Newtonian Medium Research Articles

A numerical study of Elasto-inertial particle-focusing in straight and serpentine microchannels

Microfluidic devices have gained popularity in recent years due to their low cost and simplicity. Cell focusing linked with cell counting and cell separation used in clinical tests and point-of-care devices stands towards the top of every list of microfluidic applications. To focus and manipulate particles in a Newtonian medium, inertial microfluidics has been widely used. There is a requirement to replace a significantly more efficient approach given various particle-focusing positions near the channel walls recorded by inertial microfluidics. Here, the synergetic effect of elastic and inertial forces in relatively low-concentration viscoelastic fluids was investigated numerically. Utilizing particle-focusing behavior in straight and serpentine channels with different corner angles, by applying the numerical results obtained using the developed direct numerical simulation method (DNS) for elastic and inertial forces, we created a microchannel with a remarkable performance. As the flow regime transitions to elasto-inertial, where inertial forces are comparable to elastic forces in a straight microchannel, particles tend to concentrate at the channel centerline. As the flow rate increases, however, particle defocusing develops, and shear gradient lift forces ultimately overcome elastic forces in the channel's central region. Given that fluid flows at a higher velocity in the serpentine channel with a corner angle of 75° than in other channels, the Dean drag force induced by secondary flow serves to reduce particle defocusing rate at a given flow rate, making this channel a desirable platform for Elasto-inertial microfluidics. Particle focusing in a vertical direction was noticed as a benefit of elasto-inertial microfluidics, regardless of the geometry or flow rate of the particles passing through the channel. Finally, the impact of particle diameter on focusing behavior was studied. As particle diameter decreases, focusing width decreases and the focusing band shifts to the left side of the cross-section. We expect that the numerical evaluation of using viscoelastic medium in complex geometries can contribute to accelerate the three dimensional particle-focusing mechanism and enhance the efficiency of microdevices for cell and particle manipulation.

On the linear viscoelastic behavior of semidilute polydisperse bubble suspensions in Newtonian media

In this work, we investigated the linear viscoelasticity of semidilute polydisperse bubble suspensions via small amplitude oscillatory shear (SAOS) tests performed in a rheo-optical setup. For all tested suspensions, the measured viscoelastic moduli (G′, G″) aligned with the theoretical predictions of the Jeffreys model for average dynamic capillary numbers (⟨Cd⟩) greater than unity. But at lower ⟨Cd⟩ values, experimental G′ values exceeded theoretical predictions. To investigate this, we considered the effects of suspension polydispersity and various SAOS measurement artifacts, including bubble rise, coalescence, and changes in suspension microstructure over time. Polydispersity could not cause the observed deviation, because the viscoelastic trends deviate from those of classic single-mode relaxation only for bimodal bubble size distributions with equal volume fractions of very small and very large bubbles; in any other case, the polydisperse suspension behaves as monodisperse with a bubble radius equal to the volume-weighted mean radius. Furthermore, bubble rise proved to play a minor role, while SAOS rheo-optical experiments revealed that bubble size and organization varied negligibly during our measurements. The G′ deviation at low ⟨Cd⟩ values was linked to bubble fluid dynamic interactions induced by the bubble spatial distribution. Image analysis showed that at low bubble volume fractions, stronger and prolonged preshearing reduces these interactions by increasing the average interbubble distance. But this effect is negligible in denser suspensions, which show similar G′ trends for any applied preshearing. Finally, a multimode Jeffreys model fitted to the experimental data showed that bubble interactions complicate the relaxation process, introducing multiple relaxation modes.

Wall effect on cluster particle's settling terminal velocity and drag coefficient in Newtonian and non-Newtonian fluid medium

The experimental investigation of the wall effect on the cluster particle settling in the Newtonian and non-Newtonian fluid medium is carried out in three different diameter flow channels. The cluster usually forms during the sedimentation of particles in a fluid medium, so it becomes necessary to study the behavior of the cluster particles. Different cluster particles are considered depending on the number of spheres (N) attached to the cluster and the cluster's shape. The present experiment covers the following range of conditions: 0.05 ≤ deq/D ≤ 0.24, 2 ≤ N ≤ 7, 0.64 ≤ n ≤ 1, and 0.14 ≤ K ≤ 1.81. The results reveal that the terminal velocity varies with the blockage ratio (deq/D), N, and cluster shape. For a particular deq/D ratio and the same N, the terminal velocity of polyhedron particles is high compared to the planar and chain shape particles. The blockage ratio and the Reynolds number affect the wall factor of the cluster particles. However, from the experiment, it is observed that the wall effect also depends on the orientation of the particles. The impact of the wall on the cluster particle is high in Newtonian fluid when compared to cluster particles in non-Newtonian fluid. The present work additionally investigates the influence of drag on cluster particles in the presence and absence of the wall effect. The numerical relationships are developed to predict experimental results in Newtonian and non-Newtonian fluid mediums.

Deformation of a Hele–Shaw drop undergoing quadratic flow

A Hele–Shaw quadratic flow in the form of a six-port microfluidic device is employed to study the deformation of a single channel-spanning or Hele–Shaw Newtonian drop suspended in a Newtonian medium. An initially circular drop in a quadratic flow deforms into a regular triarcle, i.e., a rounded-corner triangle. Theoretically, the deformation is calculated in the limit of small capillary number, which is defined as Ca=Cμa4/(4γb2), where C is the quadratic flow rate, μ is the suspending fluid viscosity, a is the drop radius, γ is the interfacial tension, and b is the channel depth. The theory matches reasonably well with the experiment for small capillary numbers. This is the new way of deforming drops on a Hele–Shaw quadratic platform experimentally and may lead to measurements such as complex interfacial properties and breakup.

Effect of polydispersity and bubble clustering on the steady shear viscosity of semidilute bubble suspensions in Newtonian media

In this work, we examine the steady shear rheology of semidilute polydisperse bubble suspensions to elucidate the role of polydispersity on the viscosity of these systems. We prove theoretically that the effect of polydispersity on suspension viscosity becomes apparent only if the bubble size distribution is bimodal, with very small and very large bubbles having similar volume fractions. In any other case, we can consider the polydisperse suspension as monodisperse, with the average bubble diameter equal to the De Brouckere mean diameter (d43). To confirm the theoretical results, we carried out steady shear rheological tests. Our measurements revealed an unexpected double power-law decay of the suspension relative viscosity at average capillary numbers between 0.01 and 1. To investigate this behavior further, we visualized the produced bubble suspensions under shear. The visualization experiments revealed that bubbles started forming clusters and threads at an average capillary number around 0.01, where we observed the first decay of viscosity. Clustering and alignment have been associated with shear-thinning behavior in particle suspensions. We believe that the same holds for bubble suspensions, where bubble clusters and threads align with the imposed shear flow, reducing the streamline distortions and, in turn, resulting in a decrease in the suspension viscosity. Consequently, we can attribute the first decay of the relative viscosity to the formation of bubble clusters and threads, proving that the novel shear-thinning behavior we observed is due to a combination of bubble clustering and deformation.

Binary coalescence of non-Newtonian droplets under an electric field: A numerical study

We numerically investigate the effect of electrohydrodynamics on a non-Newtonian droplet pair suspended in a Newtonian medium. The leaky dielectric model is implemented to study the response of emulsion drops in an externally applied electric field. Subsequently, the non-Newtonian drop behavior is incorporated using the power law model, whereby three different fluid behaviors are considered for the drops: Newtonian, Shear thinning, and Shear thickening. We validated our numerical model with the available literature data, and the results are in good agreement. The droplets' deformation and net motion are investigated for a range of electrical permittivity ratios of the droplet medium with respect to the surrounding fluid. In this study, four distinct regimes are identified based on the net drop pair motion and the circulation pattern that develops due to the electric stresses inside and around the drops. Furthermore, it is observed that the droplet deformation and their net motion are fastest for the pseudo-plastic drops and slowest for dilatant drops. We devised a simple ratio-based model to understand this behavior. The inferences drawn from this study will help contribute to a better understanding of the behavior of nonlinear fluids under an electric field.

The method of experimental determination of tangential stresses on the surface of flow around with a gaseous medium

Experimental research of the force load on the flow around surface is important for the design of aircraft and gas transport systems. The main parameter is the tangential stress, which is maximum in the boundary layer on the flow around surface. The velocity in the boundary layer and its thickness affect the tangential stresses that arise during the flow of the medium. They also have an influence on the friction coefficient, which characterizes the pressure losses. In the work, the method of experimental measurement of velocity on the flow around surface is considered, and an information and measurement system is proposed for determining the tangential stresses on the flow around surface based on the flow velocity measurement in the boundary layer. The algorithm of the measurement technique is presented in the form of a functional structural scheme of the laminar boundary layer on the surface of the flow around. The sensor for measuring the velocity in the boundary layer is installed at characteristic points namely at x distance from the front border of the flow around surface, and also at y height from the surface of the boundary layer. The results of the planned factorial experiment of air flow around the surface are given. The factors were the v 0 velocity of the air flow entering the flow around surface, the x distance from the edge of the surface, the y distance from the surface along the thickness of the layer. The response criterion was the calculated value of the tangential stress based on the vx measured flow velocity at a given point of the boundary layer. The developed method of studying the velocity distribution and determining the tangential stresses on the surface of the flow around under a dynamic boundary layer makes it possible to determine with high discreteness the parameters of the process of the surface around flow of a Newtonian gaseous medium.

Application of a combined PIV/PTV method to analyze suspensions in (non‐)Newtonian media in stirred tanks

Application of a combined PIV/PTV method to analyze suspensions in (non‐)Newtonian media in stirred tanks

Immersed buoyant viscoplastic injections

In this study, the injection of a viscoplastic (yield stress) fluid into a quiescent Newtonian medium is experimentally investigated, using a range of experimental methods, including high-speed camera imaging, laser-induced fluorescence (LIF), and ultrasound Doppler velocimetry (UDV) techniques. The case of our consideration is the continuous downward injection of a Carbopol solution, as a core fluid, through an eccentric inner pipe, placed inside a vertical closed-end pipe filled with a glycerin–water solution. The injected fluid is always heavier than the in-situ fluid and the fluids are miscible. As the injection continues, a filament of the core fluid immersed in the in-situ fluid is formed, for which it has been shown that there exist three distinct flow patterns, namely, the breakup, coiling, and bulging (buckling) regimes. Our focus in this study is to analyse the detailed flow features and sub-regimes at longer times, from an experimental perspective. To generalize our results, they are presented versus the governing dimensionless numbers of the flow, i.e. the buoyancy number, the Bingham number, the viscosity ratio, and the falling height. The breakup regime is analysed in terms of the yielding, necking and pinch-off process, the length of filaments, and their subsequent falling behaviours, the coiling regime in terms of the regular, free and irregular coiling behaviours, and the bulging (buckling) regime in terms of the appearance of a detached falling part and the maximum penetration depth. The results of this work can find applications, in particular, in the plug and abandonment (e.g. cementing processes) of oil and gas wells and 3D printing technologies. • Experimental analysis of immersed buoyant viscoplastic injection in Newtonian fluid. • Observed flow regimes classified into breakup, coiling, and bulging. • Analysis of yielding, necking/pinch-off, and length of filaments for breakup regime. • Coiling subregimes categorized as regular, free, and irregular coiling. • Analysis of maximum penetration depth and subregimes in bulging regime.

An Overview of Heat Transfer Enhancement Based Upon Nanoparticles Influenced By Induced Magnetic Field with Slip Condition Via Finite Element Strategy

Abstract The mathematical model of heat generation and dissipation during thermal energy transmission employing nanoparticles in a Newtonian medium is investigated. Dimensionless boundary layer equations with correlations for titanium dioxide, copper oxide, and aluminium oxide are solved by the finite element method. Parameters are varied to analyze their impact on the flow fields. Various numerical experiments are performed consecutively to explore the phenomenon of thermal performance of the combination fluid. A remarkable enhancement in thermal performance is noticed when solid structures are dispersed in the working fluid. The Biot number determines the convective nature of the boundary. When the Biot number is increased, the fluid temperature decreases significantly. Among copper oxide, aluminium oxide, and titanium oxide nanoparticles, copper oxide nanoparticles are found to be the most effective thermal enhancers.

Нeat and mass transfer models at boundary lubrication to determine the transition temperatures

At present, kinetic and thermodynamic methods for assessing the lubricating effect of oils are being increasingly developed. At the limit friction, the reduction of friction and wear of surfaces is due to the ability of the lubricant to form layers of adsorption or chemical origin on the surface. Analytical models of transition temperatures and wear in the limit lubrication mode must be used to mathematically describe the processes in the subsystems and the transition between them. The Fourier equation of thermal conductivity is accepted as the basic calculated dependence. It is assumed that the process of heat propagation under the conditions of formation of lubricating films is not Markovian, i.e. the magnitude of the heat flux is determined by the entire "history" of heat transfer in a certain elementary volume. The equation of motion of a lubricating film over the surface of a body that is being lubricated is obtained from the equation of motion for a Newtonian continuous medium. As a result, nonlinear heat and mass transfer models are obtained to determine the transition temperatures in the formation of boundary lubricating films in the concept of structural-thermodynamic approaches to describe the processes of boundary lubrication of surfaces

Viscoplastic toroidal drop in compressional Stokes flow

We report the dynamics of a viscoplastic (Bingham) torus suspended in an unbounded Newtonian medium. In this study, the immiscible ambient fluid is subjected to an axisymmetric compressional (biaxial-extensional) flow. Numerical simulations applying the boundary integral formulation to the Stokes flow are performed for the torus drop having initially a circular cross section. The quasi-stationary dynamic simulation reveals that depending on the initial conditions and the capillary number (Ca), the Bingham number (Bn), and the viscosity ratio (λ), three different scenarios of the drop evolution are obtained for a range of viscosity ratio and Bingham number: collapsing toward the axis of symmetry, expanding infinitely, and having a stationary toroidal shape.

Electrokinetics of polymeric fluids in narrow rectangular confinements.

The flow of polymeric liquids in narrow confinements with a rectangular cross section, in the presence of electrical double layers is analyzed here. Our analysis is motivated by the fact that many of the previous studies on the flow of complex fluids tend to focus on highly idealized parallel plate channels, which are markedly different from the rectangular ducts, used in many experiments and devices. We consider the combined electroosmotic and pressure driven flows as well as the streaming potential resulting from a mechanically driven flow. We use two distinct constitutive relations to model the polymeric liquids, namely the simplified exponential Phan-Thien-Tanner (sePTT) model and the Giesekus model, both of which are non-linear viscoelastic models, capable of capturing the shear thinning behavior. We establish that the applied electric field may have a strong influence on the overall flow rate, which rapidly increases with the field strength as well as the extent of viscoelasticity of the fluid. Viscoelasticity and shear thinning behavior also enhance the streaming potential by several fold as compared to a Newtonian medium. We demonstrate that the aspect ratio of a channel has a bigger influence on the net throughput and the streaming potential, when the extent of viscoelasticity is relatively large. We illustrate that for sePTT fluids, the flow is strictly unidirectional, while for Giesekus fluids, secondary flows are inevitably present on account of their non-zero second normal stress coefficient. Although the electric field does not change the overall patterns of these secondary flows, their magnitude does depend on the imposed field strength for combined flows.

Dynamics of an elastoviscoplastic droplet in a Newtonian medium under shear flow

The dynamics of a single elastoviscoplastic (EVP) drop immersed in plane shear flow of a Newtonian fluid is studied by 3D direct numerical simulations using a finite-difference and level-set method combined with the Saramito model for the EVP fluid. This model gives rise to a yield stress behavior, where the unyielded state of the material is described as a Kelvin-Voigt viscoelastic solid and the yielded state as a viscoelastic Oldroyd-B fluid. Yielding of an initially solid drop of Carbopol is simulated under successively increasing shear rates. We proceed to examine the roles of nondimensional parameters on the yielding process; in particular, the Bingham number ($Bi$), capillary number ($Ca$), Weissenberg number ($Wi$), and ratio of solvent and total drop viscosity. We find that all of these parameters, and not only $Bi$, have a significant influence on the drop dynamics. Numerical simulations predict that the volume of the unyielded region inside the droplet increases with $Bi$ and $Wi$, while it decreases with $Ca$ at low $Wi$ and $Bi$. A new regime map is obtained for the prediction of the yielded, unyielded, and partly yielded modes as a function of $Bi$ and $Wi$. The drop deformation is studied and explained by examining the stresses in the vicinity of the drop interface. The deformation has a complex dependence on $Bi$ and $Wi$. At low $Bi$, the droplet deformation shows a nonmonotonic behavior with an increasing $Wi$. In contrast, at moderate and high $Bi$, droplet deformation always increases with $Wi$. Moreover, it is found that the deformation increases with $Ca$ and with the solvent to total drop viscosity ratio. A simple ordinary differential equation model is developed to explain the various behaviours observed numerically. The presented results are in contrast with the heuristic idea that viscoelasticity in the dispersed phase always inhibits deformation.

Scalable Chemical Synthesis Route to Manufacture pH-Responsive Janus CaCO3 Micromotors.

A cost-effective scalable chemical route to produce pH-responsive active colloids (ACs) is developed here. For the first time, calcium carbonate particles are half-coated with a silica layer via Pickering emulsion methodology. This methodology allows to create anisotropy on the particles' surfaces and benefit from the decomposition of the calcium carbonate in acidic media to generate self-propulsion. The coupling between the self-diffusiophoretic motion of these ACs and acid concentrations is experimentally investigated in Newtonian media via optical microscopy. With increasing hydrogen-ion concentrations, the pH-responsive colloids experience higher mean-square displacements because of self-propulsion velocities and enhanced long-time diffusivities. Because they are biocompatible and environmentally friendly, these ACs constitute a platform for advanced diagnostics, targeted drug delivery, and water/soil remediation.

Stability of porous-Poiseuille flow with uniform vertical throughflow: High accurate solution

This paper investigates the influence of a uniform vertical throughflow on the stability of Poiseuille flow in a Newtonian fluid-saturated Brinkman porous medium. The solution to the stability eigenvalue problem is obtained numerically using the Chebyshev collocation and the Galerkin methods. The vertical throughflow, irrespective of its direction, imparts an identical stabilizing/destabilizing effect on the fluid flow. The throughflow dependent Reynolds number RT instills both stabilizing and destabilizing effects in a fluid saturated channel of porous medium and the range of RT up to which the system gets destabilized increases with increasing porous parameter. The results for the clear fluid case are also obtained as a particular case from the present study, and contrary to the porous case, throughflow always displays a stabilizing effect. Moreover, the numerical solutions presented will be useful for checking the performance and accuracy of any numerical methodologies.

Steady finite-amplitude Rayleigh–Bénard convection of ethylene glycol–copper nanoliquid in a high-porosity medium made of 30% glass fiber-reinforced polycarbonate

In the paper, we make linear and nonlinear stability analyses of Rayleigh–Benard convection in a Newtonian nanoliquid-saturated high-porosity medium. Single-phase model is used for nanoliquids, and values of thermophysical quantities concerning ethylene glycol–copper nanoliquid-saturated porous medium are calculated using mixture theory or phenomenological relations. The study is carried out for free-free, rigid-rigid and rigid-free isothermal boundaries. Boundary effects on onset of convection are shown to conform to classical predictions. The addition of copper nanoparticles to ethylene glycol is shown to lead to advanced onset of convection in the porous medium and thereby to a substantial increase in heat transport. Theoretical explanation is provided for the enhanced heat transfer situation in the medium. With suitable scaling in quantities, the result concerning heat transfer in ethylene glycol–copper nanoliquid-saturated porous medium is shown to be obtainable from those of ethylene glycol-saturated porous medium without copper nanoparticles. Nanoparticles serve the purpose of cooling and porous matrix retains the heat, thereby meaning that residence time of heat in the system can be regulated by using nanoparticles and porous matrix.

Increasing the Homogeneity of Liquid Aerated Concrete Mixtures from Dispersed Brittle Materials by Their Vibromixing in the Manufacture of Building Products

Technical re-equipment in construction industry tends toward the wide use of cellular concrete. The formulation and advantages of these materials were analyzed. The technological difficulties in their manufacture were mentioned. The most difficult problem is to ensure uniformity by quickly mixing the aerated concrete mixtures of components that differ in composition and mass fraction before the start of active gas formation, gas retention, and especially solidification. These difficulties can be eliminated by using the new technique of vibrojet mixing of both Bingham and Newtonian media. Forced high-frequency vibration actually increases the thixotropy of the mixture regardless of the number of components and their mass and physicomechanical properties. At the same time, monitoring the vibration parameters increases the circulation and interaction of the generated submerged jet flows. Rheological models were developed to determine the technical characteristics of the mixer vibrodrive and the process parameters. Supercomputer modeling of various scenarios of vibrojet mixing in the FlowVision program allowed visualization and optimization of modes to completely exclude stagnant zones in the mixer. In a full-scale experiment, test samples were prepared from the aerated concrete mixtures obtained by centrifuging and vibromixing. Their growth and strength characteristics were compared in accordance with GOST (State Standard) 10180-2012. The results showed that the new method for the preparation of mixtures and products from aerated concrete is highly effective.

Interfacial instabilities in active viscous films

The interfacial stability of an active viscous film is analyzed theoretically. The film, which rests on a flat substrate and is bounded from above by an air-liquid interface, contains a suspension of active particles such as swimming microorganisms that self-propel, diffuse, and exert active stresses on the suspending Newtonian medium. Using a continuum model for the configuration of the suspension coupled to the forced Stokes equations for the fluid motion, we analyze the growth of linearized normal mode fluctuations with respect to the quiescent base state. In the absence of gravity, puller suspensions are found to be always stable, whereas films containing pushers can become unstable above a critical activity level where active stresses overcome the damping effects of viscosity and surface tension and drive interfacial deformations. Confinement, diffusion and capillary forces all act to stabilize the system, and we characterize the transition to instability in terms of the dimensionless parameters of the problem. We also address the case of inverted films subject to the Rayleigh-Taylor instability, where we demonstrate that active stresses generated by pullers have the ability to stabilize gravitationally unstable films by counteracting the effect of the gravitational body force.

An experimental investigation on inertia motion and deformation of Boger drops falling through Newtonian media

In the present study, deformations and drag coefficients of viscoelastic drops (Boger drops) with different elasticity falling into the air and oil are investigated experimentally. The main object of the present study is an evaluation of the fluid elasticity effects on drop dynamics by comparing the results with Newtonian cases (water, ethanol and aqueous solution of sodium dodecyl benzene sulfonate). A CMOS high-speed camera is used to capture images of falling drops and the cinematic calculations are performed using image processing. Here, a theoretical correlation is presented to describe the drag coefficient as a function of Reynolds number based on Newton’s second-law. In addition, effects of viscosity ratio ( $$ k $$ ), elasticity number ( $$ En $$ ) and Bond number ( $$ Bo $$ ) on the drag coefficient are studied. The results express that the drag coefficient increases by increasing viscosity ratio. For drops with the same viscosities, by increasing elasticity number and reducing Bond number, the drag coefficient is enhanced. It is also observed that the deformation of Newtonian drops falling in the air is periodic. These oscillations are caused by the interaction between surface tension forces and hydrodynamic pressure and the presence of internal circulation flows leading to instability on the surface of droplets. The amplitude oscillation for viscoelastic drops is remarkably lower than Newtonian drops. By increasing the viscosity of exterior fluid (when drops fall in the oil), periodic deformation of Newtonian drops is gradually damped and spherical drops are changed to elliptical ones. These changes are also observed for viscoelastic droplets.