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2 pages, 143 KiB

Open AccessEditorial

Editorial for Special Issue “Drop, Bubble and Particle Dynamics in Complex Fluids”

by Shahriar Afkhami and Pengtao Yue

Fluids 2020, 5(1), 4; https://doi.org/10.3390/fluids5010004 - 2 Jan 2020

Cited by 1 | Viewed by 1896

Abstract

The presence of drops, bubbles, and particles affects the behavior and response of complex multiphase fluids [...] Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

Research

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14 pages, 1656 KiB

Open AccessArticle

Dynamics of an Ellipse-Shaped Meniscus on a Substrate-Supported Drop under an Electric Field

by Philip Zaleski and Shahriar Afkhami

Fluids 2019, 4(4), 200; https://doi.org/10.3390/fluids4040200 - 29 Nov 2019

Cited by 2 | Viewed by 3526

Abstract

The behavior of a conducting droplet and a dielectric droplet placed under an electric potential is analyzed. Expressions for drop height based on electrode separation and the applied voltage are found, and problem parameters associated with breakup and droplet ejection are classified. Similar [...] Read more.

The behavior of a conducting droplet and a dielectric droplet placed under an electric potential is analyzed. Expressions for drop height based on electrode separation and the applied voltage are found, and problem parameters associated with breakup and droplet ejection are classified. Similar to previous theoretical work, the droplet interface is restricted to an ellipse shape. However, contrary to previous work, the added complexity of the boundary condition at the electrode is taken into account. To gain insight into this problem, a two-dimensional droplet is addressed. This allows for conformal maps to be used to solve for the potential surrounding the drop, which gives the total upward electrical force on the drop that is then balanced by surface tension and gravitational forces. For the conducting case, the maximum droplet height is attained when the distance between the electrode and the drop becomes sufficiently large, in which case, the droplet can stably grow to about 2.31 times its initial height before instabilities occur. In the dielectric case, hysteresis can occur for certain values of electrode separation and relative permittivity. Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

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21 pages, 1062 KiB

Open AccessArticle

Modeling the Excess Velocity of Low-Viscous Taylor Droplets in Square Microchannels

by Thorben Helmers, Philip Kemper, Jorg Thöming and Ulrich Mießner

Fluids 2019, 4(3), 162; https://doi.org/10.3390/fluids4030162 - 2 Sep 2019

Cited by 13 | Viewed by 3491

Abstract

Microscopic multiphase flows have gained broad interest due to their capability to transfer processes into new operational windows and achieving significant process intensification. However, the hydrodynamic behavior of Taylor droplets is not yet entirely understood. In this work, we introduce a model to [...] Read more.

Microscopic multiphase flows have gained broad interest due to their capability to transfer processes into new operational windows and achieving significant process intensification. However, the hydrodynamic behavior of Taylor droplets is not yet entirely understood. In this work, we introduce a model to determine the excess velocity of Taylor droplets in square microchannels. This velocity difference between the droplet and the total superficial velocity of the flow has a direct influence on the droplet residence time and is linked to the pressure drop. Since the droplet does not occupy the entire channel cross-section, it enables the continuous phase to bypass the droplet through the corners. A consideration of the continuity equation generally relates the excess velocity to the mean flow velocity. We base the quantification of the bypass flow on a correlation for the droplet cap deformation from its static shape. The cap deformation reveals the forces of the flowing liquids exerted onto the interface and allows estimating the local driving pressure gradient for the bypass flow. The characterizing parameters are identified as the bypass length, the wall film thickness, the viscosity ratio between both phases and the

C a

number. The proposed model is adapted with a stochastic, metaheuristic optimization approach based on genetic algorithms. In addition, our model was successfully verified with high-speed camera measurements and published empirical data. Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

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27 pages, 7413 KiB

Open AccessArticle

DEM/CFD Simulations of a Pseudo-2D Fluidized Bed: Comparison with Experiments

by Ziad Hamidouche, Yann Dufresne, Jean-Lou Pierson, Rim Brahem, Ghislain Lartigue and Vincent Moureau

Fluids 2019, 4(1), 51; https://doi.org/10.3390/fluids4010051 - 15 Mar 2019

Cited by 7 | Viewed by 4674

Abstract

The present work investigates the performance of a mesoscopic Lagrangian approach for the prediction of gas-particle flows under the influence of different physical and numerical parameters. To this end, Geldart D particles with 1 mm diameter and density of 2500 kg/m

^{3}

are [...] Read more.

The present work investigates the performance of a mesoscopic Lagrangian approach for the prediction of gas-particle flows under the influence of different physical and numerical parameters. To this end, Geldart D particles with 1 mm diameter and density of 2500 kg/m

^{3}

are simulated in a pseudo-2D fluidized bed using a Discrete Element Method (DEM)/Large-Eddy Simulation (LES) solver called YALES2. Time-averaged quantities are computed and compared with experimental results reported in the literature. A mesh sensitivity analysis showed that better predictions regarding the particulate phase are achieved when the mesh is finer. This is due to a better description of the local and instantaneous gas-particle interactions, leading to an accurate prediction of the particle dynamics. Slip and no slip wall conditions regarding the gas phase were tested and their effect was found negligible for the simulated regimes. Additional simulations showed that increasing either the particle-particle or the particle-wall friction coefficients tends to reduce bed expansion and to initiate bubble formation. A set of friction coefficients was retained for which the predictions were in good agreement with the experiments. Simulations for other Reynolds number and bed weight conditions are then carried out and satisfactory results were obtained. Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

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20 pages, 1509 KiB

Open AccessArticle

A Numerical Study of Particle Migration and Sedimentation in Viscoelastic Couette Flow

by Michelle M. A. Spanjaards, Nick O. Jaensson, Martien A. Hulsen and Patrick D. Anderson

Fluids 2019, 4(1), 25; https://doi.org/10.3390/fluids4010025 - 11 Feb 2019

Cited by 8 | Viewed by 3427

Abstract

In this work, a systematic investigation of the migration of sedimenting particles in a viscoelastic Couette flow is presented, using finite element 3D simulations. To this end, a novel computational approach is presented, which allows us to simulate a periodic configuration of rigid [...] Read more.

In this work, a systematic investigation of the migration of sedimenting particles in a viscoelastic Couette flow is presented, using finite element 3D simulations. To this end, a novel computational approach is presented, which allows us to simulate a periodic configuration of rigid spherical particles accurately and efficiently. To study the different contributions to the particle migration, we first investigate the migration of particles sedimenting near the inner wall, without an externally-imposed Couette flow, followed by the migration of non-sedimenting particles in an externally-imposed Couette flow. Then, both flows are combined, i.e., sedimenting particles with an externally-imposed Couette flow, which was found to increase the migration velocity significantly, yielding migration velocities that are higher than the sum of the combined flows. It was also found that the trace of the conformation tensor becomes asymmetric with respect to the particle center when the particle is initially placed close to the inner cylinder. We conclude by investigating the sedimentation velocity with an imposed orthogonal shear flow. It is found that the sedimentation velocity can be both higher or lower then the Newtonian case, depending on the rheology of the suspending fluid. Specifically, a shear-thinning viscosity is shown to play an important role, which is in-line with previously-published results. Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

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13 pages, 636 KiB

Open AccessArticle

Encapsulation of Droplets Using Cusp Formation behind a Drop Rising in a Non-Newtonian Fluid

by Raphaël Poryles and Roberto Zenit

Fluids 2018, 3(3), 54; https://doi.org/10.3390/fluids3030054 - 1 Aug 2018

Cited by 6 | Viewed by 3779

Abstract

The rising of a Newtonian oil drop in a non-Newtonian viscous solution is studied experimentally. In this case, the shape of the ascending drop is strongly affected by the viscoelastic and shear-thinning properties of the surrounding liquid. We found that the so-called velocity [...] Read more.

The rising of a Newtonian oil drop in a non-Newtonian viscous solution is studied experimentally. In this case, the shape of the ascending drop is strongly affected by the viscoelastic and shear-thinning properties of the surrounding liquid. We found that the so-called velocity discontinuity phenomena is observed for drops larger than a certain critical size. Beyond the critical velocity, the formation of a long tail is observed, from which small droplets are continuously emitted. We determined that the fragmentation of the tail results mainly from the effect of capillary effects. We explore the idea of using this configuration as a new encapsulation technique, where the size and frequency of droplets are directly related to the volume of the main rising drop, for the particular pair of fluids used. These experimental results could lead to other investigations, which could help to predict the droplet formation process by tuning the two fluids’ properties, and adjusting only the volume of the main drop. Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

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18 pages, 3787 KiB

Open AccessArticle

Direct Numerical Simulation of Particles in Spatially Varying Electric Fields ^†

by Edison Amah, Muhammad Janjua and Pushpendra Singh

Fluids 2018, 3(3), 52; https://doi.org/10.3390/fluids3030052 - 24 Jul 2018

Cited by 5 | Viewed by 4435

Abstract

A numerical scheme is developed to simulate the motion of dielectric particles in the uniform and nonuniform electric fields of microfluidic devices. The motion of particles is simulated using a distributed Lagrange multiplier method (DLM) and the electric force acting on the particles [...] Read more.

A numerical scheme is developed to simulate the motion of dielectric particles in the uniform and nonuniform electric fields of microfluidic devices. The motion of particles is simulated using a distributed Lagrange multiplier method (DLM) and the electric force acting on the particles is calculated by integrating the Maxwell stress tensor (MST) over the particle surfaces. One of the key features of the DLM method used is that the fluid-particle system is treated implicitly by using a combined weak formulation, where the forces and moments between the particles and fluid cancel, as they are internal to the combined system. The MST is obtained from the electric potential, which, in turn, is obtained by solving the electrostatic problem. In our numerical scheme, the domain is discretized using a finite element scheme and the Marchuk-Yanenko operator-splitting technique is used to decouple the difficulties associated with the incompressibility constraint, the nonlinear convection term, the rigid-body motion constraint and the electric force term. The numerical code is used to study the motion of particles in a dielectrophoretic cage which can be used to trap and hold particles at its center. If the particles moves away from the center of the cage, a resorting force acts on them towards the center. The MST results show that the ratio of the particle-particle interaction and dielectrophoretic forces decreases with increasing particle size. Therefore, larger particles move primarily under the action of the dielectrophoretic (DEP) force, especially in the high electric field gradient regions. Consequently, when the spacing between the electrodes is comparable to the particle size, instead of collecting on the same electrode by forming chains, they collect at different electrodes. Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

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14 pages, 3048 KiB

Open AccessArticle

Evolution of the Size Distribution of an Emulsion under a Simple Shear Flow

by Jairo M. Leiva and Enrique Geffroy

Fluids 2018, 3(3), 46; https://doi.org/10.3390/fluids3030046 - 25 Jun 2018

Cited by 17 | Viewed by 4353

Abstract

Understanding the rheology of immiscible liquids mixtures, as well as the role played by its micro-structures are important criteria for the production of new materials and processes in industry. Here, we study changes over time of the droplet size distributions of emulsions induced [...] Read more.

Understanding the rheology of immiscible liquids mixtures, as well as the role played by its micro-structures are important criteria for the production of new materials and processes in industry. Here, we study changes over time of the droplet size distributions of emulsions induced by slow shearing flows. We observe that the initial heterogeneous microstructure may evolve toward more complex structures (such as bimodal distribution) as a result of coalescence and rupture of droplets. These dynamic structures were produced using a flow cell made up of two parallel disks, separated by a gap of 100 µm. The steady rotation of the lower disk generates a simple shear flow of

\dot{γ} = 0.75 s^{- 1}

, during

~ 400

s. After a brief rest time, this procedure was repeated by applying a step ramp until the maximum shear rate of

4.5 s^{- 1}

was reached, using step increments of

0.75 s^{- 1}

. During the last portion of the flow and during the rest time in between flows, structures of emulsions were characterized. Initially, a broad single-peak distribution of drops was observed, which evolved toward a rather narrower bimodal distribution, at first due to the coalescence of the smaller droplets and subsequently of the larger drops. The rupture of drops at higher shear rates was also observed. The observed evolutions also presented global structures such as “pearl necklaces” or “bands of particles”, the latter characterized by alternating bands of a high density of particles and regions of the continuous phase with only a few droplets. These changes may indicate complex, time-dependent rheological properties of these mixtures. Full article

(This article belongs to the Special Issue Drop, Bubble and Particle Dynamics in Complex Fluids )

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