Dispersed Phase Viscosity Research Articles

Investigation of viscous effects on droplet generation in a co-flowing step emulsification device

In the previous articles, we proposed a co-flowing step emulsification method to generate microdroplets. Various influencing factors, including interfacial tension, continuous and dispersed phase flow rates, and critical device dimensions, were analysed, however, without considering the effects of viscosities. Here, the volume-of-fluid (VOF) based CFD simulation method is used to investigate the effects of viscosities on the co-flowing step emulsification in this paper. The continuous and dispersed phase viscosities researched vary from 0.05 Pa s to 2.0 Pa s and 0.001 Pa s to 30 Pa s, respectively. The resulting droplet diameter ranges from 35.6 µm to 522.6 µm. It is found that the continuous phase viscosity has a significant effect on the droplet diameter, frequency and generation regime. As the continuous phase viscosity increases, the droplet diameter decreases, and the droplet generation regime changes from dripping to tip-streaming gradually. Whereas the dispersed phase viscosity increase nearly does not cause droplet diameter change, besides satellite droplets generation. The results are corresponding to the droplet diameter prediction model we proposed. This work effectively provides fundamental insights into microfluidic droplet generation characteristics in co-flowing step emulsification.

Improving Impact Toughness of Polylactide/Ethylene-co-vinyl-acetate Blends via Adding Fumed Silica Nanoparticles: Effects of Specific Surface Area-dependent Interfacial Selective Distribution of Silica

Adding fumed silica (SiO2) has been considered as an effective method for tailoring the phase morphology and performance of elastomer-toughened plastic binary blends. It has been demonstrated that the selective distribution of SiO2 plays a decisive role in the mechanical properties of plastic/elastomer/SiO2 nanocomposites, especially for the impact toughness. In this work, we aim to illuminate the role of specific surface area in controlling their selective distribution of fumed SiO2 and consequent mechanical properties of plastic/elastomer binary blends. Three types of SiO2 with different specific surface areas were incorporated into polylactide/ethylene-co-vinyl-acetate (PLA/EVA) model blends by melt blending directly. It was found that the selective distribution of SiO2 is largely determined by their specific surface areas, i.e. SiO2 nanoparticles with low specific surface area has a stronger tendency to be located at the interface between PLA matrix and EVA dispersed phase as compared to those with high specific surface area. The specific surface area-dependent interfacial selective distribution of SiO2 is mainly attributed to the extent of increased viscosity of EVA dispersed phase in which SiO2 nanoparticles are initially dispersed and resultant migration rate of SiO2 nanoparticles. The interfacial localized SiO2 nanoparticles induce an obvious enhancement in the impact toughness with strength and modulus well maintained. More importantly, in the case of the same interfacial distribution, toughening efficiency is increased with the specific surface area of SiO2. Therefore, this is an optimum specific surface area of SiO2 for the toughening. This work not only provides a novel way to manipulate the selective distribution of SiO2 in elastomer-toughened plastic blends toward high-performance, but also gives a deep insight into the role of interfacial localized nanoparticles in the toughening mechanism.

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

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.

Mathematical modeling of the droplet formation process in a microfluidic device

The results of droplet formation modeling in a microfluidic device based on the force balance method are described in the paper. The Navier-Stokes and continuity equations in an axisymmetric formulation were used to describe the hydrodynamic situation in a liquid film between a growing droplet and a channel wall. Shear stresses, pressure gradient along the droplet, as well as interfacial tension forces were included in the force balance. The obtained results allow to analyze the influence of phase flow rates and the properties of both phases on the flow regimes boundaries. It is shown that the continuous phase flow rate significantly determines the hydrodynamic situation both in the formed droplet of the dispersed phase and in the continuous phase film. On the other hand, the effect of the dispersed phase viscosity on the position of the flow regimes boundaries is much stronger than the viscosity of the continuous phase.

Viscosity Ratio Influence on Liquid‐Liquid Flow in a T‐shaped Microchannel

AbstractThe ratio of the dispersed phase viscosity to that of the continuous phase is a critical parameter for microfluidic two‐phase flows. Here, the influence of the viscosity ratio on liquid‐liquid flow in T‐shaped microchannels is studied experimentally. Three basic flow patterns, i.e., parallel, plug, and droplet flow, are observed for different sets of immiscible liquids. Flow pattern maps are plotted and generalized using a combination of Weber and Ohnesorge dimensionless numbers. In the plug flow pattern, interface deformations occur for low viscosity ratios. Existing correlations from the literature are tested against experimental data for plug velocity and lengths. Despite the significant plug interface deformations, the influence of the viscosity ratio on the plug length and velocity is negligible.

Thin wall injection‐overmoulding of polyamide 6/polypropylene multilayer parts: PA6/PP‐g‐ma interfacial adhesion investigations

AbstractThis work investigates the interfacial adhesion of PA6/PP‐g‐MA in a thin‐walled multilayer part obtained by injection overmoulding. This process is a good solution to the need of weight reduction for the sustainable and economic growth of food packaging. A rigid plastic food package is a complex multilayer structure obtained from the association of polymers with different properties to achieve good barrier, environment, or thermo‐mechanical performance. The present study aims to obtain a complete understanding of the interfacial adhesion of PA6/PP‐g‐MA interface in a three‐layer PA6/sealant/PP assembly through microscopy analysis, mechanical characterization, and inter‐diffusion models. Optical microscopy, XPS, and AFM analyses showed the presence of a copolymer interphase. This layer is greatly dependent on the holding pressure applied, which revealed to be the key parameter to obtain the best results in terms of failure strength. A better interface wettability improves the inter‐diffusion distance and thus chemical grafting efficiency. Other interface improvements were achieved by the addition of EVA in the PP‐g‐MA layer. The lowest viscosity of the EVA dispersed phase has led to a fully cohesive failure due to an interface diffusion efficiency. The presence of EVA increased more than 80% of the peeling performance of the assemblies.

Emulsification in a microfluidic flow-focusing device: Effect of the dispersed phase viscosity

Flow regimes of two immiscible liquids are experimentally investigated in a rectangular microchannel using a water–oil system. Oil-water viscosity ratios are varied from 0.2 to 75, accompanied by varied oil–water velocity ratios from 10−2 to 102 and varied capillary numbers from 10−3 to 10−5. Four flow regimes are distinguished, including threading, jetting, dripping and squeezing regimes, based on the continuous phase’s capillary number (Ca) and the dispersed phase’s Weber number (We). An exponential correlation exists between the viscosity ratio at which the flow regime transits and the velocity ratio. Specifically, the viscosity ratio is found to have a larger effect on the slug length in the squeezing regime than the droplet size in the dripping regime. Therefore, two expressions are proposed to separately scale emulsions in squeezing and dripping regimes: one for prediction of droplet dimension in the dripping regime based on the continuous phase’s oil–water velocity ratio and Ca, and the other one for prediction of slug lengths in the squeezing regime, which involves the viscosity ratio in addition to abovementioned two parameters. Both scaling expressions are validated by available data in the existing literature.

Poly(butylene adipate-co-terephthalate)/thermoplastic starch/zeolite 5A films: Effects of compounding sequence and plasticizer content

This work investigated the effect of the compounding sequence and the glycerol content on poly(butylene adipate-co-terephthalate)/thermoplastic starch/zeolite 5A (PBAT/TPS/Z5A) composites. The composite pellets and films were prepared by an extrusion process using a PBAT:TPS ratio of 60:40, Z5A loading of 3 wt%, and glycerol contents of 35 and 40 parts per hundred parts of starch (phs). Prior to blown film extrusion, the composite pellets were produced by two compounding sequences: sequence I (SI)–mixing PBAT with Z5A prior to blending with TPS; sequence II (SII)–mixing TPS with Z5A before blending with PBAT. The SII compounding sequence provided improved mixing between PBAT and TPS, leading to increased continuous phase region and a reduced TPS dispersed phase size. Increasing the glycerol content decreased the viscosity and size of the TPS dispersed phase and gave rise to a more uniform dispersion of the TPS domains and Z5A particles. Compounding Z5A via the SII sequence with a glycerol content of 40 phs effectively improved the mixing and the performance of the PBAT/TPS blend.

Preparation of drug delivery system based on poly (lactide-glycolide) and evaluation of parameters affecting its structure for cancer treatment

In this present work, synthesized of poly (lactide-glycolide) was carried out via two separate solvent evaporation methods for evaluating the effect of particle size in cancer treatment. By changing the percentage of polymer in dispersed phase solution and percentage of poly (vinyl alcohol) in continuous phase, particles with different morphology and dimensions were prepared. The particles with a low percentage of stabilizer had rough surfaces and larger sizes. By increasing the percentage of polymer in the dispersed phase from 1 to 2%, the particle size first decreased and then increased. As the process conditions changed, sub-micron-sized particles were made from PLGA solutions of varying concentrations in 1% poly (vinyl alcohol) solution. The results indicate the high sensitivity of the morphology and dimensions of poly (lactide-glycolide) particles to the manufacturing process conditions. The sub-micron particles were loaded with the anti-cancer drug Oxaliplatin. Drug loading rate was a function of dispersed phase viscosity and droplet dimensions. The release test showed the ability of the particles prepared to control and delay release of Oxaliplatin in the release medium for 96 h.

A rheo-optic study of liquid/liquid systems with varying phases, volume fractions and viscosity ratios

There is a need to understand the rheology and structure of emulsion-based systems under flow, as they determine quality attributes of foods such as texture, stability and taste. However, simultaneous rheo-optical investigations are challenging partially due the limited (and expensive) equipment available. An in-house rheo-optical device has been used to study the effect of shear (steady shear at 250s−1; increasing-decreasing shear at 0-500-0s−1) on rheology and structure of O/W and W/O systems at viscosity ratios (viscosity of dispersed phase over viscosity of continuous phase) λ = 0.2−68 and volume fractions of dispersed phase φ = 0.05−0.3. The O/W and W/O systems had identical ingredients but inversed phases. First, development of the device and its potential to capture clear images under shear is reported. Then, results for O/W systems at steady shear are compared with predictive rheological models, which typically link relative viscosity with φ and λ. The effective medium model showed best agreement. On increasing-decreasing shear, strong link between rheology and structure was indicated. Finally, relative viscosities of the W/O systems were invariably lower than those of the O/W dispersions with similar φ and λ. The related obtained microstructures appeared to be largely determined by drop breakage, controlled by the viscosities of the individual phases rather than λ. This work indicates a close link between rheology and structure for the investigated systems, and it suggests that the link between morphology and viscosity is complex and depends on the nature of the investigated material.

Influence of Dispersed-Phase Viscosity on Droplet Breakup in a Continuous Pump-Mixer

The influence of dispersed-phase viscosity on droplet breakup in a pump-mixer was determined using a direct measurement approach, which was performed using a high-speed online camera. The experimen...

An effective method to facile coalescence of microdroplet in the symmetrical T-junction with expanded convergence

The coalescence process of microdroplets in the expanded convergence of symmetrical T-junction was investigated. Five types of droplet coalescence regimes were observed. The existence of expanded convergence slows down the moving speed of droplets, which accordingly allows a certain time difference between the two microdroplets arriving at the convergence for coalescence, and the maximum allowable arriving time difference of the two droplets was predicted. The passive coalescence of two droplets is mainly described by the liquid film draining model. The critical capillary number (Cac), which determines whether symmetrical colliding droplets would coalesce, decreases with the increase of the viscosity of the dispersed phase, it means that the increase of the viscosity of dispersed phase is not conducive to coalescence. A correlation equation was proposed to predict the critical capillary number, which may provide valuable guidance for the design and application of microdevices.

Integrated droplet microfluidic device for magnetic particles handling: Application to DNA size selection in NGS libraries preparation

We present an innovative droplet microfluidic device based on integrated soft-magnetic structures allowing extraction, redispersion and depletion processes of magnetic particles. Following a numerically-assisted conception, the device was at first fabricated and experimentally characterized, showing reliable and efficient magnetic particles extraction with a low carryover of the initial matrix (<3.5%), in a broad range of dispersed phase viscosities (between 1 and 15 mPa·s) and particles loads (between 0.1 μg and 1.5 μg). Its functionalities were successfully exploited to perform DNA size selection processes for NGS libraries preparation. The device achieved final concentrations and size distributions comparable with conventional methods, demonstrating compatibility with downstream sequencing and providing improvements in automation, throughput and workflow simplification. Finally, this device represents a universal solution for high throughput multi-steps magnetic solid phase extraction workflows in a wide range of applications, e.g. immunoassays, nucleic acids and proteins purification, and sample preparation.

On the role of the surface rheology in film drainage between fluid particles

Coalescence in the presence of low surfactant concentrations is investigated via a film drainage model, where the interface is a Boussinesq surface fluid with surfactant concentration dependent physical properties. Three cases are considered representing the systems where water is the continuous phase and the dispersed phase is either high viscosity droplets, droplets of comparable viscosity to water or gas bubbles. In the former, the immobilization of the interface is due to high dispersed phase viscosity and surface viscosities, whereas in the latter two, the Marangoni flow plays an important role, too. When droplets of comparable viscosity to water or gas bubbles are considered, it is seen that both the Marangoni flow and the surface viscosities can change the coalescence time significantly for the experimentally encountered values of the initial surfactant concentration, and the Boussinesq and surface Péclet numbers. In all cases, the impact of the surface phenomena amplifies with the approach velocity, especially for the dimpled interfaces. A complete immobilization criterion that is independent of the dispersed phase viscosity is proposed as a function of the continuous phase and surface viscosities, and the particle radii.

Study on the stability of heavy crude oil-in-water emulsions stabilized by two different hydrophobic amphiphilic polymers

Emulsification is an important oil recovery mechanism for amphiphilic polymer flooding, with a good viscosity reduction effect for heavy oil. Hence, the comparison study, aimed to elucidate the stabilization mechanism of amphiphilic polymers with single-branched and multi-branched hydrophobic groups, was proposed for the first time. Two amphiphilic polymers with different hydrophobic groups, poly (acrylamide/2-acryloylamino-2-methyl-1-propane sulfonic acid/N, N-dodecyl acrylamide) (PAAD) and poly (acrylamide/2-acryloylamino-2-methyl-1-propane sulfonic acid/N-dodecyl acrylamide) (PAAC), were employed to generate heavy crude oil-in-water emulsion. A laser particle size analyzer and Turbiscan Lab Expert stability analyzer were employed to characterize the stability of emulsions stabilized by the amphiphilic polymers. The results showed that the stability of emulsion stabilized by PAAD (Em-PD) was better than that of emulsion stabilized by PAAC (Em-PC). The particle size of Em-PD ranged from 0.9 μm to 10.5 μm while that of Em-PC ranged from 1.6 μm to 16.5 μm, which indicated that Em-PD showed a smaller average particle size and narrower distribution. It is found that the decline of oil-water interfacial tension is not the key factor for emulsion stability. The multi-branched amphiphilic polymer shows the enhancement of both the viscosity of dispersion medium and the interfacial viscoelasticity, producing the emulsion with stronger stability. The current work shows by designing the molecular structure of amphiphilic polymer to adjust its hydrophobicity can achieve the desired stability of heavy crude oil emulsion which is good for enhanced oil recovery.

The Influence of Internal Intermittency, Large Scale Inhomogeneity, and Impeller Type on Drop Size Distribution in Turbulent Liquid-Liquid Dispersions.

The influence of the impeller type on drop size distribution (DSD) in turbulent liquid-liquid dispersion is considered in this paper. The effects of the application of two impellers, high power number, high shear impeller (six blade Rushton turbine, RT) and three blade low power number, and a high efficiency impeller (HE3) are compared. Large-scale and fine-scale inhomogeneity are taken into account. The flow field and the properties of the turbulence (energy dissipation rate and integral scale of turbulence) in the agitated vessel are determined using the k-ε model. The intermittency of turbulence is taken into account in droplet breakage and coalescence models by using multifractal formalism. The solution of the population balance equation for lean dispersions (when the only breakage takes place) with a dispersed phase of low viscosity (pure system or system containing surfactant), as well as high viscosity, show that at the same power input per unit mass HE3 impeller produces much smaller droplets. In the case of fast coalescence (low dispersed phase viscosity, no surfactant), the model predicts similar droplets generated by both impellers. In the case of a dispersed phase of high viscosity, when the mobility of the drop surface is reduced, HE3 produces slightly smaller droplets.

Analysis of centrifugal homogenization and its applications for emulsification & mechanical cell lysis

We detail the analysis of centrifugal homogenization process by a hydrodynamic model and the model-guided design of a low-cost centrifugal homogenizer. During operation, centrifugal force pushes a multiphase solution to be homogenized through a thin nozzle, consequently homogenizing its contents. We demonstrate and assess the homogenization of coarse emulsions into relatively monodisperse emulsions, as well as the application of centrifugal homogenization in the mechanical lysis of mpkCCD mouse kidney cells. To gain insight into the homogenization mechanism, we investigate the dependence of emulsion droplet size on geometrical parameters, centrifugal acceleration, and dispersed phase viscosity. Our experimental results are in qualitative agreement with models predicting the droplet size. Furthermore, they indicate that high shear rates kept constant throughout operation produce more monodisperse droplets. We show this ideal homogenization condition can be realized through hydrodynamic model-guided design minimizing transient effects inherent to centrifugal homogenization. Moreover, we achieved power densities comparable to commercial homogenizers by model guided optimization of homogenizer design and experimental conditions. Centrifugal homogenization using the proposed homogenizer design thus offers a low-cost alternative to existing technologies as it is constructed from off-the-shelf parts (Falcon tubes, syringe, needles) and used with a centrifuge, readily available in standard laboratory environment.

On the steady‐state drop size distribution in stirred vessels. Part II: Effect of continuous phase viscosity

AbstractIn Part I, we used silicon oils with viscosities across six orders of magnitude to investigate the effect of the dispersed phase viscosity on the droplet size distribution of dilute emulsions. In this study, we extended Part I by using three glucose aqueous solutions to thicken the continuous phases, approximately an order of magnitude while keeping the Power number constant. It was found that increasing the continuous phase viscosity decreases the maximum drop size despite having drops well above the Kolmogorov lengthscale. Our results are in disagreement with the mechanistic models for the turbulent inertia regime. The results were explained using the full turbulent energy spectrum proposed by Pope instead of the Kolmogorov −5/3 spectrum. Our analysis revealed that most of the steady‐state drop sizes do not fall in the isotropic turbulence size range.

Study of droplet breakage in a pulsed disc and doughnut column-Part I: Experiments and correlations

We have developed a direct measurement method for the breakage of droplet swarm in our previous work. However, experimental data for droplet breakage is still extremely indispensable for the development and employment of population balance model (PBM). In this paper, the direct measurement of droplet breakage for a series of liquid-liquid systems has been carried out in a pulsed disc and doughnut column (PDDC) to meet the requirement. The data together with that in our previous work cover the main factors affecting the droplet breakage in the PDDC and are sufficient to support the establishment of useful empirical correlations for breakup frequency and daughter droplet size distribution. The experimental results and correlations all indicate that the breakup frequency function increases as the dispersed-phase viscosity or the interfacial tension decreases. The daughter droplet size distribution broadens considerably with decreasing interfacial tension.

Apparent viscosity characteristics and prediction model of an unstable oil-in-water or water-in-oil dispersion system

A series of theoretical analysis and experimental measurements were done to analyze the characteristic of the apparent viscosity of oil-water unstable mixtures which display the similar distributions as the mixtures in pipeline flow. Based on the experimental results, it can be concluded that the unstable oil-water dispersed mixtures are always show the characteristic of non-Newtonian fluid. The shear rate has a great influence on the apparent viscosity, particular for the water-in-oil mixtures. The effect of dispersed phase volume fraction on the apparent viscosity of the unstable mixtures displays the similar rules as the stable emulsions. Also, the influences of the dispersed phase distribution and viscosity on the apparent viscosity were studied through the corresponded experiments systematically. Furthermore, a new kind of prediction model was obtained in the present study. The unstable and non-Newtonian fluid characteristics of oil-water two-phase dispersed mixtures were considered in this model to improve their range of application and the prediction accuracy. Moreover, this kind of apparent viscosity prediction model was validated by using the experimental data from some other literatures including different kinds of oils such as white oil and crude oil, and the average deviation was always less than 9% which was a great accuracy of apparent viscosity prediction.