Investigation of microbubble/droplet formation in cross-flow and co-flow micro devices

The primary goal of the work presented in this dissertation is to generate microbubbles of a diameter of less than 15μm inside micro channels. In order to achieve this detailed understanding, the facts and limitations behind the generation of microbubbles inside a micro channel are determined from existing literature. The major limitations of the current bubble/droplet generators are found to be the bubble confinement effect, the merging of bubbles, the difficulty in determining bubble diameter and the need for smaller channels to generate smaller bubbles. A device eliminating these drawbacks is conceptualized, and its feasibility is studied using COMSOL® and found to be successful. Based on the findings of the initial studies, prototypes of a new generation of microbubble/droplet generators are developed in this work. This generator utilizes a fused silica tube as the channel carrying the secondary fluid. The micro channel into which the bubbles/droplets are formed is made on a silicon wafer and is sealed using a glass plate. Both cross-flow and co-flow bubble generation devices are analyzed as part of this work. The new generation devices have the advantage of generating unconfined spherical bubbles inside a micro channel while keeping the pressure drop across the channel as low as possible. The new generation cross-flow bubble generator is able to produce bubbles smaller than that possible with existing devices. The cross-flow devices were found to be more efficient in producing microbubbles of smaller diameter in comparison to a similar co-flow device operating under identical conditions. In order to produce smaller microbubbles inside micro channels, a flow focusing technique is introduced in both cross-flow and co-flow devices. Using flow focusing techniques has made it possible to produce microbubbles smaller than that possible without flow focusing. The major parameters that affect bubble formation inside micro channels are determined using a parametric study of both the fluid properties and the geometry of micro channels. A mathematical model is developed to predict the bubble diameter at its detachment from the orifice in a cross-flow device and is validated using experimental data. Surface tension and drag force are found to be the major factors in determining the bubble diameter at detachment. Major achievements of this work are summarized below: 1) A new generation of a microbubble/droplet generator capable of producing unconfined microbubble/droplet is developed in this study. 2) The merging problem of bubbles inside the micro channel immediately after the bubble detaches from orifice is observed for the first time during this study. 3) A microbubble of diameter 11μm is generated in the micro channel of hydraulic diameter 162µm. 4) The bubbly region of bubble formation inside a micro channel is further divided into three in this work: confined region, active region and saturation region. 5) The flow focusing technique inside the cross-flow devices is introduced and studied for the first time during this work. 6) Bubbles of diameter 6µm were produced using the flow focusing technique inside the micro channel of hydraulic diameter 162μm. 7) Circular micro channels were found to be more efficient than straight channels when used in a cross-flow device

Generation and control of monodisperse bubble suspensions in microgravity

A new experimental setup for the generation of homogeneous, monodisperse bubble suspensions in turbulent duct flows in microgravity has been designed and tested in drop tower experiments. The setup provides independent control of bubble size, void fraction and degree of turbulence. The device combines several slug-flow injectors that produce monodisperse bubble jets, with a turbulent co-flow that ensures homogeneous spatial spreading. Bubble separation in the scale of the most energetic eddies of the flow, and bubble size sufficiently smaller, ensure that turbulence is most efficient as a mechanism for spatial spreading of bubbles while preventing coalescence, thus optimizing the homogeneous and monodisperse character of the suspension. The setup works in a regime for which bubbles are spherical, but sufficiently large compared to the turbulent dissipative scales to allow for two-way coupling between bubbles and carrying flow. The volume fraction is kept relatively small to facilitate particle tracking techniques. To illustrate the potential uses of the method we characterize the statistics of bubble velocity fluctuations in steady regimes and we characterize the transient relaxation of the buoyancy-driven pseudo-turbulence when gravity is switched-off.We acknowledge the support from ESA for the funding of the drop tower experiments that provided the raw data analyzed and the ZARM crew, in particular to Dieter Bischoff, for their valuable support all along the experiments and their hospitality. We acknowledges financial support from Ministerio de Economia y Competividad (Spain) under projects FIS2013-41144-P, FIS2016-78507-C2-2-P (J.C.), FIS2015-66503-C3-2-P (L.R.-P., also financed by FEDER, European Union), ESP2014-53603-P (X.R.), and Generalitat de Catalunya under projects 2014-SGR-878 (J.C.), 2014-SGR-365 (X.R.). P.B. acknowledges Ministerio de Ciencia y Tecnología (Spain) for a pre-doctoral fellowshipPreprin

PHANGS-JWST First Results: Multiwavelength View of Feedback-driven Bubbles (the Phantom Voids) across NGC 628

We present a high-resolution view of bubbles within the Phantom Galaxy (NGC 628), a nearby (similar to 10 Mpc), star-forming (similar to 2 M (circle dot) yr(-1)), face-on (i similar to 9 degrees) grand-design spiral galaxy. With new data obtained as part of the Physics at High Angular resolution in Nearby GalaxieS (PHANGS)-JWST treasury program, we perform a detailed case study of two regions of interest, one of which contains the largest and most prominent bubble in the galaxy (the Phantom Void, over 1 kpc in diameter), and the other being a smaller region that may be the precursor to such a large bubble (the Precursor Phantom Void). When comparing to matched-resolution H alpha observations from the Hubble Space Telescope, we see that the ionized gas is brightest in the shells of both bubbles, and is coincident with the youngest (similar to 1 Myr) and most massive (similar to 10(5) M (circle dot)) stellar associations. We also find an older generation (similar to 20 Myr) of stellar associations is present within the bubble of the Phantom Void. From our kinematic analysis of the H I, H-2 (CO), and H ii gas across the Phantom Void, we infer a high expansion speed of around 15 to 50 km s(-1). The large size and high expansion speed of the Phantom Void suggest that the driving mechanism is sustained stellar feedback due to multiple mechanisms, where early feedback first cleared a bubble (as we observe now in the Precursor Phantom Void), and since then supernovae have been exploding within the cavity and have accelerated the shell. Finally, comparison to simulations shows a striking resemblance to our JWST observations, and suggests that such large-scale, stellar-feedback-driven bubbles should be common within other galaxies

A suitable parametrization to simulate slug flows with the Volume-Of-Fluid method

Diffuse–interface methods, such as the Volume-Of-Fluid method, are often used to simulate complex multiphase flows even if they require significant computation time. Moreover, it can be difficult to simulate some particular two-phase flows such as slug flows with thin liquid films. Suitable parametrization is necessary to provide accuracy and computation speed. Based on a numerical study of slug flows in capillary tubes, we show that it is not trivial to optimize the parametrization of these methods. Some simulation problems described in the literature are directly related to a poor model parametrization, such as an unsuitable discretization scheme or too large time steps. The weak influence of the mesh irregularity is also highlighted. It is shown how to capture accurately thin liquid films with reasonably low computation times

GigaGauss solenoidal magnetic field inside of bubbles excited in under-dense plasma

Magnetic fields have a crucial role in physics at all scales, from astrophysics to nanoscale phenomena. Large fields, constant or pulsed, allow investigation of material in extreme conditions, opening up plethora of practical applications based on ultra-fast process, and studying phenomena existing only in exotic astro-objects like neutron stars or pulsars. Magnetic fields are indispensable in particle accelerators, for guiding the relativistic particles along a curved trajectory and for making them radiate in synchrotron light sources and in free electron lasers. In the presented paper we propose a novel and effective method for generating solenoidal quasi-static magnetic field on the GigaGauss level and beyond, in under-dense plasma, using screw-shaped high intensity laser pulses. In comparison with already known techniques which typically rely on interaction with over-dense or solid targets, where radial or toroidal magnetic field localized at the stationary target were generated, our method allows to produce gigantic solenoidal fields, which is co-moving with the driving laser pulse and collinear with accelerated electrons. The solenoidal field is quasi-stationary in the reference frame of the laser pulse and can be used for guiding electron beams and providing synchrotron radiation beam emittance cooling for laser-plasma accelerated electron and positron beams, opening up novel opportunities for designs of the light sources, free electron lasers, and high energy colliders based on laser plasma acceleration.Comment: 15 pages, 9 figures. Main text (without abstract, References and Appendix): 12 page

Bubble formation at a flexible orifice with liquid cross-flow

In waste water treatment, biological processes for denitrification and nitrification are performed using oxidation ditches. In these reactors, the mixing and the aeration functions are dissociated: a bubble cloud is generated from flexible membrane spargers and is subjected to a horizontal liquid flow. The objective of this paper is to study the effects of the liquid crossflow on the bubble formation at a single flexible orifice in water. The several forces acting on the forming bubble have been modelled in order to understand the dynamics of the bubble growth and detachment. The bubble detachment is controlled by the drag force due to the liquid motion and not by the buoyancy force. The experimental analysis of the bubble growth has shown that, under liquid cross-flow conditions, the bubbles move downstream and are flattened during their growth (position of the bubble centre of gravity, bubble inclination angle). The bubbles spread over the orifice surface, and the advancing and the receding bubble angles were measured. The detached bubbles have significantly smaller sizes and higher frequencies when compared to bubble formation under quiescent liquid conditions

Dynamics of bubble growth and detachment from rigid and flexible orifices

The objective of this paper is to understand how and why the orifice nature (rigid or flexible) governs the bubble generation. The differences in orifice nature and properties have strong consequences on the bubbles generated. Indeed, the dynamics of the formation and the nature of the detached bubbles are fundamentally different depending on whether the bubbles are generated from the rigid orifice or from the flexible orifice. Keywords. Gas-Liquid reactors, aeration, rigid and flexible orifices, bubble formation dynamics. Résumé. L’objectif de cette étude est de comprendre comment et pourquoi la nature de l’orifice (rigide ou flexible) contrôle la génération de bulles. Les différences de nature et de propriétés entre les deux orifices ont des conséquences notables sur les bulles générées. En effet, la dynamique de formation et la nature des bulles détachées sont fondamentalement différentes selon si elles sont générées par un orifice rigide ou par un orifice flexible

Reactive oxygen species induced by water containing nano-bubbles and its role in the improvement of barley seed germination

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The study of reactive oxygen species (ROS) generation caused by nano-bubbles (NBs) is of great importance for its application in both physiological activity promotion aspect and sterilization aspect. In this paper, Aminophenyl Fluorescein (APF) was used as a fluorescent reagent for the detection of ROS generation by NBs. The NBs scattering could cause the decrease of fluorescence intensity. Meanwhile, the quenching effect of oxygen could also cause the decrease of fluorescence intensity. Although the above two factors masked the fluorescence intensity generation by ROS, the fluorescence intensity of the water containing NBs still increased with NBs generation time, which proved that oxygen NBs could generate ROS. Moreover, the endogenous ROS in the barley seed cells were measured in the seed that germinated in the water containing NBs and the distilled water respectively. According to the results of seed staining experiments, both the microscope images and the absorbance at 560nm proved that the seeds dipped in the water containing NBs could generate more ROS compared to those in the distilled water. These findings greatly increase the NBs potential use both in agricultural field and environmental field

Bubble size prediction in co-flowing streams

In this paper, the size of bubbles formed through the breakup of a gaseous jet in a co-axial microfluidic device is derived. The gaseous jet surrounded by a co-flowing liquid stream breaks up into monodisperse microbubbles and the size of the bubbles is determined by the radius of the inner gas jet and the bubble formation frequency. We obtain the radius of the gas jet by solving the Navier-Stokes equations for low Reynolds number flows and by minimization of the dissipation energy. The prediction of the bubble size is based on the system's control parameters only, i.e. the inner gas flow rate $Q_i$, the outer liquid flow rate $Q_o$, and the tube radius $R$. For a very low gas-to-liquid flow rate ratio ($Q_i / Q_o \rightarrow 0$) the bubble radius scales as $r_b / R \propto \sqrt{Q_i / Q_o}$, independently of the inner to outer viscosity ratio $\eta_i/\eta_o$ and of the type of the velocity profile in the gas, which can be either flat or parabolic, depending on whether high-molecular-weight surfactants cover the gas-liquid interface or not. However, in the case in which the gas velocity profiles are parabolic and the viscosity ratio is sufficiently low, i.e. $\eta_i/\eta_o \ll 1$, the bubble diameter scales as $r_b \propto (Q_i/Q_o)^\beta$, with $\beta$ smaller than 1/2