“…In contrast to the effect of elevated viscosity, shear-thinning should exert an effect to accelerate the breakup (by improving the exponent α). However, our results report the decreased exponent α, suggesting the screened effect of shear-thinning in the current experiments, which agrees with the observations of previous experiments for purely shear-thinning fluids [38,39].…”
Section: Micromachines 2020 11 X For Peer Review 9 Of 14supporting
confidence: 93%
“…It is noted that most of the existing studies consider dilute polymer solutions, while more concentrated polymer solutions are frequently encounter in practical applications [5,[14][15][16], which often exhibit non-Newtonian properties more than elasticity. Although several studies have also examined the effect of shear-thinning on filament breakup [38,39], the breakup behavior of dense polymer solutions exhibiting both elasticity and shear-thinning has rarely been addressed in microfluidic devices. In the present study, the breakup dynamics of semi-dilute polymer solutions in a flow-focusing microchannel are highlighted.…”
Droplet microfluidics involving non-Newtonian fluids is of great importance in both fundamental mechanisms and practical applications. In the present study, breakup dynamics in droplet generation of semi-dilute polymer solutions in a microfluidic flow-focusing device were experimentally investigated. We found that the filament thinning experiences a transition from a flow-driven to a capillary-driven regime, analogous to that of purely elastic fluids, while the highly elevated viscosity and complex network structures in the semi-dilute polymer solutions induce the breakup stages with a smaller power-law exponent and extensional relaxation time. It is elucidated that the elevated viscosity of the semi-dilute solution decelerates filament thinning in the flow-driven regime and the incomplete stretch of polymer molecules results in the smaller extensional relaxation time in the capillary-driven regime. These results extend the understanding of breakup dynamics in droplet generation of non-Newtonian fluids and provide guidance for microfluidic synthesis applications involving dense polymeric fluids.
“…In contrast to the effect of elevated viscosity, shear-thinning should exert an effect to accelerate the breakup (by improving the exponent α). However, our results report the decreased exponent α, suggesting the screened effect of shear-thinning in the current experiments, which agrees with the observations of previous experiments for purely shear-thinning fluids [38,39].…”
Section: Micromachines 2020 11 X For Peer Review 9 Of 14supporting
confidence: 93%
“…It is noted that most of the existing studies consider dilute polymer solutions, while more concentrated polymer solutions are frequently encounter in practical applications [5,[14][15][16], which often exhibit non-Newtonian properties more than elasticity. Although several studies have also examined the effect of shear-thinning on filament breakup [38,39], the breakup behavior of dense polymer solutions exhibiting both elasticity and shear-thinning has rarely been addressed in microfluidic devices. In the present study, the breakup dynamics of semi-dilute polymer solutions in a flow-focusing microchannel are highlighted.…”
Droplet microfluidics involving non-Newtonian fluids is of great importance in both fundamental mechanisms and practical applications. In the present study, breakup dynamics in droplet generation of semi-dilute polymer solutions in a microfluidic flow-focusing device were experimentally investigated. We found that the filament thinning experiences a transition from a flow-driven to a capillary-driven regime, analogous to that of purely elastic fluids, while the highly elevated viscosity and complex network structures in the semi-dilute polymer solutions induce the breakup stages with a smaller power-law exponent and extensional relaxation time. It is elucidated that the elevated viscosity of the semi-dilute solution decelerates filament thinning in the flow-driven regime and the incomplete stretch of polymer molecules results in the smaller extensional relaxation time in the capillary-driven regime. These results extend the understanding of breakup dynamics in droplet generation of non-Newtonian fluids and provide guidance for microfluidic synthesis applications involving dense polymeric fluids.
“…Thus, higher viscous resistance is experienced by increasing the weight percentage of SCMC. Moreover, effective viscosity of the continuous phase is also calculated based on equation (18) 51,52 and compared to the previously obtained average viscosity. In this equation, U c and D are inlet velocity and inlet diameter of the continuous phase.…”
Section: Resultsmentioning
confidence: 99%
“…A similar data fashion has been reported by other researches who investigated droplet generation in other geometries with inner phase considered as non-Newtonian. 52 Thus, it can be said that shear-thinning property of the dispersed phase does not impose a strong deviation from the Newtonian behaviour in the coflowing geometry. Figure 14.Droplet formation and propagation in the coflowing geometry with inner phase assumed to be non-Newtonian (left), droplet generation and droplet diameter versus SCMC concentration in the inner phase (right).…”
This study aims to assess the droplet formation process in microfluidic devices using an all Mach-number multi-phase flow solver. Harten-Lax-van Leer-Contact (HLLC) Riemann solver is implemented for solving the discretized equations while Tangent of Hyperbola for INterface Capturing (THINC) method is applied to reduce the excessive diffusion of the method at the interface. The multi-block strategy is implemented in the flow solver to improve its capability of simulating flows in more complicated, multi-port droplet formation geometries. First, the computational performance of the numerical solver is validated through simulating the benchmark Rayleigh-Taylor instability problem and the droplet formation in a planar flow-focusing geometry. The comparison of numerical results with available analytical/experimental data indicates a precise prediction of flow features. Then, the droplet generation process in coflowing devices with Newtonian liquid is numerically studied. Finally, the shear-thinning effects of the dispersed and continuous phase on the drop formation characteristics are investigated. It is shown that deviation of the continuous phase from Newtonian behavior has a strong impact on droplet size, speed, and generation rate. On the other hand, assuming the dispersed phase as non-Newtonian does not strongly affect the aforementioned properties of the droplet formation regime.
“…Abundant literature is available on fundamental emulsification phenomena in a single microchannel. These studies distinguish themselves by the mechanisms of droplets/plugs formation (Garstecki et al (2006); Steegmans et al (2009); Nabavi et al (2017)), by different configurations for liquid-liquid contact (T-junction (Chong et al (2016); Ma et al (2017)), cross-junction (Belkadi et al (2015); Van Loo et al (2016)), or flow focusing (Josephides and Sajjadi (2015); Du et al (2018)), by the use of computational fluid dynamics (CFD) (Kashid et al (2010); Delteil et al (2011); Chen et al (2015); Soh et al (2016); Sontti and Atta (2017)) or experimental approach (Priest et al (2011); Xi et al (2016); Teo et al (2017)), and by different adjustments to achieve highly monodisperse emulsions (Maan et al (2011); Tanaka et al (2015); Martins et al (2017); Akamatsu et al (2017)). Nevertheless, to overcome the identified barrier of low throughput (or production rate) for a single microchannel, the parallelization of micro/mini-channels is required aiming at industrial-scale mass production.…”
This paper presents a numerical study on the water-in-oil emulsification process through the parallelization of micro/mini-channels. Firstly, the single-phase fluid flow distribution uniformity in the bifurcated tree-like fluidic network is discussed. Secondly, the separate roles of the oil viscosity and the interfacial tension on the plug details are clarified. Finally, the impact of flow distribution non-uniformity among the parallel mini-channels on polydispersity of the produced emulsion is evaluated by modelling. The obtained results show that for bifurcated tree-like fluidic network, the single phase flow distribution is the flow distribution non-uniformity increases linearly with the increasing mean Re ch once a transitional Re is reached. The water plug length and volume increase with the increasing interfacial tension and the decreasing oil viscosity under our tested conditions. Correcting factors relevant to the modified liquid properties have to be added in the predictive correlations for plug length and volume. For water-in-oil emulsification in parallel channels, its polydispersity P w is affected by the single phase flow distribution uniformity, the total water/oil flow-rate ratio and most importantly by the flow-rate ratio distribution non-uniformity. An empirical correlation has been proposed to predict the emulsion P w based on these influencing factors.
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