Electrohydrodynamic linear stability of two immiscible fluids in channel flow

The electrohydrodynamic instability of the interface between two viscous fluids with different electrical properties in plane Poiseuille flow has recently found applications in mixing and droplet formation in microfluidic devices. In this paper, we perform the stability analysis in the case where the fluids are assumed to be leaky dielectrics. The two-layer system is subjected to an electric field normal to the interface between the two fluids. We make no assumption on the magnitude of the ratio of fluid to electric time scales, and thus solve the full conservation equation for the interfacial charge. The electric field is found to be either stabilizing or destabilizing, and the influence of the various parameters of the problem on the interface stability is thoroughly analyzed.     

Deformation and breakup of a second-order fluid droplet in an electric field

The effects of elastic property on the deformation and breakup of an uncharged drop in a uniform electric field are investigated theoretically using the second-order fluid model as a constitutive equation. Two dimensionless numbers, the electric capillary number (C) and the Deborah number (De), the dimensionless parammeters governing the problem. The asymptotic analytic solution of the nonlinear free boundary problem is determined by utilizing the method of domain perturbation in the limit of small mathcal C and small De. The asymptotic solution provides the limiting point of C above which no steady-state drop shape exists. The linear stability theory shows that the elastic property of fluids give either stabilizing or destabilizing effect on the drop, depending on the deformation mode.     

Thermal Instability of Oldroydian Viscoelastic Fluid with Suspended Particles in Hydromagnetics in Porous Medium

The effect of suspended particles on the thermal instability of an Oldroydian viscoelastic fluid in hydromagnetics in porous medium is considered. The magnetic field, suspended particles, viscoelasticity, and porous medium effects create oscillatory modes in the system which did not exist previously. For stationary convection, the viscoelastic fluid behaves like a Newtonian fluid and the magnetic field has a stabilizing effect, whereas medium-permeability, suspended particles have destabilizing effects on the system. The sufficient conditions for the avoidance of overstability are obtained, and these also hold good for the case of a Maxwellian viscoelastic fluid.     

Combined Effect of Magnetic Field and Rotation on Thermosolutal Instability of a Compressible Fluid in Porous Medium

The combined effect of magnetic field and rotation on thermosolutal instability of a compressible fluid in porous medium is considered. The system is found to be stable for (C_p/g)β < 1 where C_p, β, and g stand for specific heat at constant pressure, uniform adverse temperature gradient, and acceleration due to gravity, respectively. The stable solute gradient, magnetic field, and rotation introduce oscillatory modes in the system for (C_p/g)β > 1, which were nonexistent in their absence. For stationary convection, the stable solute gradient and rotation have a stabilizing effect on the system for (C_p/g)β > 1. In the presence of rotation, the magnetic field has a stabilizing (or destabilizing) effect, and the medium permeability has a destabilizing (or stabilizing) effect under certain condition, whereas in the absence of rotation, the magnetic field and rotation have stabilizing and destabilizing effects for (C_p/g)β > 1, respectively, on the system. The sufficient conditions for the existence of overs-lability are obtained.     

Instability of Two Maxwellian Viscoelastic Superposed Fluids with Suspended Particles and Variable Magnetic Field in Porous Medium

Instability of the plane interface between two viscoelastic (Maxwellian) superposed conducting fluids in the presence of suspended particles and variable horizontal magnetic field in porous medium is studied. The cases of two fluids of uniform densities, viscosities, magnetic fields, and suspended particles number densities separated by a horizontal boundary; and of exponentially varying density, viscosity, suspended particles number density, and magnetic field are considered. It is found that the stability criterion is independent of the effects of viscoelasticity, medium porosity, and suspended particles but is dependent on the orientation and magnitude of the magnetic field. The magnetic field succeeds in stabilizing a certain range of wavenumbers which were unstable in the absence of the magnetic field. The system is found to be stable for potentially stable configuration/stratifications. The growth rates are found to increase (for certain wavenumbers) and decrease (for other wavenumbers) with the increase in kinematic viscosity, suspended particles number density, magnetic field, medium permeability, and stress relaxation time.     

Compressibility and Hall Effects on Thermosolutal Instability of a Partially Ionized Plasma in Porous Medium

The thermosolutal instability of a compressible and partially ionized plasma in porous medium is considered in the presence of a uniform vertical magnetic field to include the effects of collisions and Hall currents. For the case of stationary convection, Hall currents and medium permeability have destabilizing effects whereas the magnetic field and stable solute gradient have stabilizing effects on the system. The effect of compressibility is found to postpone the onset of convection. The collisional effects disappear for stationary convection. The magnetic field, stable solute gradient, and collisions are found to introduce oscillatory modes in the system for (C _p/g)β > 1, which were nonexistent in their absence. The case of overstability is also studied, leading to the establishment of conditions sufficient for the existence of overstability.     

Shear-thinning film on a porous substrate: Stability analysis of a one-sided model

The temporal stability of a Carreau fluid flowing down an inclined porous substrate is considered. A reduced model is derived under the assumption of small permeability which decouples the flow in the liquid layer from the filtration flow in the porous medium and incorporates the effect of the porous medium by means of an effective slip condition at the liquid–solid interface. The slip coefficient in the effective slip condition is a function of the structure, permeability of the porous medium and the rheology of the fluid saturating the porous medium. The effects of shear-thinning rheology and permeability of the substrate on the stability of the film flow system are investigated. This problem gives rise to a generalized eigenvalue formulation which is solved through two approaches. The problem is solved analytically for long waves in the limiting cases of weakly and strongly non-Newtonian behaviors (power-law limit). A numerical investigation is carried out in the general case. The results are shown to agree well for the weakly non-Newtonian limit. Further, the power-law model and the Carreau model agree on a wide range of shear-thinning parameter values for a thin film over a rigid substrate. However, when considering a porous medium, this trend is not observed. The Carreau model gives valid results for the entire range of shear-thinning parameter values for a film over a rigid/porous substrate. The novelty of the present investigation lies in the inclusion of both the effects of bottom permeability and shear-thinning rheology. Both permeability and shear-thinning rheology have a destabilizing effect on the film flow system. The numerical results indicate the correlation between the effects due to shear-thinning properties and permeability. An energy balance analysis performed on the perturbation fields shows that destabilization induced by both shear-thinning and permeability is linked to the viscous shear work rate on the free surface.     

Kelvin-Helmholtz Instability of a Partially Ionized Plasma in Porous Medium

The Kelvin-Helmholtz instability of a partially ionized plasma in porous medium to include the effects of collisions and medium porosity is considered. The effect of a uniform horizontal magnetic field on the problem is also studied. In the absence of surface tension, perturbations transverse to the direction of streaming are found to be unaffected by the presence of streaming if perturbations in the direction of streaming are ignored, whereas for perturbations in every other direction, there exists instability for a certain wave-number range. When the surface tension is present, it has a stabilizing effect. The instability of this system is postponed by the presence of magnetic field. The magnetic field and surface tension are able to suppress this Kelvin-Helmholtz instability for small wavelength perturbations and medium porosity reduces the stability range given in terms of a difference in streaming velocities and the Alfvén's velocity. The collisions between ionized and neutral particles do not have any qualitative effect on the nature of the stability or instability.     

Numerical Investigation of Third‐Body Behavior in Dry and Wet Environments under Plane Shearing

A coupled discrete element method and computational fluid dynamics (DEM‐CFD) approach to model and assess third‐body behavior in dry and wet environments under plane shearing is presented. DEM is used to model the granular media, while the fluid side of the system is simulated with CFD, which is based on the finite volume method. The applied model is extended to consider buoyancy as well as lubrication effects. The third body is confined and compressed between two walls, which are sheared in opposite direction with a constant velocity. The influence of different shear velocities, fluid viscosities, and gravity orientations on particle and fluid rheology is investigated. Obtained results of both dry and lubricated systems are compared regarding velocity and porosity distribution across the gap, sliding friction, and particle interaction.     

THE EFFECTS OF MEDIUM THICKNESS ON THE ISLAND SIZE DISTRIBUTION IN IMMISCIBLE DISPLACEMENT FLOW IN POROUS MEDIA

The invasion percolation algorithm is used to simulate two-fluid immiscible displacement of a wetting fluid by a non-wetting fluid in various porous media represented by two-dimensional and three-dimensional networks of interconnected capillaries. Trapping of the displaced fluid occurs, thereby creating isolated islands. The effects of the thickness of the porous medium on the island size distribution are studied for capillary displacements for the case in which buoyancy effects are negligible. It was found in a previous study that the number of islands of size s scales approximately as s~" in two-dimensional porous media, where a is a function of the fluid viscosity ratio. The present work reveals that there is a cross-over behavior between the two-dimensional and the three-dimensional problems.     

EFFECT OF MAGNETIC FIELD-DEPENDENT VISCOSITY ON THERMAL CONVECTION IN A FERROMAGNETIC FLUID

This article deals with the theoretical investigation of the effect of magnetic field-dependent (MFD) viscosity on a layer of ferromagnetic fluid heated from below subject to a transverse uniform magnetic field. For a flat fluid layer contained between two free boundaries, an exact solution is obtained using a linear stability analysis and normal mode analysis method. For the case of stationary convection, the MFD viscosity has a stabilizing effect, whereas the departure of linearity in the magnetic equation of state has a destabilizing (or stabilizing) effect on the system under certain conditions. The critical wave number and critical magnetic thermal Rayleigh number for the onset of instability are also determined numerically for large values of buoyancy magnetization, and results are depicted graphically. The principle of exchange of stabilities is valid for the ferromagnetic fluid heated from below.     

Experimental studies on the flow of two immiscible fluids in porous media using X-ray computed tomography: The case when the viscosities of the fluids are equal

Experiments were carried out to demonstrate the dispersion that occurs at the interface between fluids when two immiscible fluids flow in porous structures. In this work the porous medium was cellulosic absorbent and the fluids, decyl alcohol and water, were modified so as to cover a range of flow rates and identical fluid viscosities. Computerized Tomography (CT) was used to generate dynamic three-dimensional images of two-phase saturations and provided quantitative information of time evolution of fluid saturation at each position. Thus, use of CT made characterization of two-phase displacement history in cellulosic porous media possible. This work could relate experimental fluid saturation to theoretical model of immiscible fluids flow.     

NMR imaging of mass transport processes and catalytic reactions

NMR imaging and spectroscopy techniques are applied to study flow and filtration of liquids, gases and granular solids in various geometries and to the in situ studies of the interplay of mass transport and catalytic reactions in porous media. In particular, quantitative spatially resolved maps of flow velocities of liquids and gases in the channels of monoliths have been obtained. A comparative study of the filtration of water and propane through model porous media has revealed that the dispersion coefficients for water are dominated by the holdup effects even in a bed of nonporous glass beads. Similar experiments performed with the gravity driven flow of liquid-containing fine solid particles through a porous bed have yielded the distributions of particle velocities for various flow rates. The NMR imaging technique was employed to visualize the propagation of autocatalytic waves for the Belousov–Zhabotinsky reaction carried out in a model porous medium. It was demonstrated that the wave propagation velocity decreases as the wave crosses the boundary between the bulk liquid and the flooded bead pack. The images detected during the catalytic hydrogenation of -methylstyrene on a single catalyst pellet at elevated temperatures have revealed that the reaction and the accompanying phase transition alter the distribution of the liquid phase within the pellet.     

Effect of dispersant on the rheological properties and slip casting of concentrated sialon precursor suspensions

The ability of Hypermer KD1 to disperse high solids loading reaction sialon suspensions for slip casting has been characterised. It has been found to be a very effective dispersant in organic media of 60-vol.% MEK and 40-vol.% Ethanol, yielding fluid and highly homogeneous suspensions. The effects of added amounts of KD1 have been observed through adsorption data, sedimentation tests and rheology measurements. KD1 imparts low viscosity and stability to the suspension. It has been found that 3-wt.% addition of KD1, based on the weight of reaction sialon powders, results in a very stable and high flowable suspension with near Newtonian flow behavior. Less amounts of dispersant lead to unstable suspensions with obvious shear thinning flow behaviors, while adding excessive dispersant leads to high viscosities, especially at high solids loading. Measuring the pore size distribution of green bodies from different suspensions has proved the effects of dispersant amounts on dispersing the slurries and on slip casting performance.     

Donor impurity-related linear and nonlinear intraband optical absorption coefficients in quantum ring: effects of applied electric field and hydrostatic pressure

The linear and nonlinear intraband optical absorption coefficients in GaAs three-dimensional single quantum rings are investigated. Taking into account the combined effects of hydrostatic pressure and electric field, applied along the growth direction of the heterostructure, the energies of the ground and first excited states of a donor impurity have been found using the effective mass approximation and a variational method. The energies of these states are examined as functions of the dimensions of the structure, electric field, and hydrostatic pressure. We have also investigated the dependencies of the linear, nonlinear, and total optical absorption coefficients as a function of incident photon energy for several configurations of the system. It is found that the variation of distinct sizes of the structure leads to either a redshift and/or a blueshift of the resonant peaks of the intraband optical spectrum. In addition, we have found that the application of an electric field leads to a redshift, whereas the influence of hydrostatic pressure leads to a blueshift (in the case of on-ring-center donor impurity position) of the resonant peaks of the intraband optical spectrum.     

Effect of the injection-nozzle geometry on the interaction between a gas–liquid jet and a gas–solid fluidized bed

In industrial fluid cokers, bitumen is first mixed with steam in a premixer, and then fed to the atomization nozzle. The objective of this work was to evaluate the impact of both the premixer and the nozzle geometrical configuration on the quality of the liquid–solid contact resulting from injections of liquid into a gas–solid fluidized bed. To assess the quality of the liquid–solid contact a method based on electric conductance measurements of the bed material previously developed by the authors [9] was used. Liquid atomization efficiency in open air, spray geometry, and spray stability were also characterized to evaluate their effects on the nozzle spraying performance within the fluidized bed. This study indicated that spray stability is highly beneficial to the liquid–solid contact efficiency. In particular, fluid constrictions such as the series of converging and diverging sections within the nozzle have a stabilizing effect on the spray. Future optimization of the existing liquid-injection systems should consider alternative gas–liquid premixers and nozzle geometries to enhance the jet stability.     

Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures

A two-dimensional elliptic, computational fluid dynamics (CFD) model of a microburner is solved to study the effects of microburner dimensions, conductivity and thickness of wall materials, external heat losses, and operating conditions on combustion characteristics and flame stability. We have found that the wall conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Two modes of flame extinction occur: a spatially global type for large wall thermal conductivities and/or low flow velocities and blowout. It is shown that there exists a narrow range of flow velocities that permit sustained combustion within a microburner. Large transverse and axial gradients are observed even at these small scales under certain conditions. Periodic oscillations are observed near extinction in cases of high heat loss. Engineering maps that delineate flame stability, extinction, and blowout are constructed. Design recommendations are finally made.     

Spectroscopic and In Silico Studies on the Interaction of Substituted Pyrazolo[1,2-a]benzo[1,2,3,4]tetrazine-3-one Derivatives with c-Myc G4-DNA

Herein we describe a combined experimental and in silico study of the interaction of a series of pyrazolo[1,2-a]benzo[1,2,3,4]tetrazin-3-one derivatives (PBTs) with parallel G-quadruplex (GQ) DNA aimed at correlating their previously reported anticancer activities and the stabilizing effects observed by us on c-myc oncogene promoter GQ structure. Circular dichroism (CD) melting experiments were performed to characterize the effect of the studied PBTs on the GQ thermal stability. CD measurements indicate that two out of the eight compounds under investigation induced a slight stabilizing effect (2–4 °C) on GQ depending on the nature and position of the substituents. Molecular docking results allowed us to verify the modes of interaction of the ligands with the GQ and estimate the binding affinities. The highest binding affinity was observed for ligands with the experimental melting temperatures (T_ms). However, both stabilizing and destabilizing ligands showed similar scores, whilst Molecular Dynamics (MD) simulations, performed across a wide range of temperatures on the GQ in water solution, either unliganded or complexed with two model PBT ligands with the opposite effect on the T_ms, consistently confirmed their stabilizing or destabilizing ability ascertained by CD. Clues about a relation between the reported anticancer activity of some PBTs and their ability to stabilize the GQ structure of c-myc emerged from our study. Furthermore, Molecular Dynamics simulations at high temperatures are herein proposed for the first time as a means to verify the stabilizing or destabilizing effect of ligands on the GQ, also disclosing predictive potential in GQ-targeting drug discovery.     

Experimental and Theoretical Study of an Autowave Process in a Magnetic Fluid

Magnetic fluid (MF) is a colloidal system consisting of ferromagnetic particles (magnetite) with a diameter of ~10 nm suspended in a dispersion medium of a carrier fluid (for example, kerosene). A distinctive feature of magnetic fluid is the fact that when an electric field is applied to it using two electrodes, thin layers consisting of close-packed particles of the dispersed phase are formed in the regions near the surface of both electrodes. These layers significantly affect the macroscopic properties of the colloidal system. In this work, the interpretation of the near-electrode layer is for the first time given as a new type of liquid membrane, in which the particles of the dispersed phase become charged with the opposite sign. On the basis of experimental studies, we propose a physicochemical mechanism of the autowave process in a cell with a magnetic fluid. It is based on the idea of oppositely recharging colloidal particles of magnetite in a liquid membrane. A mathematical model of an autowave process, which is described by a system of coupled partial differential equations of Nernst–Planck–Poisson and Navier–Stokes with appropriate boundary conditions, is proposed for the first time. One-dimensional, two-dimensional, and three-dimensional versions of the model are considered. The dependence of the frequency of concentration fluctuations on the stationary voltage between the electrodes was obtained, and the time of formation of a liquid membrane was estimated. Qualitative agreement between theoretical and experimental results has been established.     

CRITICAL POINTS IN THE SOLIDIFICATION OF A PURE MATERIAL

Patterns in the solidification of a pure material are dealt with in this article. Results, deduced from a simple model based on heat conduction in the two phases and the effect of surface tension on the equilibrium temperature at the moving front, present a guide for experimental work. By introducing far-field conditions imitating what can be achieved in an experiment, we explain how the depths of the phases and the width of the container influence the patterns that can be seen if one advances the control variable to the critical point and then just beyond. Our new result is the existence of a third critical point. It occurs at small wave numbers and it is independent of surface tension. It appears because we take the depths of the phases into account. These depths are input values that offer the possibility of controlling crest-to-trough conduction, stabilizing in the solid, destabilizing in the liquid. The new critical point, and the patterns attending its appearance, can be found in cells of easily attainable widths.