Subsequent Bubble Research Articles

Effects of gravity level on bubble formation and rise in low-viscosity liquids.

We present an experimental analysis of the effects of gravity level on the formation and rise dynamics of bubbles. Experiments were carried out with millimeter-diameter bubbles in the hypergravity environment provided by the large-diameter centrifuge of the European Space Agency. Bubble detachment from a nozzle is determined by buoyancy and surface tension forces regardless of the gravity level. Immediately after detachment, bubble trajectory is deviated by the Coriolis force. Subsequent bubble rise is dominated by inertial forces and follows a zig-zag trajectory with amplitude and frequency dependent on the gravity level. Vorticity production is enhanced as gravity increases, which destabilizes the flow and therefore the bubble path.

Bubble the wave or waive the bubble: Why seawater waves foam and freshwater waves do not?

The natural phenomena of wave breaking, subsequent bubble entrainment and coalescence, and eventual foam formation or the lack of it are simulated by a simple system of a jet penetrating into a bath of the same liquid. The mechanisms involved in the process, ion specificity, and dependence on concentration of the electrolyte are demonstrated to be very similar to industrial dense bubble systems. Thus, our recent theoretical explanation of the specificity of the effect of ions in industrial systems is valid also for the natural process of breaking sea waves.

CO2 bubble generation and migration during magma–carbonate interaction

We conducted quantitative textural analysis of vesicles in high temperature and pressure carbonate assimilation experiments (1200 °C, 0.5 GPa) to investigate CO2 generation and subsequent bubble migration from carbonate into magma. We employed Mt. Merapi (Indonesia) and Mt. Vesuvius (Italy) compositions as magmatic starting materials and present three experimental series using (1) a dry basaltic-andesite, (2) a hydrous basaltic-andesite (2 wt% H2O), and (3) a hydrous shoshonite (2 wt% H2O). The duration of the experiments was varied from 0 to 300 s, and carbonate assimilation produced a CO2-rich fluid and CaO-enriched melts in all cases. The rate of carbonate assimilation, however, changed as a function of melt viscosity, which affected the 2D vesicle number, vesicle volume, and vesicle size distribution within each experiment. Relatively low-viscosity melts (i.e. Vesuvius experiments) facilitated efficient removal of bubbles from the reaction site. This allowed carbonate assimilation to continue unhindered and large volumes of CO2 to be liberated, a scenario thought to fuel sustained CO2-driven eruptions at the surface. Conversely, at higher viscosity (i.e. Merapi experiments), bubble migration became progressively inhibited and bubble concentration at the reaction site caused localised volatile over-pressure that can eventually trigger short-lived explosive outbursts. Melt viscosity therefore exerts a fundamental control on carbonate assimilation rates and, by consequence, the style of CO2-fuelled eruptions.

Influence of Transient Thermal Response of Solid Wall on Bubble Dynamics in Pool Boiling

Numerical simulation of single bubble pool boiling process including transient thermal response of solid wall is performed using the ghost fluid method and the level set method for the sharp interface representation. The results show that non-physical initial condition in the numerical simulation deeply affects the process of bubble growth, and then multicycle simulation is necessary to eliminate its influence. It is shown by the present results that two nucleate criteria, i.e. constant waiting time and constant nucleate superheat, for determining the appearance time for the subsequent bubble lead to the same quasi-steady process of bubble growth if they are matched with each other. A periodically expanding and receding thermal “hollow” can be observed inside solid wall underneath the growing bubble. The recovery of the temperature on the nucleate site and the thermal boundary layer near the heating surface is influenced by transient heat conduction inside solid wall, which can affect evidently bubble thermal dynamics and heat transfer.

Direct simulations of homogeneous bubble nucleation: Agreement with classical nucleation theory and no local hot spots.

We present results from direct, large-scale molecular dynamics simulations of homogeneous bubble (liquid-to-vapor) nucleation. The simulations contain half a billion Lennard-Jones atoms and cover up to 56 million time steps. The unprecedented size of the simulated volumes allows us to resolve the nucleation and growth of many bubbles per run in simple direct micro-canonical simulations while the ambient pressure and temperature remain almost perfectly constant. We find bubble nucleation rates which are lower than in most of the previous, smaller simulations. It is widely believed that classical nucleation theory (CNT) generally underestimates bubble nucleation rates by very large factors. However, our measured rates are within two orders of magnitude of CNT predictions; only at very low temperatures does CNT underestimate the nucleation rate significantly. Introducing a small, positive Tolman length leads to very good agreement at all temperatures, as found in our recent vapor-to-liquid nucleation simulations. The critical bubbles sizes derived with the nucleation theorem agree well with the CNT predictions at all temperatures. Local hot spots reported in the literature are not seen: Regions where a bubble nucleation event will occur are not above the average temperature, and no correlation of temperature fluctuations with subsequent bubble formation is seen.

Influence of Transient Thermal Response of Solid Wall on Bubble Dynamics in Pool Boiling

Numerical simulation of single bubble pool boiling process including transient thermal response of solid wall is performed using the ghost fluid method and the level set method for the sharp interface representation. The results show that non-physical initial condition in the numerical simulation deeply affects the process of bubble growth, and then multi-cycle simulation is necessary to eliminate its influence. It is shown by the present results that two nucleate criteria, i.e. constant waiting time and constant nucleate superheat, for determining the appearance time for the subsequent bubble lead to the same quasi-steady process of bubble growth if they are matched with each other. A periodically expanding and receding thermal “hollow” can be observed inside solid wall underneath the growing bubble. The recovery of the temperature on the nucleate site and the thermal boundary layer near the heating surface is influenced by transient heat conduction inside solid wall, which can affect evidently bubble thermal dynamics and heat transfer.

Inertial cavitation enhancement using confocal ultrasound

Inertial cavitation has been shown to be useful in many therapeutic applications; thus, controlling this phenomenon is of great therapeutic interest. However, the stochastic nature of cavitation often proves problematic for predicting its location and extent. Traditional solutions to this problem are the of use dedicated detection apparatuses, or to use injectable microbubbles (MBs), which act as nuclei for the initiation of cavitation. We hypothesize here that cavitation can be reliably controlled without the use of MBs using a confocal system, which produces a lobular focal zone due to acoustic interference. This interference pattern was studied both in simulation and hydrophone measurement. Cavitation extent was confirmed chemometrically with an assay for hydroxyl radical formation, and by passive cavitation detection with a hydrophone. A high speed camera was used to image the initiation of cavitation within the focal zone, the evolution of the bubble cloud, and the subsequent bubble rebound after pulse cessation. The experiments in this work confirm that cavitation is produced more reliably in the confocal setup as opposed to a single transducer, as well as illuminating the mechanisms for this enhancement. [Work supported by the European Union through the Eurostars program (project E!6173) and Caviskills SAS.]

Thermally assisted spatially directed pore formation in poly-dimethylsiloxane (PDMS)

This paper reports a method for spatially directed, self-assembled pore formation in poly-dimethylsiloxane (PDMS) that results in through-membrane pores only at desired locations. This method is based on bubble generation in uncured PDMS and subsequent bubble motion towards a hot region in a microfabricated chip containing a heater device due to thermocapillary effect. Pore formation in straight-line and semi-circular shapes is demonstrated. Coupled physics behind the pore formation and alignment process is described. Results show that the mechanism of pore formation can be controlled by changing the microheater temperature.

Numerical simulations of bubble formation from submerged needles under non-uniform direct current electric field

In several chemical and space industries, small bubbles are desired for efficient interaction between the liquid and gas phases. In the present study, we show that non-uniform electric field with appropriate electrode configurations can reduce the volume of the bubbles forming at submerged needles by up to three orders of magnitude. We show that localized high electric stresses at the base of the bubbles result in slipping of the contact line on the inner surface of the needle and subsequent bubble formation occurs with contact line inside the needle. We also show that for bubble formation in the presence of highly non-uniform electric field, due to high detachment frequency, the bubbles go through multiple coalescences and thus increase the apparent volume of the detached bubbles.

Phase transitions of perfluorocarbon nanoemulsion induced with ultrasound: A mathematical model

While ultrasound has been used in many medical and industrial applications, only recently has research been done on phase transformations induced by ultrasound. This paper presents a numerical model and the predicted results of the phase transformation of a spherical nanosized droplet of perfluorocarbon in water. Such a model has applications in acoustic droplet vaporization, the generation of gas bubbles for medical imaging, therapeutic delivery and other biomedical applications.The formation of a gas phase and the subsequent bubble dynamics were studied as a function of acoustic parameters, such as frequency and amplitude, and of the physical aspects of the perfluorocarbon nanodroplets, such as chemical species, temperature, droplet size and interfacial energy. The model involves simultaneous applications of mass, energy and momentum balances to describe bubble formation and collapse, and was developed and solved numerically. It was found that, all other parameters being constant, the maximum bubble size and collapse velocity increases with increasing ultrasound amplitude, droplet size, vapor pressure and temperature. The bubble size and collapse velocity decreased with increasing surface tension and frequency. These results correlate with experimental observations of acoustic droplet vaporization.

The size range of bubbles that produce ash during explosive volcanic eruptions

Volcanic eruptions can produce ash particles with a range of sizes and morphologies. Here we morphologically distinguish two textural types: Simple (generally smaller) ash particles, where the observable surface displays a single measureable bubble because there is at most one vesicle imprint preserved on each facet of the particle; and complex ash particles, which display multiple vesicle imprints on their surfaces for measurement and may contain complete, unfragmented vesicles in their interiors. Digital elevation models from stereo-scanning electron microscopic images of complex ash particles from the 14 October 1974 sub-Plinian eruption of Volcán Fuego, Guatemala and the 18 May 1980 Plinian eruption of Mount St. Helens, Washington, U.S.A. reveal size distributions of bubbles that burst during magma fragmentation. Results were compared between these two well-characterized eruptions of different explosivities and magma compositions and indicate that bubble size distributions (BSDs) are bimodal, suggesting a minimum of two nucleation events during both eruptions. The larger size mode has a much lower bubble number density (BND) than the smaller size mode, yet these few larger bubbles represent the bulk of the total bubble volume. We infer that the larger bubbles reflect an earlier nucleation event (at depth within the conduit) with subsequent diffusive and decompressive bubble growth and possible coalescence during magma ascent, while the smaller bubbles reflect a relatively later nucleation event occurring closer in time to the point of fragmentation. Bubbles in the Mount St. Helens complex ash particles are generally smaller, but have a total number density roughly one order of magnitude higher, compared to the Fuego samples. Results demonstrate that because ash from explosive eruptions preserves the size of bubbles that nucleated in the magma, grew, and then burst during fragmentation, the analysis of the ash-sized component of tephra can provide insights into the spatial distribution of bubbles in the magma prior to fragmentation, enabling better parameterization of numerical eruption models and improved understanding of ash transport phenomena that result in pyroclastic volcanic hazards. Additionally, the fact that the ash-sized component of tephra preserves BSDs and BNDs consistent with those preserved in larger pyroclasts indicates that these values can be obtained in cases where only distal ash samples from particular eruptions are obtainable.

Influences of initial and boundary conditions on underwater explosion bubble dynamics

Numerical and experimental methods are combined in this paper to study the bubble dynamics generated by an underwater charge explosion with different initial charge shape, detonating styles and boundary conditions. Outdoor experiments are carried out to employ sunlight with the assistant of lamplight to get clearer pictures of bubble motion. The results show that the charge detonating stage is not finished instantly but takes some time to explode fully. The explosion begins from its detonating end and finishes at the other end, which results in its uneven distribution of initial normal velocity. So instead of following the traditional method of treating the charge explosion instantly, a real charge explosion model is built in this paper with the combination of LS-DYNA and the boundary element method (BEM). LS-DYNA is used to solve the charge initial detonation and the BEM to solve its subsequent bubble motion. The linking relationship between these two methods is modeled. The convergence study has been firstly taken with different meshes. In this paper, charges with a different slenderness ratio from 0.99 to 20 are chosen to carry out the experiments and their corresponding numerical results are put forward. It can be found in both experimental and numerical results that the initial charge shape and its detonating style would affect its following bubble dynamic behavior. When the cylindrical charge is horizontally installed and end detonated, an oblique jet leaned to the detonating end would be formed and a horizontal migration process is also observed during the whole bubble pulsating stage. The bubble dynamic behaviors near different solid boundaries are further studied and the numerical results coincide well with the experimental ones.

Ultrafast dynamics of the acoustic vaporization of phase-change microdroplets

Acoustically sensitive emulsions are a promising tool for medical applications such as localized drug delivery. The physical mechanisms underlying the ultrasound-triggered nucleation and subsequent vaporization of these phase-change emulsions are largely unexplored. Here, the acoustic vaporization of individual micron-sized perfluoropentane (PFP) droplets is studied at a nanoseconds timescale. Highly diluted emulsions of PFP-in-water and oil-in-PFP-in-water droplets, ranging from 3.5 to 11 μm in radius, were prepared and the nucleation and growth of the vapor bubbles was imaged at frame rates of up to 20 Mfps. The droplet vaporization dynamics was observed to have three distinct regimes: (1) prior to nucleation, a regime of droplet deformation and oscillatory translations within the surrounding fluid along the propagation direction of the applied ultrasound; (2) a regime characterized by the rapid growth of a vapor bubble enhanced by ultrasound-driven rectified heat transfer; and (3) a final phase characterized by a relatively slow expansion, after ultrasound stops, that is fully dominated by heat transfer. A method to measure the moment of inception of the nucleation event with respect to the phase of the ultrasound wave is proposed. A simple physical model captures quantitatively all of the features of the subsequent vapor bubble growth.

Ultrafast dynamics of the acoustic vaporization of phase-change microdroplets

Superheated emulsion droplets are a promising tool for localized drug delivery. The physical mechanisms underlying the ultrasound-triggered vaporization of phase-change emulsions are largely unexplored. Here we study the acoustic vaporization of individual micron-sized perfluoropentane droplets at a nanoseconds timescale. The nucleation and growth of the vapor bubbles was imaged at frame rates up to 20 Mfps. The droplet vaporization dynamics was observed to have three distinct regimes: (1) prior to nucleation, a regime of droplet deformation and oscillatory translations; (2) a rapid growth of a vapor bubble enhanced by ultrasound-driven rectified heat transfer; and (3) a final phase characterized by a relatively slow expansion that is fully dominated by heat transfer. A method to measure the moment of inception of the nucleation event with respect to the phase of the ultrasound wave is proposed. A simple physical model captures quantitatively all of the features of the subsequent vapor bubble growth. In addition, we study the role of gas through a model for a vapor-gas bubble, including thermal diffusion and gas diffusion inside the liquid and we find good agreement with the experimental data. We underline the fundamental role of gas diffusion to prevent total recondensation of the bubble at collapse.

Pore shape development from a bubble captured by a solidification front

Development of the pore shape from a tiny bubble captured by a solidification front is fundamentally and systematically investigated in this study. Pore formation and its shape in solid influence not only microstructure of materials, but also contemporary issues of various sciences of biology, engineering, foods, geophysics and climate change, etc. In this work, the tiny bubble cap beyond the solidification front is considered to be spherical. As the dominant parameter, the bubble growth rate-solidification rate ratio, decreases, contact angle is found to approach 90°. An accepted criterion, stating that a pore becomes closed as long as the solidification rate is greater than bubble growth rate, is incorrect. This study also finds that the pore can be closed if the bubble radius at contact angle of 90° exhibits a local minimum. Since contact angle of 90° can maintain for a period of time, a subsequent positive bubble growth rate-to-solidification rate ratio readily gives rise to an isolated pore. The pore can be elongated, expanded, shrunk, rippled or closed, depending on the bubble growth rate-to-solidification rate ratio. Manipulating the bubble growth rate or solidification rate to control the pore shape in solid is therefore provided.

Experimental study on CuO nanoparticles in distilled water and its effect on heat transfer

The pool boiling characteristics of dilute dispersions of CuO nanoparticles in water were studied using a 36 gauge NiCr wire at atmospheric pressure. Significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (< 0.1% by volume) without facing any problem of agglomeration and fouling. During experimentation and subsequent inspection, formation of a porous layer of nanoparticles on the heater surface was observed. Boiling of nanofluid resulted in nanoparticle deposition smoothing the surface at lower concentration. To substantiate the nanoparticle deposition and its effect on critical heat flux, investigation was done by measuring the surface roughness and SEM images of the wire surface. While SEM images revealed particle coating, measurement of surface roughness indicated surface modification. Formation of the porous layer on the heater surface as revelled by SEM images provided an excellent location for nucleation sites and subsequent bubble formation. Flow visualisation of the CuO nanofluid (0.1 g/l) up to critical heat flux gave an idea of bubble growth. The experimental results showed the evidence of nanoparticle deposition on the wire surface enhancing critical heat flux.

Characterization of plunging liquid jets: A combined experimental and numerical investigation

This paper presents a combined experimental and numerical study of the flow characteristics of round vertical liquid jets plunging into a cylindrical liquid bath. The main objective of the experimental work consists in determining the plunging jet flow patterns, entrained air bubble sizes and the influence of the jet velocity and variations of jet falling lengths on the jet penetration depth. The instability of the jet influenced by the jet velocity and falling length is also probed. On the numerical side, two different approaches were used, namely the mixture model approach and interface-tracking approach using the level-set technique with the standard two-equation turbulence model. The numerical results are contrasted with the experimental data. Good agreements were found between experiments and the two modelling approaches on the jet penetration depth and entraining flow characteristics, with interface tracking rendering better predictions. However, visible differences are observed as to the jet instability, free surface deformation and subsequent air bubble entrainment, where interface tracking is seen to be more accurate. The CFD results support the notion that the jet with the higher flow rate thus more susceptible to surface instabilities, entrains more bubbles, reflecting in turn a smaller penetration depth as a result of momentum diffusion due to bubble concentration and generated fluctuations. The liquid average velocity field and air concentration under tank water surface were compared to existing semi-analytical correlations. Noticeable differences were revealed as to the maximum velocity at the jet centreline and associated bubble concentration. The mixture model predicts a higher velocity than the level-set and the theory at the early stage of jet penetration, due to a higher concentration of air that cannot rise to the surface and remain trapped around the jet head. The location of the maximum air content and the peak value of air holdup are also predicted differently.

Characteristics of acoustic emissions during nucleation of superheated droplets

Superheated droplet nucleation and subsequent bubble oscillation produces an acoustic pressure pulse that contains valuable information about the nucleation process. Spectral analysis of the pressure pulse indicates excitation of different modes of bubble oscillations in the nucleation process. In the present study it is observed that gamma induced droplet nucleation excites higher modal oscillations and also emits higher intensity acoustic emission compared to that of spontaneous nucleations.

Effect of Gold Nanoparticles on the Dynamics of Gas Bubbles

The effect of gold nanoparticles on the formation of gas bubbles on top of a stainless steel tube is investigated in this work. Unlike other observations of bubble dynamics under evaporation or boiling conditions, which are caused by the surface modification due to particle sedimentation, this work reveals a unique phenomenon of enhanced pinning of the triple line and improved wetting by nanoparticles suspended in the liquid phase. Detailed characteristics related to bubble growth inside pure water and gold nanofluids, including the dynamics of the triple line, the variation of instantaneous contact angle, bubble height, and bubble volume expansion rate, are analyzed. The shape of the bubble is found to be in good agreement with predictions of the Young-Laplace equation by using experimental captured radius of contact line and bubble height as the two known inputs. The variation of surface tensions and the resultant force balance at the triple line are believed to be responsible for the modified dynamics of the triple line and subsequent bubble formation.

On the interaction of a bubble and a vortex ring at high Reynolds numbers

The interaction of a bubble and a vortex ring at high Reynolds numbers could be considered a simplified model of the interaction of a bubble and a turbulent structure of similar size, with the possible subsequent bubble breakup. In this paper, some results from axisymmetric and 3D simulations of the interaction of a bubble and a vortex ring at high Reynolds numbers are presented for different values of the Weber number and vortex ring sizes. Some bubble breakup patterns that could not be obtained by previous axisymmetric boundary integral models are shown. Results obtained are discussed into the framework of the classical Kolgomorov–Hinze theory on bubble breakup and some recent experimental investigations.