Numerical, experimental, and theoretical investigation of bubble aggregation and deformation in magnetic fluids (original) (raw)
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Influence of magnetic fields on the behavior of bubbles in liquid metals
The European Physical Journal Special Topics, 2013
In this work the ultrasound-transit time technique is introduced as a versatile method to analyze the bubble dynamics in liquidmetal-gas flows. After discussing the principle of operation and the implementation of the technique, the methods used to extract the positions of the bubbles, their velocities, or their diameters are explained. Finally, the performance of the method is demonstrated for a liquidmetal-gas flow with and without a magnetic field.
Generation and manipulation of bubbles and foams stabilised by magnetic nanoparticles
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2011
Much interest is being given these days to the development of techniques, which render liquid foams not only super-stable, but also responsive to non-contact stimulation. Both of these goals can be met simultaneously by the application of magnetic particles as stabilising agents, which not only suppress the traditional ageing mechanisms of foams (such as coalescence and coarsening), but also provide means of non-contact manipulation and characterisation by the application of magnetic fields. In this spirit we provide here detailed investigations into the use of commercially available MAGSILICA ® H8 nanoparticles, which consist of single-domain iron oxides surrounded by a silica shell, for the generation of superstable bubbles and foams, and study their response to various types of magnetic fields. More specifically, we investigate the foaming behaviour of hydrophobic magnetic particles in water/ethanol mixtures for which we confirm that best foaming behaviour is achieved for intermediate wettabilities of the particles. We characterise the magnetisation of these foams and use magnetic field gradients to not only validate the super-stability of bubbles, but also to demonstrate the ease of their magnetic manipulation. Both kinds of measurements provide us with more detailed information on particle-detachment forces and the presence of a thick shell of aggregated particles at the gas/liquid interface. Last but not least, we demonstrate how such foams may conveniently be heated and kept at well-controlled temperatures through the application of oscillating magnetic fields.
Journal of Magnetism and Magnetic Materials, 2018
Multiphase flow is one of the most complicated problems, considering the multiplicity of the related parameters, especially the external factors influences. Thus, despite the recent developments more investigations are still required. The effect of a uniform magnetic field on the hydrodynamics behavior of a two-phase flow with different magnetic permeability is presented in this article. A single water vapor bubble which is rising inside a channel filled with ferrofluid has been simulated numerically. To capture the phases interface, the Volume of Fluid (VOF) model, and to solve the governing equations, the finite volume method has been employed. Contrary to the prior anticipations, while the consisting fluids of the flow are dielectric, uniform magnetic field causes a force acting normal to the interface toward to the inside of the bubble. With respect to the applied magnetic field direction, the bubble deformation due to the magnetic force increases the bubble rising velocity. Moreover, the higher values of applied magnetic field strength and magnetic permeability ratio resulted in the further increase of the bubble rising velocity. Also it is indicated that the flow mixing and the heat transfer rate is increased by a bubble injection and applying a magnetic field. The obtained results have been concluded that the presented phenomenon with applying a magnetic field can be used to control the related characteristics of the multiphase flows. Compared to the previous studies, implementing the applicable cases using the common and actual materials and a significant reduction of the CPU time are the most remarkable advantages of the current study.
Fluids
Rising bubbles in liquid metals in the presence of magnetic fields is an important phenomenon in many engineering processes. The nonlinear behavior of the terminal rise velocities of the bubbles as a function of increasing field strength has been observed experimentally, but it remains poorly understood. We offer an explanation of the phenomenon through numerical calculations. A single rising bubble in stagnant liquid metal in the presence of an applied horizontal magnetic field is simulated. The observed nonlinear behavior is successfully reproduced; the terminal velocity increases with the increase in the magnetic field strength in the lower magnetic field regions but decreases in higher regions. It is shown that, in the lower region, the increase in the average bubble rise velocity results from the suppression of the fluctuations in the bubble trajectory in the vertical plane perpendicular to the magnetic field, as a consequence of the Lorentz force resulting from the component o...
Morphology of Rising Hydrodynamic and Magnetohydrodynamic Bubbles from Numerical Simulations
Astrophysical Journal, 2004
Recent Chandra and XMM-Newton observations of galaxy cluster cooling flows have revealed X-ray emission voids of up to 30 kpc in size that have been identified with buoyant, magnetized bubbles. Motivated by these observations, we have investigated the behavior of rising bubbles in stratified atmospheres using the Flash adaptive-mesh simulation code. We present results from 2-D simulations with and without the effects of magnetic fields, and with varying bubble sizes and background stratifications. We find purely hydrodynamic bubbles to be unstable; a dynamically important magnetic field is required to maintain a bubble's integrity. This suggests that, even absent thermal conduction, for bubbles to be persistent enough to be regularly observed, they must be supported in large part by magnetic fields. Thermal conduction unmitigated by magnetic fields can dissipate the bubbles even faster. We also observe that the bubbles leave a tail as they rise; the structure of these tails can indicate the history of the dynamics of the rising bubble.
Direct observations of field-induced assemblies in magnetite ferrofluids
Journal of applied physics, 2015
Evolution of microstructures in magnetite-based ferrofluids with weak dipolar moments (particle size ≤ 10 nm) is studied with an emphasis on examining the effects of particle concentration (ϕ) and magnetic field strength (H) on the structures. Nanoparticles are dispersed in water at three different concentrations, ϕ = 0.15%, 0.48%, and 0.59% (w/v) [g/ml%] and exposed to uniform magnetic fields in the range of H = 0.05-0.42 T. Cryogenic transmission electron microscopy is employed to provide in-situ observations of the field-induced assemblies in such systems. As the magnetic field increases, the Brownian colloids are observed to form randomly distributed chains aligned in the field direction, followed by head-to-tail chain aggregation and then lateral aggregation of chains termed as zippering. By increasing the field in low concentration samples, the number of chains increases, though their length does not change dramatically. Increasing concentration increases the length of the lin...
Field-induced self-assembled ferrofluid aggregation in pulsatile flow
Physics of fluids, 2005
Ferrofluid aggregation and dispersion occurs at several length scales in pulsatile flow applications, e.g., in ferrofluidic pumps, valves, and biomedical applications such as magnetic drug targeting. Because of a yet limited understanding, ferrohydrodynamic investigations involving laboratoryscale studies in idealized geometries are of considerable use. We have injected a ferrofluid into a pulsatile host flow and produced field-induced dissolution ͑aggregation͒ using external magnets. A comparison is made with ferrofluid aggregation in a steady flow. Subsequently, the accumulation and dispersion of the ferrofluid aggregates in pulsatile flow are characterized by analyzing their size, mean position, and the flow frequency spectrum. The maximum aggregate size A max , time to form it t max , and the aggregate half-life t half are found to scale according to the relations A max ϰ Re −0.71 , t max ϰ Re −2.1 , and t half ϰ Re −2.2 . While the experiments are conducted at a macroscopic length scale for useful experimental resolution, the results also enable an understanding of the micro-and mesoscale field-assisted self-assembly of magnetic nanoparticles.
Journal of Magnetism and Magnetic Materials, 2015
We present a dynamic study of soft magnetic, commercial Fe and Ni micrometer-sized particles dispersed in oleic acid and subjected to a variable (rotating) magnetic field in the horizontal plane. A very complex structure is formed after the particles decant towards the bottom liquid-solid interface and the magnetic field is applied for several minutes. The dynamics of structure formation was studied by means of the registration and analysis of microscopic video images, through a Matlab image analysis script. Several parameters, such as the number of clusters, the perimeter-based fractal dimension and circularity, were calculated as a function of time. The time evolution of the number of clusters was found to follow a power-law behavior, with an exponent consistent with that found in other studies for magnetic systems, whereas the typical formation time depends on the particle diameter and field configuration. Complementarily, the magnetic properties of the formed structure were studied, reproducing the experiment with liquid paraffin as the containing fluid, and then letting it solidify. The sample obtained was studied by vibrating sample magnetometry. The magnetization curves show that the material obtained is a planar magnetically anisotropic material, which could eventually be used as an anisotropic magnetic sensor or actuator.