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The bubble dynamic behaviors on the electrode surface are significantly affected by the magnetohydrodynamic (MHD) flow induced by the Lorentz force resulting from the applied magnetic field. However, the mechanism of the impact of a magnetic field perpendicular to the electrode on the dynamic behaviors of bubble remain unclear. In this study, the evolution law of a single oxygen bubble on the surface of a TiO2 photoelectrode under the influence of a vertical magnetic field with different magnetic induction intensities was investigated. The presence of a vertical magnetic field effectively promoted the rapid detachment of bubbles from the electrode surface and greatly improved the gas evolution efficiency. The mass transfer was dominated by single-phase free convection during bubble evolution. Under the influence of the vertical magnetic field, the mass transfer coefficient near the electrode was enhanced, and the adverse effects of bubbles on mass transfer were counteracted when they entered the reaction-controlled growth phase. The further multiphysics simulation found that the MHD convection decreased the local supersaturation of reaction product oxygen molecules near the electrode, thereby reducing concentration overpotential and increasing the reaction rate. This study provides an experimental and theoretical basis for promoting the bubble detachment and the reaction rate on the gas evolving electrode surface during photoelectrochemical water splitting. [ABSTRACT FROM AUTHOR]

The liquid carry-over (LCO) phenomenon brings about the performance deterioration of gas–liquid cyclone separator and an increase in pressure drop. However, the formation mechanism of the LCO and its manifestation in the separator cylinder and the overflow pipe have not been fully understood. This work investigated the flow process of the LCO by visual observation and quantitative measurement of the overflow liquid flow rate and liquid holdup. The transient gas–liquid flow feature in the overflow pipe and spatiotemporal relationship between the separator inlet and outlet were characterized by time-frequency analysis and wavelet coherence of liquid holdup, respectively. The results showed that the size of air core determines two kinds of sources of the LCO, including the surrounding liquid direct entry into the overflow pipe and the film short-circuit flow beneath the top wall of the separator. When the air core can continuously wrap up the overflow inlet, the film short-circuit flow became the primary source of the LCO, which was embodied in the significant reduction of the overflow liquid flow rate. Three flow patterns, namely, slug flow, churn flow, and annular flow, were classified in the overflow pipe. The inlet intermittent flow of the separator led to the distribution of churn flow expanding toward higher gas velocity, which was interpreted by flow pattern transition theory. The time-averaged overflow liquid holdup was well predicted by drift-flux model. The results are beneficial to the proposal of inhibition methods of the LCO and structure design of the separator. [ABSTRACT FROM AUTHOR]

One falling liquid droplet impacting a sessile droplet and then undergoing oscillation is a common phenomenon in both nature and industry. However, to the best of our knowledge, the head-on collision of two nanofluid drops has not been studied when both the nanoparticle and surfactant exist in the drop. In this work, the characteristics of oscillation after head-on collision of two TiO2-water nanofluid drops were investigated experimentally. The effects of impact velocity, drop size, and nanoparticle concentration have been considered to understand how they influence the drop spreading, recoiling, and rebounding. For the influence of the drop size, it shows that in 0% and 0.001% of nanoparticle concentrations, the relative spreading radius of small drop size is higher than that with large size, while it is reversed in 0.01% and 0.1% of nanoparticle mass fractions. Interestingly, it was found that there is an optimal nanoparticle mass fraction of 0.1% for nanofluid droplets where the oscillation of head-on collision can be damped much more rapidly than that for nanoparticle-free droplets. A hypothesis considering the comprehensive interaction of surfactant redistribution, surfactant gradient, and particle initial effect was proposed to explain the observed phenomenon. This work will be of significant reference to various practical applications especially when the drop collision oscillation characteristics are supposed to be under control. [ABSTRACT FROM AUTHOR]

The motion, deformation, and stability of compound droplets in extensional flows are investigated numerically via a three-dimensional spectral boundary element method. We examine the droplet stability under the influences of the capillary number, the inner droplet size and the relative magnitude of the surface tension of the two interfaces composing the compound droplet. The influence of viscosity on the droplet deformation is also discussed. We conclude that a compound droplet with a larger inner droplet and/or smaller inner surface tension is less stable and cannot withstand strong flow. For moderate viscosity ratios, a compound droplet with a more viscous 'shell' exhibits larger deformation at steady state. In addition, for an eccentric compound droplet, both the inner and outer droplets tend to migrate away from its original location due to the asymmetry of the problem. The initial location of the inner droplet also influences the droplet stability as well as the migration velocity of the compound droplet. [ABSTRACT FROM AUTHOR]

A 3D spectral boundary element method is employed to compute the dynamics of a single droplet in a microconfined shear flow. Comparisons have been made for the motion of an initially spherical droplet near a single wall and that between two parallel plates. Investigations are conducted for the influences of the capillary number, viscosity ratio, and initial location of the droplet on the droplet deformation, orientation, velocities, as well as the transition between the initial rapid deformation and the subsequent relaxation stage. Computational results for the deformation and velocities are compared with analytical predictions. It is found that the analytical predictions are limited for small deformations, large droplet-wall distances, and near equiviscous droplets. [ABSTRACT FROM AUTHOR]

The hemodynamic forces on endothelial cells or leukocytes attached to the inner surface of blood vessels affect the physiological behavior of these systems via mechano-transduction or receptor-ligand binding. The present study investigates the relative importance and nature of the two components of the hemodynamic force, i.e., the shear and normal force, on a cell and its vicinity. We consider a wide range of blood vessels (from capillaries to arteries) and cell’s spreading angles. Based on computational investigation and analysis, our study demonstrates that the normal force contributes significantly to the total force on the cell; the influence is much more pronounced in small vessels. We also show that the spreading of the cell on the vessel surface should not be discounted, especially in small vessels. Our results are also applicable to the fluid forces on other protuberances of biological nature attached to vascular vessels (e.g., cancer cells and biofilm) as well as to the fluid forces over protuberances in microfluidics and porous media. [ABSTRACT FROM AUTHOR]

In the present study we describe a novel three-dimensional spectral boundary element algorithm for interfacial dynamics in Stokes flow and/or gravity. The main attraction of this approach is that it exploits all the benefits of the spectral methods (i.e., exponential convergence and numerical stability) with the versatility of the finite element method. In addition, it is not affected by the disadvantage of the spectral methods used in volume discretization to create denser systems. Our algorithm also exploits all the benefits of the boundary element techniques, i.e., a reduction of the problem dimensionality and great parallel scalability. To achieve continuity of the interfacial geometry and its derivatives at the edges of the spectral elements during the droplet deformation, a suitable interfacial smoothing is developed based on a Hermitian-like interpolation. An adaptive mesh reconstructing procedure based on the relevant lengths of the spectral elements is also described. In addition, we consider the inertialess motion of a buoyant droplet left to rise (or sediment) near a vertical solid wall and compare our numerical results with analytical predictions. In our study we emphasize the need for computational studies for the accurate determination of droplet migration near solid walls. [ABSTRACT FROM AUTHOR]