scholarly journals Generation and Dynamics of Janus Droplets in Shear-Thinning Fluid Flow in a Double Y-Type Microchannel

Micromachines ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 149
Author(s):  
Fan Bai ◽  
Hongna Zhang ◽  
Xiaobin Li ◽  
Fengchen Li ◽  
Sang Woo Joo
Keyword(s):  
Surface Tension ◽  
Formation Mechanism ◽  
Computational Study ◽  
Shear Thinning ◽  
Droplet Formation ◽  
Continuous Phase ◽  
Viscous Force ◽  
Rheological Parameters ◽  
Janus Droplet ◽  
Shear Thinning Fluid

Droplets composed of two different materials, or Janus droplets, have diverse applications, including microfluidic digital laboratory systems, DNA chips, and self-assembly systems. A three-dimensional computational study of Janus droplet formation in a double Y-type microfluidic device filled with a shear-thinning fluid is performed by using the multiphaseInterDyMFoam solver of the OpenFOAM, based on a finite-volume method. The bi-phase volume-of-fluid method is adopted to track the interface with an adaptive dynamic mesh refinement for moving interfaces. The formation of Janus droplets in the shear-thinning fluid is characterized in five different states of tubbing, jetting, intermediate, dripping and unstable dripping in a multiphase microsystem under various flow conditions. The formation mechanism of Janus droplets is understood by analyzing the influencing factors, including the flow rates of the continuous phase and of the dispersed phase, surface tension, and non-Newtonian rheological parameters. Studies have found that the formation of the Janus droplets and their sizes are related to the flow rate at the inlet under low capillary numbers. The rheological parameters of shear-thinning fluid have a significant impact on the size of Janus droplets and their formation mechanism. As the apparent viscosity increases, the frequency of Janus droplet formation increases, while the droplet volume decreases. Compared with Newtonian fluid, the Janus droplet is more readily generated in shear-thinning fluid due to the interlay of diminishing viscous force, surface tension, and pressure drop.

scholarly journals Liquid–Liquid Flows with Non-Newtonian Dispersed Phase in a T-Junction Microchannel

Micromachines ◽  
2021 ◽  
Vol 12 (3) ◽  
pp. 335
Author(s):  
Anna Yagodnitsyna ◽  
Alexander Kovalev ◽  
Artur Bilsky
Keyword(s):  
Flow Pattern ◽  
Dynamic Viscosity ◽  
Slug Flow ◽  
Shear Thinning ◽  
Continuous Phase ◽  
Immiscible Liquid ◽  
Dispersed Phase ◽  
Effective Dynamic ◽  
Liquid Flows ◽  
Shear Thinning Fluid

Immiscible liquid–liquid flows in microchannels are used extensively in various chemical and biological lab-on-a-chip systems when it is very important to predict the expected flow pattern for a variety of fluids and channel geometries. Commonly, biological and other complex liquids express non-Newtonian properties in a dispersed phase. Features and behavior of such systems are not clear to date. In this paper, immiscible liquid–liquid flow in a T-shaped microchannel was studied by means of high-speed visualization, with an aim to reveal the shear-thinning effect on the flow patterns and slug-flow features. Three shear-thinning and three Newtonian fluids were used as dispersed phases, while Newtonian castor oil was a continuous phase. For the first time, the influence of the non-Newtonian dispersed phase on the transition from segmented to continuous flow is shown and quantitatively described. Flow-pattern maps were constructed using nondimensional complex We0.4·Oh0.6 depicting similarity in the continuous-to-segmented flow transition line. Using available experimental data, the proposed nondimensional complex is shown to be effectively applied for flow-pattern map construction when the continuous phase exhibits non-Newtonian properties as well. The models to evaluate an effective dynamic viscosity of a shear-thinning fluid are discussed. The most appropriate model of average-shear-rate estimation based on bulk velocity was chosen and applied to evaluate an effective dynamic viscosity of a shear-thinning fluid. For a slug flow, it was found that in the case of shear-thinning dispersed phase at low flow rates of both phases, a jetting regime of slug formation was established, leading to a dramatic increase in slug length.

Droplet Formation Under the Effect of a Flexible Nozzle Plate

2007 ◽  
Author(s):  
S. Sangplung ◽  
J. A. Liburdy
Keyword(s):  
Surface Tension ◽  
Droplet Formation ◽  
Step Function ◽  
Droplet Velocity ◽  
Viscous Force ◽  
Flexible Plate ◽  
Computational Domain ◽  
Cfd Model ◽  
Drop On Demand ◽  
Nozzle Plate

Droplet formation from a flexible nozzle plate driven by a prescribed-waveform excitation of a piezoelectric is numerically investigated using a computational fluid dynamics (CFD) model with the VOF method. The droplet generator with a flexible nozzle plate, which is free to vibrate due to the pressure acting on the plate, is modeled in a CFD computational domain. The CFD analysis includes the fluid-structure interaction between fluid and a flexible plate using large deflection theory. The problem is characterized by the nondimensional variables based on the capillary parameters of time, velocity, and pressure. The CFD model is validated with the experiment results. This study examines the characteristics of the applied waveforms and nozzle plate material properties to change the vibrational characteristics of the nozzle plate. The effect of fluid properties on the droplet formation process is also investigated focusing on surface tension and viscous forces. The mechanism of the droplet formation excited by a drop-on-demand piezoelectric waveform is investigated using a step-function and a pulse waveform. The piezoelectric displacement plays an important role in generating either forward-driven momentum or a suction pressure inside the chamber. For the step-function waveform, the nondimensional applied impulse is defined and used to characterize the post-breakoff droplet volume. Increasing the impulse of the piezoelectric can be used to cause a faster droplet velocity and it is shown that the vibration of the nozzle plate has a strong effect on the droplet velocity, shape, and volume. Surface tension has strong influence to the droplet formation characteristics which is contrast to a viscous force that makes no difference on the droplet formation for different viscosities. For the combination of a fluid with high surface tension and the most flexible nozzle plate, this system can not cause the droplet ejected out of the nozzle.

scholarly journals Lattice Boltzmann Simulation of Droplet Generation in a Microfluidic Cross-Junction

2011 ◽  
Vol 9 (5) ◽  
pp. 1235-1256 ◽  
Author(s):  
Haihu Liu ◽  
Yonghao Zhang
Keyword(s):  
Droplet Size ◽  
Flow Rate ◽  
Lattice Boltzmann ◽  
Capillary Number ◽  
Rate Ratio ◽  
Viscosity Ratio ◽  
Droplet Formation ◽  
Continuous Phase ◽  
Viscous Force ◽  
Flow Rate Ratio

AbstractUsing the lattice Boltzmann multiphase model, numerical simulations have been performed to understand the dynamics of droplet formation in a microfluidic cross-junction. The influence of capillary number, flow rate ratio, viscosity ratio, and viscosity of the continuous phase on droplet formation has been systematically studied over a wide range of capillary numbers. Two different regimes, namely the squeezinglike regime and the dripping regime, are clearly identified with the transition occurring at a critical capillary number Cacr. Generally, large flow rate ratio is expected to produce big droplets, while increasing capillary number will reduce droplet size. In the squeezing-like regime (Ca ≤ Cacr), droplet breakup process is dominated by the squeezing pressure and the viscous force; while in the dripping regime (Ca ≤ Cacr), the viscous force is dominant and the droplet size becomes independent of the flow rate ratio as the capillary number increases. In addition, the droplet size weakly depends on the viscosity ratio in both regimes and decreases when the viscosity of the continuous phase increases. Finally, a scaling law is established to predict the droplet size.

Effect of Carreau-Yasuda rheological parameters on subcritical Lapwood convection in horizontal porous cavity saturated by shear-thinning fluid

2017 ◽  
Vol 29 (6) ◽  
pp. 063101 ◽  
Author(s):  
Khaled Khechiba ◽  
Mahmoud Mamou ◽  
Madjid Hachemi ◽  
Nassim Delenda ◽  
Redha Rebhi
Keyword(s):  
Shear Thinning ◽  
Rheological Parameters ◽  
Porous Cavity ◽  
Shear Thinning Fluid

scholarly journals Non-Newtonian Droplet Generation in a Cross-Junction Microfluidic Channel

Polymers ◽  
2021 ◽  
Vol 13 (12) ◽  
pp. 1915
Author(s):  
Maryam Fatehifar ◽  
Alistair Revell ◽  
Masoud Jabbari
Keyword(s):  
Droplet Size ◽  
Power Law ◽  
Microfluidic Channel ◽  
Shear Thinning ◽  
Droplet Formation ◽  
Considerable Effect ◽  
Droplet Generation ◽  
Rheological Parameters ◽  
Polymeric Solutions ◽  
Detachment Time

A two-dimensional CFD model based on volume-of-fluid (VOF) is introduced to examine droplet generation in a cross-junction microfluidic using an open-source software, OpenFOAM together with an interFoam solver. Non-Newtonian power-law droplets in Newtonian liquid is numerically studied and its effect on droplet size and detachment time in three different regimes, i.e., squeezing, dripping and jetting, are investigated. To understand the droplet formation mechanism, the shear-thinning behaviour was enhanced by increasing the polymer concentrations in the dispersed phase. It is observed that by choosing a shear-dependent fluid, droplet size decreases compared to Newtonian fluids while detachment time increases due to higher apparent viscosity. Moreover, the rheological parameters—n and K in the power-law model—impose a considerable effect on the droplet size and detachment time, especially in the dripping and jetting regimes. Those parameters also have the potential to change the formation regime if the capillary number (Ca) is high enough. This work extends the understanding of non-Newtonian droplet formation in microfluidics to control the droplet characteristics in applications involving shear-thinning polymeric solutions.

Numerical Simulation of the Effect of Rheological Parameters on Shear-Thinning Droplet Formation

2014 ◽  
Author(s):  
Voon-Loong Wong ◽  
Katerina Loizou ◽  
Phei-Li Lau ◽  
Richard S. Graham ◽  
Buddhika N. Hewakandamby
Keyword(s):  
Relaxation Time ◽  
Steady State ◽  
Shear Rate ◽  
Level Set ◽  
Droplet Diameter ◽  
Shear Thinning ◽  
Droplet Formation ◽  
Droplet Breakup ◽  
Rheological Parameters ◽  
Characteristic Relaxation Time

Immiscible non-Newtonian-Newtonian fluid systems in microfluidics constitute an essential study as non-Newtonian fluids consistently met in medical and biological systems. Although a large number of experimental investigations have been reported in this area, attempts to develop predictive models appear to be limited. This paper is an attempt to incorporate a non-Newtonian stress model together with front-tracking scheme used in computational fluid dynamics. A conservative two-phase level set method (LSM) was applied for capturing the droplet breakup dynamics and relevant hydrodynamics of shear-thinning carboxymethylcellulose (CMC) droplets. Our droplets comprise of 0.02wt% to 1.2wt% CMC solutions in a Newtonian continuous fluids system (olive oil) employed in a T-shaped microfluidic cell. A Carreau-Yasuda viscosity model for shear-thinning CMC droplets has been implemented. This shear-dependent constitutive model fitted well to our steady state non-linear shear measurements for polymeric CMC solutions, with asymptotic viscosities at zero and infinite shear rates, and with different degrees of shear thinning (η0/η∞) in steady state. The particular focus of this study was to systematically undergo parametric studies on the influence of rheological parameters of the specified model such as zero (η0) and infinite shear viscosity (η∞), and relaxation time (λ) on the droplet formation processes. The level set simulation predicted that the droplet diameter increases with increasing η0/η∞. The effect of η0/η∞ has been found to have more prominent impact on droplet diameter for higher CMC concentrations. The variation in droplet diameter becomes less significant at the higher degrees of shear-thinning for all concentrations of CMC dispersed solutions. In the limit of zero shear-thinning effect, the droplet diameter increases when the dispersed phase viscosity decreases. Additionally, the effect of λ on the droplet diameter is also discussed. The reciprocal of the characteristic relaxation time (1/λ) corresponds to a critical shear rate that indicates the onset shear rate for shear-thinning. As λ increases, the numerical studies clearly reveal that the droplet diameter is increasing until it reaches a plateau for larger values of λ. The influence of λ leads to a more significant impact on droplet diameter for higher CMC concentration. These findings will ultimately help in understanding the sensitivity of rheological parameters to the microdroplet formation.

scholarly journals Modeling the motion of a Taylor bubble in a microchannel through a shear-thinning fluid

2021 ◽  
Vol 312 ◽  
pp. 05006
Author(s):  
Andrea Aquino ◽  
Davide Picchi ◽  
Pietro Poesio
Keyword(s):  
Shear Thinning ◽  
Current Theory ◽  
Continuous Phase ◽  
Bubble Velocity ◽  
Recent Theory ◽  
Taylor Bubble ◽  
Lubrication Film ◽  
Shear Thinning Fluids ◽  
Validity Range ◽  
Shear Thinning Fluid

Applications of multiphase flows in microchannels as chemical and biological reactors and cooling systems for microelectronic devices typically present liquid slugs alternated with bubbles of elongated shape, the Taylor bubbles. These occupy almost entirely the cross-section of the channel and present a hemispherical front and a liquid layer, the lubrication film, which separates the gas from the tube wall. The Taylor bubble perturbs the surrounding fluids activating many transport mechanisms in the proximity of the gas-liquid interface; therefore, the bubble motion significantly influences the heat and mass transfer rates. Although many works deeply investigate the bubble hydrodynamics in Newtonian fluids, the knowledge about the relation between bubble hydrodynamics and rheological properties is insufficient, and studies where the continuous phase exhibits a shear-thinning behavior are missing. Our numerical analysis tries to fill this gap by investigating the motion of a Taylor bubble in a non-Newtonian shear-thinning fluid, modeled by the Carreau viscosity model. First, we validate the results against the Newtonian case and a recent theory for shear-thinning fluids (Picchi et al., Journal of Fluid Mechanics, 2021, 918). Then, we investigate the bubble hydrodynamics far from the validity range of the current models. Finally, we study the scaling of the bubble velocity and lubrication film thickness, extending the current theory to shear-thinning fluids.

scholarly journals Experimental Studies of Droplet Formation Process and Length for Liquid–Liquid Two-Phase Flows in a Microchannel

Energies ◽  
2021 ◽  
Vol 14 (5) ◽  
pp. 1341
Author(s):  
Li Lei ◽  
Yuting Zhao ◽  
Wukai Chen ◽  
Huiling Li ◽  
Xinyu Wang ◽  
...  
Keyword(s):  
Formation Mechanism ◽  
Flow Rate ◽  
Volume Flow Rate ◽  
Scaling Law ◽  
Experimental Studies ◽  
Droplet Formation ◽  
Continuous Phase ◽  
Volume Flow ◽  
Dispersed Phase ◽  
Two Phase

In this study, changes in the droplet formation mechanism and the law of droplet length in a two-phase liquid–liquid system in 400 × 400 μm standard T-junction microchannels were experimentally studied using a high-speed camera. The study investigated the effects of various dispersed phase viscosities, various continuous phase viscosities, and two-phase flow parameters on droplet length. Two basic flow patterns were observed: slug flow dominated by the squeezing mechanism, and droplet flow dominated by the shear mechanism. The dispersed phase viscosity had almost no effect on droplet length. However, the droplet length decreased with increasing continuous phase viscosity, increasing volume flow rate in the continuous phase, and the continuous-phase capillary number Cac. Droplet length also increased with increasing volume flow rate in the dispersed phase and with the volume flow rate ratio. Based on the droplet formation mechanism, a scaling law governing slug and droplet length was proposed and achieved a good fit with experimental data.

scholarly journals The Hele-Shaw injection problem for an extremely shear-thinning fluid

2015 ◽  
Vol 26 (5) ◽  
pp. 563-594 ◽  
Author(s):  
G. RICHARDSON ◽  
J. R. KING
Keyword(s):  
Surface Tension ◽  
Approximate Solution ◽  
Free Boundary ◽  
Power Law ◽  
Shear Thinning ◽  
Small Region ◽  
Power Law Fluid ◽  
Injection Point ◽  
Laplacian Equation ◽  
Shear Thinning Fluid

We consider Hele-Shaw flows driven by injection of a highly shear-thinning power-law fluid (of exponentn) in the absence of surface tension. We formulate the problem in terms of the streamfunction ψ, which satisfies thep-Laplacian equation ∇·(|∇ψ|p−2∇ψ) = 0 (withp= (n+1)/n) and use the method of matched asymptotic expansions in the largen(extreme-shear-thinning) limit to find an approximate solution. The results show that significant flow occurs only in (I) segments of a (single) circle centred on the injection point, whose perimeters comprise the portion of free boundary closest to the injection point and (II) an exponentially small region around the injection point and (III) a transition region to the rest of the fluid: while the flow in the latter is exponentially slow it can be characterised in detail.

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