of the application of these liquids in high voltage and pulsed power systems.

Stochastic models of partial discharge activity in solid and liquid dielectrics

A new model that can reproduce main stochastic features of partial discharge (PD) activity at AC and DC voltages was proposed. The type of PD activity because of microdischarges in small cavities present in dielectric materials was considered. Three different criteria were used to simulate an initiation of partial discharge inside voids. The simplest criterion of threshold type was used also to describe a decay of plasma in voids and subsequent decrease in conductivity to zero. After AC voltage was applied to solid dielectric, the narrow peaks of current in external circuit were observed in our simulations. Every peak corresponds to a moment of PD in a void. The behaviour of cavities in dielectric liquid under DC voltage was also simulated. In this case, PD activity is possible even under DC voltage because of both elongation of microbubbles present in a liquid and diffusion of charge carriers from the surface of a bubble into a liquid.

Spherical shape stability of shells and cavities

Thin-walled shells characteristic of their light weight and strength are used everywhere – in household items to ocean liners and space rockets. Their applications are so diversified and uncountable that one can safely say, ”The world consists of shells; the world rests on shells.” After all, the Earth’s crust is also a shell. The wide application of thin-walled structures has triggered the need for developing reliable methods to calculate their strength and stability, and this is just the research subject of the shell theory, a new branch of mechanics arisen in the past century. The important role is found to be played by their interaction with working media. The present paper discusses the issues on the stability of spherical thin-walled shells and gas cavities in liquid.

Cavities in ionic liquids

The formation of cavities in ionic liquids in the vicinity of the liquid binodal curve is investigated by means of Monte Carlo simulations of the restricted primitive model (RPM). Analysis of the cavity size distribution functions provides a quantitative view of the hole sizes arising in ionic liquids when approaching the coexistence region. Cavities of sizes 0.1–1 nm are formed, the larger cavities being favored by the Coulombic forces. The mean cavity size grows with the square root of the temperature. We compute the reversible work needed to create a cavity in the ionic liquid and it is used to estimate the surface tension of the ionic liquid–vapor interface. The accuracy of theoretical approaches based on the scaled particle theory and Ornstein–Zernike equation to estimate the cavity work of formation in ionic liquids is discussed. We find that both simulations and integral equations predict density depletion with increasing cavity size, suggesting the existence of surface drying in ionic liquids.

Acoustically-driven spherical implosions and the possibility of thermonuclear reactions

Acoustically driven, gas-filled cavities in liquids have been known to collapse violently, generating short flashes of light of ∼100 psec duration. This phenomenon has been known as Sonoluminescence (SL) and was first observed by Marinesco et al. in 1933. Ten years ago the author pioneered a technique for observing the oscillations of a single, gas-filled cavity (termed Single-Bubble Sonoluminescence) which has provided new insights into the phenomenon of sonoluminescence. More recently, the possibility of generating fusion reactions using acoustics has been considered. Results of computer simulations and preliminary experimental data will be presented. Back-of-the-envelope calculations in terms of the acoustical and thermodynamic parameters necessary to achieve Thermonuclear reactions will be presented in an effort to evaluate the feasibility of Sonofusion as an energy source.