Confined Shear Flow Research Articles

Enhanced separation via confined shear flow in microchannel: A novel insight into the role of confined hydrogen bonds in microfluid and its effect on competitive mass transfer of hydrated ions

Microfluidic extraction and separation have demonstrated enormous potential in the field of purification and separation, but the essence of its enhanced separation has always been a focus of debate in the academic community. This study reveals that the interaction between extractant molecules and ions in the process of microfluidic extraction exhibits some significant differences from conventional liquid–liquid two phase stirring dispersed droplet extraction. The interaction between extractant molecules and metal ions through the hydrogen bonds (H-bonds) bridged by water molecules around the hydration shell of metal ions exhibits a noticeable change in confined flow in microchannel. It was demonstrated that in our experiments that the strength difference of this interaction is subjected to the ability of different metal ions to bind water molecules. Therefore, the differences in the interaction between extractant molecules and ions through the hydrogen bonding “water-bridge” are easier to be distinguished within the confined fluid, due to the different effects from the confined environment on mass transfer of metal ions with different abilities to bind water molecules. The confined fluid environment strengthened the difference in the interaction, leading to an amplification of difference in ion diffusion rates, thereby achieving enhanced separation of target rare earth Er3+ ions from the coexisting background ions. Hydrophilic coexisting background ions can be separated by reducing the microchannel width, so that the βEr/Al increases from 2.95 to 49.78. However, the relatively hydrophobic background ions can be separated by increasing flow rates, so that the βEr/Mg increases from 75.32 to 299.06. This work provides a new insight into the essence of microfluidic enhanced separation from the perspective of confined H-bonds.

Self-organization of autophoretic suspensions in confined shear flows

Self-organization of autophoretic suspensions in confined shear flows

Effect of overlying pressure and applied shear velocity on the nonlinear behavior of confined shear flow

Effect of overlying pressure and applied shear velocity on the nonlinear behavior of confined shear flow

Determination of Critical Point of Particle Migration Direction in a Confined Shear Flow of Giesekus Fluids

Migration of a particle in a confined shear flow of Giesekus fluids is investigated numerically with the method of direct forcing/fictitious domain. We focus on the migration direction for the particle with initial lateral position y0 and determination of critical point yc of a particle moving towards the center line or wall. The effect of viscosity ratio μr, shear-thinning parameter α, Weissenberg number Wi, and blocking rate β on the value yc is analyzed. The results showed that when μr ≤ 0.5, the particle will migrate towards the wall regardless of the value of y0. When μr > 0.5, yc increases with increasing μr, and some particles will migrate towards the center line with the increase in μr. The particle is more likely to migrate towards the center line at small values of Wi and α but at large values of μr. The impact of Wi and β on the particle migration direction is more obvious. The particle will migrate towards the wall for β = 0.3 and is more likely to migrate towards the wall with increasing β. α and Wi have little influence on the pressure distribution in the case of the same β and μr. The particle near the wall will migrate faster because large positive pressure and negative pressure appear around the particle.

Dynamics of flexible fibers in confined shear flows at finite Reynolds numbers

We carry out a numerical study on the dynamics of a single non-Brownian flexible fiber in two-dimensional confined simple shear (Couette) flows at finite Reynolds numbers. We employ the bead-spring model of flexible fibers to extend the fluid particle dynamics (FPD) method that was originally developed for rigid particles in viscous fluids. We implement the extended FPD method using a multiple-relaxation-time scheme of the lattice Boltzmann method. The numerical scheme is validated first by a series of benchmark simulations that involve fluid–solid coupling. The method is then used to study the dynamics of flexible fibers in Couette flows. We only consider the highly symmetric cases where the fibers are placed on the symmetry center of Couette flows, and we focus on the effects of the fiber stiffness, the confinement strength, and the finite Reynolds number (from 1 to 10). A diagram of the fiber shape is obtained. For fibers under weak confinement and a small Reynolds number, three distinct tumbling orbits have been identified: (1) Jeffery orbits of rigid fibers—the fibers behave like rigid rods and tumble periodically without any visible deformation; (2) S-turn orbits of slightly flexible fibers—the fiber is bent to an S-shape and is straightened again when it orients to an angle of around 45° relative to the positive x-direction; and (3) S-coiled orbits of fairly flexible fibers—the fiber is folded to an S-shape and tumbles periodically and steadily without being straightened anymore during its rotation. Moreover, the fiber tumbling is found to be hindered by increasing either the Reynolds number or the confinement strength, or both.

Probing the influence of inertia, viscosity, and initial offset on the hydrodynamic interaction of droplet pairs in confined shear flow

The collision behavior of droplet pairs subjected to confined shear flows at high density and viscosity ratios remains unexplored and unquantified, and therefore, the critical density and viscosity ratios for the collision mode of droplets under geometric parameters were unknown. Simulations have been performed using a free-energy-based lattice Boltzmann method, with the aim of determining new regimes compared to the previous study. The results demonstrate that the interaction between viscous and inertia forces plays a vital role in the coalescence nature of droplets, their deformation, course of trajectory, as well as their expedition toward the steady state.

Flow-induced "waltzing" red blood cells: Microstructural reorganization and the corresponding rheological response.

We investigate flow-induced structural organization in a dilute suspension of tumbling red blood cells (RBCs) under confined shear flow. For small Reynolds (Re = 0.1) and capillary numbers (Ca), with fully coupled hydrodynamic interaction (HI) and without interparticle adhesion, we find that HI between the biconcave discoid particles prompts the formation of layered RBC chains and synchronized rotating RBC pairs, referred here as "waltzing doublets." As the volume fraction ϕ increases, more waltzing doublets appear in RBC files. Stronger shear stress disrupts structural arrangements at higher Ca. We find that the flow-induced organization of waltzing doublets changes how the suspension viscosity varies with ϕ qualitatively. The intrinsic viscosity is particularly sensitive to microstructural rearrangement, increasing (decreasing) with ϕ at low (high) Ca that correlates with the change in the fraction of doublets. We verified flow-induced collective motion with comparison to two-cell simulations in which the cell volume fraction is controlled by varying the domain volume.

Numerical study on the engulfing behavior between immiscible droplets in a confined shear flow

This study investigates the engulfing behavior between two equal-sized immiscible droplets suspended in a confined shear flow. The finite element analysis is carried out, where a ternary diffuse-interface model and the Stokes equation are coupled. The numerical results of nearly 60 cases are studied to systematically examine the effect of shear rate, surface tension, and confinement on the engulfing behavior. In general, the partial and complete engulfing result in Janus and compound droplets, respectively, when the shear rate is low. It is found that the partial engulfing regime is significantly affected by the shear rate such that a proper condition for the Janus formation becomes narrower as the shear rate increases. We also note that the shell of a compound droplet can be broken by increasing the initial separation or confinement.

Lattice Boltzmann simulation of binary three-dimensional droplet coalescence in a confined shear flow

Small-scale microscopic phenomena determine the behavior of large-scale droplets, which brings great challenges to accurately simulate the droplet coalescence process. In this paper, the mesoscopic lattice Boltzmann method based on the phase field theory is used to simulate the collision and coalescence of binary three-dimensional droplets in a confined shear flow. The numerical prediction of droplet coalescence behavior was first compared with the experimental result, and good agreement was reported. Then, we investigated the influences of a comprehensive range of capillary numbers (0.01≤Ca≤0.5) and Reynolds numbers (0.01≤Re≤10) on the shearing dynamics of binary droplets and also provided a quantitative description of droplet behavior in terms of the droplet deformation parameter and relative trajectory. A shearing regime diagram is further constructed based on the coupling effect of Ca and Re, which reveals three distinct types of droplet behaviors, including coalescence, breakup after the coalescence, and non-coalescence. Concretely, three different patterns of droplets can be completely captured with the variation of Ca at low Re; only two types of coalescence and non-coalescence can be observed for a medium Re, and two droplets just slide over each other without the occurrence of the coalescence when Re is sufficiently large. Also, we identified two critical capillary numbers in the lower Re region and one critical capillary number in the middle Re region, respectively, characterizing flow type transitions from the coalescence to breakup, from the breakup to the non-coalescence, and from the coalescence to the non-coalescence. It is found that all the capillary numbers decrease with Re.

Ternary modeling of the interaction between immiscible droplets in a confined shear flow

The interaction between two immiscible droplets in a confined shear flow is studied numerically by using a ternary diffuse-interface model. Three different types of interaction are observed: (1) reverse motion, (2) pass-over motion, and (3) rotary Janus motion after wetting. We investigate the effects of initial droplet position, capillary number, and confinement on the droplet trajectory and the deformation.

Effect of confinement on the rotation of a two-dimensional elliptical porous particle in shear flow

The rotation of non-spherical porous particles in fluid flows is of practical relevance in various natural and industrial processes. However, despite the increasing interest in micro-scale channels and reactors, the understanding of rotation of non-spherical porous particles in a confined fluid flow is, if not blank, far from complete. In this work, we present a numerical study on the rotation of an elliptical porous particle in a confined shear flow by solving the governing equations using a lattice Boltzmann method. The particles with varying aspect ratios AR, Darcy number Da, and Reynolds number Re are examined for different confinement ratios B. Akin to its solid counterpart, the elliptical porous particle either exhibits time-periodic rotation with a non-uniform angular rate or takes a stationary orientation for different B. With finite fluid inertia, both the maximum and minimum angular rate decrease with B. For the elliptical porous particle, a higher B promotes the increasing rate of rotation period against Re, resulting in a smaller critical Reynolds number Rec (if observed) at which the particle ceases to rotate. A scaling law for solid particles was extended to correlate the rotation period and Re for porous particles, where B has a negligible effect. An empirical formula to predict Rec as a function of B, AR, and Da is established using the symbolic regression. The transition from rotating to stationary at different B can be explained by the net torque exerted on the elliptical porous particle.

Structure and rheology of suspensions of spherical strain-hardening capsules

Abstract

Electric field modulated deformation dynamics of a compound drop in the presence of confined shear flow

Dynamics of compound drops is central in several emerging applications including emulsion-fueled direct injection, targeted drug delivery, and the development of mechano-sensitive artificial cells. These applications are commonly hallmarked by high shear rates in confined fluidic environments. In the present work, we depict the role of the transverse electric field in controlling the resulting morpho-dynamics, including non-monotonic topological evolution and pinch-off phenomenon. In a tightly confined domain, we further show that the critical value of the electric field for triggering the pinch-off phenomenon may be greatly reduced as compared to larger-sized conduits. Finally, we portray a non-trivial variation of the drop pinch-off time with the degree of domain confinement, as attributed to the complex electro-hydrodynamic coupling over small scales. These results may turn out to be critical in manipulating the shape evolution of bio-mimetic soft matter in physiologically relevant fluidic pathways and on-chip applications.

Segregation models for density-bidisperse granular flows

Individual constituent balance equations are often used to derive expressions for species-specific segregation velocities in flows of dense granular mixtures. We propose a semiempirical expression for the interspecies momentum exchange in density-bidisperse granular flows as an extension of ideas from kinetic theory and compare it to a previous viscous drag approach that is analogous to particles settling in a fluid. The proposed model expands the range of the granular kinetic theory from short-duration binary collisions to the multiple enduring contacts characteristic of dense shear flows and incorporates the effects of particle friction, concentration ratio, and local flow conditions. The segregation velocities derived from the momentum balance equation using both interspecies drag models match the downward and upward segregation velocities of heavy and light particles obtained from DEM simulations through the flowing layer depth for different density ratios and constituent concentrations in confined shear flows. Predictions of the kinetic theory inspired approach are additionally compared to results from free surface heap flow simulations, and, again, a close match is observed.

Inertial bifurcation of the equilibrium position of a neutrally-buoyant circular cylinder in shear flow between parallel walls

The authors examine the inertial lift on a neutrally-buoyant circular cylinder in confined shear flow. Through two-dimensional lattice Boltzmann simulations, it is shown that the transverse equilibrium position of the cylinder will undergo a pitchfork bifurcation above a critical Reynolds number, with the stable equilibrium position shifting away from the centerline of the channel. The study demonstrates that this critical Reynolds number is dependent on the ratio of particle size to channel width, and occurs below the transition to unsteady flow.

Confinement effect on electrically induced dynamics of a droplet in shear flow.

Deformation and breakup of droplets in confined shear flows have been attracting increasing attention from the research community over the past few years, as attributable to their implications in microfluidics and emulsion processing. Reported results in this regard have demonstrated that the primary effect of confinement happens to be the inception of complex oscillating transients, monotonic variation of droplet deformation, and droplet stabilization against breakup, as attributable to wall-induced distortion of the flow field. In sharp contrast to these reported findings, here, we show that a nonintuitive nonmonotonic droplet deformation may occur in a confined shear flow, under the influence of an external electric field. In addition, we demonstrate that the orientation angle of a droplet may either increase or decrease with the domain confinement under the influence of an electric field, whereas the same trivially decreases with the increase in degree of confinement in the absence of any electrical effects. Unlike the typical oscillatory transients observed in microconfined shear flows, we further bring out the possibility of an electrohydrodynamically induced dampening effect in the oscillation characteristics, as governed by a specific regime of the relevant dimensionless electrical parameters. Our results reveal that instead of arresting droplet deformation, the unique hydrodynamics of microconfined shear flow may augment the tendency of droplet breakup, and is likely to alter the droplet breakup mode from midpoint pinching to edge pinching at high electric field strength. These results may bear far reaching implications in a wide variety of applications ranging from the processing of emulsions to droplet based microfluidic technology.

Migration of spherical particles in a confined shear flow of Giesekus fluid

The lateral migration of a spherical particle in a sheared Giesekus fluid is numerically studied by the direct-forcing fictitious domain method. The model is first validated by comparing the simulation results with the available data in the literature. Effects of the viscosity ratio, shear thinning, Weissenberg number, and wall confinement on the particle migration are examined. Results show that the particle migrates toward the wall, irrespective of whether the fluid is shear thinning. The wall confinement, shear thinning, and high Weissenberg number could respectively facilitate the particle lateral migration. While the effect of viscosity ratio on the particle migration is not monotonic, a separatrix value is found which divides the viscosity ratio into two ranges. Moreover, effects of rheological properties on the particle angular velocity and its variation throughout the migration pathway are also explored, and the position where the angular velocity starts to decrease is affected by fluid rheological properties.

Electric field induced droplet deformation and breakup in confined shear flows

Deformation or breakup of a droplet suspended in confined shear flows and subjected to an electric field is investigated numerically. A contrasting behavior of the droplet depending upon the electrical properties of the fluids is reported.

Electrohydrodynamic interaction between droplet pairs in a confined shear flow

This study deals with electrohydrodynamic interactions between two deformable droplets in a confined shear flow. We have investigated the effect of the channel confinement and the electric field strength on the droplet trajectories using numerical simulations. Two important varieties of motions are identified in the present analysis: (i) the reversing motion and (ii) the passing-over motion. This study suggests that conversion of the passing-over motion to the reversing motion or vice versa is possible by modulating the strength of the imposed electric field. Such a conversion of the pattern of droplet migration is also achieved by changing the electrical properties of the system (for instance, conductivity). Similar to the electric field strength and the electrical properties, the domain confinement is also found to play a key role in the conversion of the mode of droplet motion. Interestingly, the presence of electric field is found to prevent droplet coalescence to a certain extent depending on its strength, thus rendering the emulsion stable. Our results provide a perspective of electric field in a confined medium which can be used as a tool to modulate droplet trajectories in an emulsion.

Blood Crystal: Emergent Order of Red Blood Cells Under Wall-Confined Shear Flow.

Driven or active suspensions can display fascinating collective behavior, where coherent motions or structures arise on a scale much larger than that of the constituent particles. Here, we report numerical simulations and an analytical model revealing that deformable particles and, in particular, red blood cells (RBCs) assemble into regular patterns in a confined shear flow. The pattern wavelength concurs well with our experimental observations. The order is of a pure hydrodynamic and inertialess origin, and it emerges from a subtle interplay between (i)hydrodynamic repulsion by the bounding walls that drives deformable cells towards the channel midplane and (ii)intercellular hydrodynamic interactions that can be attractive or repulsive depending on cell-cell separation. Various crystal-like structures arise depending on the RBC concentration and confinement. Hardened RBCs in experiments and rigid particles in simulations remain disordered under the same conditions where deformable RBCs form regular patterns, highlighting the intimate link between particle deformability and the emergence of order.