Hemispherical Bubble Research Articles

Rotational manipulation of paramecium using a semi-capsule-shaped bubble with an adjustable volume actuated by acoustic waves

Acoustically actuated bubbles provide a versatile and non-invasive approach for manipulating microorganisms in fluid. However, the susceptibility of the bubble volume to environment and the complex intersecting vortices of the oscillation of hemispherical bubbles reduce the stability of micromanipulation of ellipsoid-like organisms. This study involves an on-chip rotational manipulation device for rotating ellipsoid-like organisms, which utilizes parallel microstreaming vortices that are generated with acoustically actuated semi-capsule-shaped bubbles. In addition, a relatively stable volume of the semi-capsule-shaped bubble with tolerances about 5 % is realized by adjusting the gas diffusion between the bubble and the gas channel. Characterized experiments using polystyrene particles of 10 μm demonstrate that two pairs of significant out-of-plane parallel microstreaming vortices can be generated near the short or long side of a semi-capsule-shaped bubble at acoustic driving frequencies of 11.23 kHz and 13.97 kHz, respectively. The vortices effectively induce rotation both for the spherical particles and the ellipsoid-like paramecia in fluid. Compared to oscillating hemispherical bubbles, acoustically actuated semi-capsule-shaped bubbles offer a more stable attitude of the rotation axis and even rotation velocity for paramecia. The acoustically actuated semi-capsule-shaped bubbles offer a label-free method for rotational manipulation of ellipsoid-like organisms, characterized by good stability, adaptability, and biocompatibility.

Numerical simulation for pressure fluctuations caused by the growth of a single boiling bubble

The pressure fluctuations caused by the rapid volume changes during the initial growth stage for a single bubble are crucial excitations of boiling induced vibration. Therefore, it is necessary to accurately model the dynamic behavior of bubbles during this stage to obtain reasonable excitation forces, which can be used to estimate the vibration response of structures. The present study develops a dynamics model, which is verified and validated to be applicable to the growth of spherical bubbles in infinite superheated liquid and hemispherical bubbles growing on a vertical heated surface. The effects of mass transfer are considered for the vapor-liquid interface motion, and the temperature variations at the liquid side are determined by the finite difference method. The numerical data match well with the theoretical and experimental results, and the characteristics of the excitation forces caused by a single boiling bubble are obtained and analyzed. In addition, the influences of superheat, accommodation coefficient for phase change, and the temperature gradient at the liquid side on excitation forces caused by bubble growth are studied. The results show that the temperature distributions near the heated surface strongly affect the frequency domain characteristics of the excitation forces.

The multi-cycle dynamics of the cavitation bubble near the solid wall with an air-entrapping hole or a hemispherical air bubble: A numerical study

Investigating the interaction between the near-wall cavitation bubble and the air bubble has great significance for understanding the mechanism of air entrainment to alleviate cavitation in actual hydraulic engineering. To quantify the effect of the air bubble on the multi-cycle dynamics of the near-wall cavitation bubble, a more comprehensive compressible three-phase model considering the phase-change process was developed based on OpenFOAM, and corresponding validation was performed by comparing the simulated bubble shape with the published experimental values. The key features of the multi-cyclical evolution of the cavitation bubble are nicely reproduced based on the current numerical model. For the cavitation bubble near the solid wall containing a hemispherical air bubble, the simulated results reveal that the air bubble can reflect the shock wave and thus prevent it from impacting directly on the solid wall, which will help to uncover the microscopic mechanism of aeration avoiding cavitation damage. The dynamical features of the cavitation bubble at different dimensionless distances (γ1) and dimensionless sizes (ε) are investigated and analyzed. For the near-wall cavitation bubble with an air-entrapping hole, the air hole plays a crucial role in the multi-cycle dynamics of the cavitation bubble, leading to the bubble that is always far away from both the air hole and the solid wall. Thus, the current results may provide a potential application for preventing the wall damage caused by the impact of the liquid jet.

The role of surface wettability on the growth of vapour bubbles

The formation of heterogeneous vapour bubbles is widely studied due to its importance to two-phase thermal management systems, ultrasonic cleaning, and turbomachinery performance. However, the role that the surface plays in determining the growth of a bubble is still poorly understood. Currently, theoretical understanding of heterogeneous vapour bubble growth is limited to hemispherical bubbles or completely spherical bubbles next to a surface. We have previously developed an inertio-thermal model to accurately predict how heat transfer from the surrounding fluid affects homogeneous vapour bubble growth from the nanoscale to the macroscale (Sullivan et al. J. Fluid Mech. 948(A55):1-15, 2022). By accounting for the presence of the surface and its wettability on both the geometry of the bubble and on the available thermal energy in the surrounding fluid, we extend our model to capture heterogeneous vapour bubble growth. Using molecular simulations, we show not only how the strength of fluid-solid interaction affects the growth rate, but also how the formation of an adsorbed fluid layer under the bubble on lyophilic surfaces plays a vital role in determining the bubble shape and subsequent dynamics. These insights have potential to improve the performance of systems involving a change of phase from liquid to vapour by better understanding the role of surface wettability on this process.

Laser Cavitation Peening: A Review

During submerged laser peening using a pulsed laser, a bubble that behaves like cavitation is generated after laser ablation (LA). The bubble is referred to as laser cavitation (LC). The amplitude of the shockwave in water generated by LA is larger than that of LC; however, the impact passing through the target metal during LC is larger than that of LA. LC impact can be utilized for peening at optimized conditions. Thus, submerged laser peening is referred to as “laser cavitation peening”, as the peening method using the cavitation impact is known as “cavitation peening”. The impact induced by a hemispherical bubble is more aggressive than that of a spherical bubble with a microjet. Laser cavitation peening can improve the fatigue strength of metallic materials by producing work-hardening and introducing compressive residual stress. Three-dimensional additively manufactured metals (3D metals) such as titanium alloy are attractive materials for aviation components and medical implants; however, the fatigue strength of as-built components is nearly half of that of bulk metals, and this is an obstacle for the applications of 3D metals. In the present study, published research papers are reviewed to identify the key factors of laser cavitation peening, with additional visualization of LC and data. Then, improvements in the fatigue strength of metallic materials, including 3D metals treated by laser cavitation peening, are summarized.

Temporal evolution of pressure profiles for laser-induced cavitation bubble on the metal surface

When a laser is focused on an underwater object, it experiences a large amount of pressure owing to the plasma confinement effect of water. A hemispherical bubble is generated on the surface of the object, and large pressure is generated when the bubble collapses. In this study, we conducted experiments using different laser energies to analyze the pressure–time histories associated with bubble contraction. The maximum pressure was 10%–40% of the laser ablation pressure, whereas the pressure pulse width was 5–10 times longer than the laser pulse width. Furthermore, the bubble motion could be adiabatically explained, except for the plasma interaction region. The results indicate that the pressure at which the bubble collapses does not depend on the maximum size of the generated bubble but depends on the energy of water vapor within the bubble.

Relationship between a non-spherical collapse of a bubble and a stress state inside a wall

This study performed a fluid/material coupled numerical simulation of the first stage of a non-spherical collapse of a bubble near a wall and investigated the stress state inside the elastic material of the wall according to the change in the stand-off distance γ between the bubble and the wall. The relationship between the collapse behavior of the bubble and propagation of stress waves was confirmed for typical collapse modes: pancake-shaped mode at γ = −0.3, hemispherical mode at γ = 0, microjet mode at 0.3 < γ < 1.0, and detaching mode at γ > 1.2. The stress influence area, which is an index of material damage, was estimated. At 0.3 < γ < 1.0, the stress influence area caused by the microjet is narrow and shallow in the material; in contrast, that caused by the pressure waves spreads more widely and deeply, especially inside the material. This means that the pressure wave has a larger influence than the microjet on damage to a material even though the maximum value of the equivalent stress is nearly identical between the microjet and the pressure wave. Additionally, the depth of the stress influence area at 0.3 < γ < 0.5 is larger than that at γ = 0, although the volume and the maximum stress are larger at γ = 0 than at 0.3 < γ < 0.5. This indicates that the case of toroidal bubble rebound attaching to a wall has the potential to cause a deeper damage inside a material in comparison with hemispherical bubble collapse.

Deformation of soap bubbles in uniform magnetic fields.

The deformation of hemispherical sessile bubbles made of ferrofluid soap under vertical uniform magnetic fields was studied using Helmholtz coils. The deformation and the shape of the bubbles were monitored according to the amplitude of the magnetic field, the initial volume of the bubbles and the ferrofluid volume used to create them. The meniscus was found to bear most of the deformation, reshaping into a cylinder, with the remainder of the bubble forming a spherical cap, mainly adapting to the meniscus transformation. The growth of the meniscus height was rationalised using a simple model. More precisely, the meniscus shape depends on the competition between capillary, gravity and magnetic effects. These three ingredients can be rewritten to highlight two characteristic lengths of the system: the capillary and the magnetic lengths. Depending on the magnetic field intensity, the shape of the meniscus is described by one of the two lengths, thus revealing the existence of two distinct regimes.

Microtensiometer for Confocal Microscopy Visualization of Dynamic Interfaces.

Adsorption of surface-active molecules to fluid-fluid interfaces is ubiquitous in nature. Characterizing these interfaces requires measuring surfactant adsorption rates, evaluating equilibrium surface tensions as a function of bulk surfactant concentration, and relating how surface tension changes with changes in the interfacial area following equilibration. Simultaneous visualization of the interface using fluorescence imaging with a high-speed confocal microscope allows the direct evaluation of structure-function relationships. In the capillary pressure microtensiometer (CPM), a hemispherical air bubble is pinned at the end of the capillary in a 1 mL volume liquid reservoir. The capillary pressure across the bubble interface is controlled via a commercial microfluidic flow controller that allows for model-based pressure, bubble curvature, or bubble area control based on the Laplace equation. Compared to previous techniques such as the Langmuir trough and pendant drop, the measurement and control precision and response time are greatly enhanced; capillary pressure variations can be applied and controlled in milliseconds. The dynamic response of the bubble interface is visualized via a second optical lens as the bubble expands and contracts. The bubble contour is fit to a circular profile to determine the bubble curvature radius, R, as well as any deviations from circularity that would invalidate the results. The Laplace equation is used to determine the dynamic surface tension of the interface. Following equilibration, small pressure oscillations can be imposed by the computer-controlled microfluidic pump to oscillate the bubble radius (frequencies of 0.001-100 cycles/min) to determine the dilatational modulus The overall dimensions of the system are sufficiently small that the microtensiometer fits under the lens of a high-speed confocal microscope allowing fluorescently tagged chemical species to be quantitatively tracked with submicron lateral resolution.

Dilatational and shear rheology of soluble and insoluble monolayers with a Langmuir trough

HypothesisThe surface dilatational and shear moduli of surfactant and protein interfacial layers can be derived from surface pressures measured with a Wilhelmy plate parallel, ΔΠpar and perpendicular ΔΠperp to the barriers in a Langmuir trough. ExperimentalApplying area oscillations, A0+ ΔAeiωt, in a rectangular Langmuir trough induces changes in surface pressure, ΔΠpar and ΔΠperp for monolayers of soluble palmitoyl-lysophosphatidylcholine (LysoPC), insoluble dipalmitoylphosphatidylcholine (DPPC), and the protein β-lactoglobulin to evaluate Es∗+Gs∗=A0ΔΠparΔA and Es∗-Gs∗=A0ΔΠperpΔA. Gs∗ was independently measured with a double-wall ring apparatus (DWR) and Es∗ by area oscillations of hemispherical bubbles in a capillary pressure microtensiometer (CPM) and the results were compared to the trough measurements. FindingsFor LysoPC and DPPC, A0ΔΠparΔA≅A0ΔΠperpΔA meaning Es∗≫Gs∗ and Es∗≅A0ΔΠparΔA≅A0ΔΠperpΔA. Trough values for Es∗ were quantitatively similar to CPM when corrected for interfacial curvature. DWR showed G∗ was 4 orders of magnitude smaller than Es∗ for both LysoPC and DPPC. For β-lactoglobulin films, A0ΔΠparΔA>A0ΔΠperpΔA and Es∗ and Gs∗ were in qualitative agreement with independent CPM and DWR measurements. For β-lactoglobulin, both Es∗ and Gs∗ varied with film age and history on the trough, suggesting the evolution of the protein structure.

Experimental transient cavitation bubbles in a conical-frustum shaped tube

Abstract Extreme flow acceleration in the water can induce large cavitation bubbles in the hydraulic machinery, and cause liquid column separations in the pipeline. The abrupt collapse of these large bubbles produces strong pressure oscillation and causes catastrophic destruction instantaneously. Considering the complexity of the shapes of pipelines in engineering, we experimentally study the transient bubble dynamics in a conical-frustum shaped tube by a modified tube-arrest method in the present paper. Three shapes of cavitation bubbles are observed during the transient processes: (A) conical-frustum shaped bubbles originated from the bottom; (B) hemispherical bubbles originated from the side wall; (C) conical-frustum shaped bubbles originated from the liquid bulk. It is seen that the locations of the cavitation bubbles strongly depend on the geometries and the surface roughness of the tube, and the nucleation status of the bulk liquid.

Bubble collapse and jet formation inside a liquid film

The cavitation of a hemispherical bubble nucleated inside a liquid film of microscale thickness and the subsequent formation of liquid jets are investigated numerically, using the compressible volume of fluid method, in particular, in the context of laser-induced forward transfer (LIFT). Because of the presence of both a solid wall and a free surface, the dynamics of the bubble differ notably from those near a solid wall alone or a free surface alone. When the liquid film is sufficiently thin, the shape of the expanded bubble becomes narrow along the axial direction, forging a stretched cone with a spike. The interface of the bubble near the spike opposite to the solid wall contracts much earlier than the other parts, leading to the separation of the spike. As a result of this separation, the collapse of the bubble is stronger, and the outward jet and inward jet along the axial direction are sharper and faster. However, as the film thickness increases, the tip separation of the bubble becomes weaker, and both outward and inward liquid jets become thicker and slower. In addition, an increase in liquid viscosity changes the bubble shape from an axially stretched cone shape to a round shape because of enhanced viscous force along the radial direction inside the thin film, which eventually results in disappearance of the inward jet. The fundamental insights elucidated in this work can serve as a quantitative design guideline for the LIFT.

Near-Field Probe of Thermal Fluctuations of a Hemispherical Bubble Surface.

We report measurements of resonant thermal capillary oscillations of a hemispherical liquid gas interface obtained using a half bubble deposited on a solid substrate. The thermal motion of the hemispherical interface is investigated using an atomic force microscope cantilever that probes the amplitude of vibrations of this interface versus frequency. The spectrum of such nanoscale thermal oscillations of the bubble surface presents several resonance peaks and reveals that the contact line of the hemispherical bubble is pinned on the substrate. The analysis of these peaks allows us to measure the surface viscosity of the bubble interface. Minute amounts of impurities are responsible for altering the rheology of the pure water surface.

Analysis of the blast waves from the explosions of stoichiometric, rich, and lean propane/oxygen mixtures

The blast waves from a series of explosions of stoichiometric, rich, and lean propane/oxygen mixtures have been analysed. The explosive mixtures were contained in hemispherical soap bubbles, 0.05 m in radius, with total masses in the order of 1 g. The blast waves were measured with a series of piezoelectric transducers, flush mounted in the horizontal surface supporting the charges, at various distances from the centres of the explosions. The measured time history of hydrostatic overpressure from each transducer was least-squares-fitted to the modified Friedlander equation to provide the best estimates of the peak hydrostatic overpressure immediately behind the primary shock and the positive phase duration. The times-of-arrival (TOA) of the primary shocks at each gauge location were used to determine the shock Mach numbers as functions of distance, and these values were used in a Rankine–Hugoniot relationship to calculate the peak hydrostatic overpressures, also as functions of distance. For all explosive mixtures, there was excellent agreement between the direct gauge measurements and the values from the TOA analyses. In order to compare the relative strengths of the blast waves from the three mixtures, the measured results were scaled to those for a 1-kg charge using the masses of propane and applying Hopkinson’s cube root scaling. This analysis showed that the blast wave from the lean mixture, for which there was an excess of oxygen, was stronger than that from a stoichiometric mixture, indicating a more efficient detonation. The blast wave from the rich mixture, for which there was a deficiency of oxygen, was weaker than that from the stoichiometric mixture, but gradually strengthened relatively, probably due to the afterburning of the undetonated propane in the presence of atmospheric oxygen. The peak overpressures as functions of distance and the overpressure time histories from the three types of explosive were compared with predictions by a Propane Blast interface and in all cases showed excellent agreement. The interface predictions were based on measurements from a nominal 20-ton propane/oxygen explosion, and the agreement with results from charges with masses of less than 1 g indicates the validity of Hopkinson’s cube root scaling applied to propane/oxygen explosions over a range of charge masses in excess of six orders of magnitude.

Collapse Dynamics of Hemispherical Cavitation Bubble in Contact with a Solid Boundary

The dynamics of electrochemically produced hemispherical bubble at solid-liquid interface is investigated experimentally. Time-resolved photography is applied to measure the position of bubble wall as a function of time. By means of stroboscope schlieren photography with 1 μs exposure time it is shown that the development of instabilities of the bubble shape are general feature of the bubble dynamics in contact with the solid boundaries. The acoustic transients emitted after cavitation bubble collapse were registered with the laser beam deflection technique. Duration of the transient of 50–60 ns enables to estimate maximum pressure as 2.5 kbar, when the maximum bubble radius is 1 mm.

Numerical simulation of bubble dynamics in pool boiling at heated surface

In this work, we investigate numerically the dynamics of bubble growth and departure in pool boiling at surface in low pressures using the asymptotic solution derived from our semi analytical method and considering both macrolayer and microlayer evaporation. Our asymptotic solution for bubble-growth kinetics is capable of computing bubble departure diameter Dd and growth time tg more accurately in higher superheats at atmospheric pressure, in which the existing asymptotic growth-kinetics are not adequate. It does not apply in low pressures, particularly below atmospheric pressure. We therefore propose a new asymptotic solution of growth kinetics for computing bubble departure in low pressures. A steady-state analysis of force balance equation in low pressure yields a power-law variation of Dd with Jakob number and pressure. The bubble departure with and without microlayer evaporation reveal that the method with dynamic microlayer evaporation predicts Dd and tg more accurately. The effect of superheat on the shape of growing bubbles reveals that spherical bubbles form at low superheat and hemi-spherical bubbles at high superheat.

Effects of liquid depth on the expansion and collapse of a hemispherical cavitation bubble induced in nanosecond pulsed laser ablation of a solid in liquid

This study examines the growth and collapse dynamics of a cavitation bubble induced by nanosecond pulsed laser ablation of a solid in finite liquid depths. The investigation was conducted with liquid depths that varied from 0.4 to 9 times the maximum bubble radius. The bubble dynamics were tracked using a high-speed laser stroboscopic videography system in the photoelasticity mode. When the liquid depth is sufficiently smaller than the maximum bubble radius, the bubble bursts through the liquid layer during its expansion. The bubble lifetime is substantially shortened, and no secondary shock is detected. At liquid depths that range from slightly below to twice the maximum bubble radius, the bubble is elongated during its expansion and flattened during its shrinkage. The bubble is toroidal at minimum contraction, which leads to multiple emissions of secondary shock waves. In comparison to the bubble lifetime calculated by the Rayleigh-Plesset model, the lifetime of a bubble developed in a finite liquid depth is shortened by a factor that is a function of the non-dimensional liquid depth measured by the maximum bubble radius. These findings contribute to increasing the control and the productivity of applications for nanosecond pulsed laser ablation in liquid.

Experimental investigation on the condensation patterns and pressure oscillation characteristics of steam submerged jet through a horizontal pipe at low steam mass flux

Steam submerged jet condensation is a direct contact condensation widely used in many industries. However, this process may cause damage to related equipment due to pressure oscillation, especially at low steam mass flux. This study synchronously investigated the steam–water interface fluctuation of unstable condensation jet and its corresponding pressure oscillation through a horizontal pipe at low steam mass flux to research the condensation oscillation mechanism. Three typical flow patterns, namely, Chugging, hemispherical bubble oscillation (HBO), and encapsulating bubble oscillation (EBO), were distinguished based on the dynamic behavior of steam–water interface and oscillation pressure characteristics. Only the dynamic pressure in EBO region was periodic among the three flow patterns. In the EBO region, the pressure oscillation frequency increased with the increase of steam mass flux and decreased with the increase of water temperature. A flow pattern regime map for horizontal jet condensation related to steam mass flux and water temperature was presented based on experiments. Moreover, the influence of pipe diameters on low steam mass flux was analyzed. For the flow pattern regime map at low steam mass flux, the critical steam mass fluxes showed an inverse relationship with pipe diameter. An empirical criterion was introduced to determine the flow regime map at low steam mass flux.

Unusual Molecular Diffusion in an Induced SmA Phase Composed of Charge-Transferred Liquid Crystalline Mixtures

We investigated molecular diffusion in charge-transferred (CT) liquid crystalline (LC) mixtures composed of a cyano-biphenyl compound (8CB, electron acceptor) and an azobenzene derivative (7AB7, electron donor). When the molar ratio of 7AB7 is as small as 0–0.1, the mixture forms a uniform two-dimensional (2D) fluid phase called induced SmA. Statistically, the induced SmA phase possesses the same structure as pure 8CB, but in the dynamics, it shows unusual molecular diffusion. With hemispherical bubbles made of the induced SmA phase, we examined gas transport across the films, and found that the oxygen diffusion abruptly fell when the 7AB7 ratio exceeded 0.05. The result suggests that even in the macroscopically uniform 2D fluid phase, small CT crystalline domains can be formed and stably dispersed, which should disturb the one-dimensional molecular transport normal to the smectic layer.

In‐Air Bubble Phobicity and Bubble Philicity Depending on the Interfacial Air Cushion: Toward Bubbles Manipulation Using Superhydrophilic Substrates

AbstractBubbles, a typical air‐in‐liquid system including both underwater and in‐air bubbles, are important phenomena involved in our daily life and engineering fields. So far, the fundamental and dynamics for in‐air bubbles largely remain unclear. Here, two typical in‐air wetting states of the bubble are revealed: the bubble‐phobic state in which a bubble with a perfect spherical shape stays on the liquid surface but without coalescence with the liquid; and the bubble‐philic state in which a bubble on the liquid substrate gives a quasi‐hemispherical shape. It is proposed that the bubble phobicity is a typical high‐energy state where the air cushion at the bubble/liquid interface plays a crucial role in preventing their coalescence by generating an energy barrier. By applying an external force to overcome the energy barrier, the bubble will distort and finally be stabilized in a bubble‐philic state. It is demonstrated that superhydrophilic substrates enable manipulating bubbles in air: a superhydrophilic surface with microscale hairy textures shows robust in‐air repellence to the bubbles, while a hemispherical bubble is capable of transport directionally in air on a superhydrophilic conical fiber. Here, the concept of superwetting into air‐in‐air system (liquid films) is promoted, which may open a new perspective in surface and interface chemistry.