Fluid Dispersion Research Articles

Active sorting of droplets by using an ECF (Electro-conjugate Fluid) micropump

This paper proposes an on-demand method to sort water-in-oil (W/O) droplets by using an ECF (Electro-Conjugate Fluid) micropump on a chip. ECF is a kind of functional and dielectric oil, which can generate a strong and active flow (called ECF jet) when a high DC voltage is applied to its corresponding anode & cathode in ECF. Targeting at on-demand droplets sorting, we design a new sorter including several parts: a T-junction droplet generator made of SU-8, a sorting area, an ECF micropump, and I/O ports. The ECF combined with a suitable concentration of a surfactant (Span 80) and DI (deionized) water are utilized as the continuous and the dispersed fluids, respectively. The mixture of ECF and Span 80 can also be pumped by our ECF micropump and then forms the ECF jet. The fabrication of the sorter is based on the photolithographic patterning of thick resists (KMPR and SU-8) and the electroforming. We model the ECF micropump and also analyze the sorting performance of our sorter based on the FEM analysis. To validate the sorter, we investigate the influence of two vital parameters of the strength of ECF jets and the droplet sizes on the sorter. Then, we also research the working zones (areas), throughput and sorting efficiency in terms of flow rate ratios and the applied voltages of our device.

Thermo-physical properties of solar thermic fluids dispersed with carbon nanotubes

The paper examines the influence of dispersion of multi-walled carbon nanotubes on the thermo-physical properties of working fluids used in solar thermal engineering systems. Thermo-physical property evaluation is very vital to evaluate the performance of solar thermal systems. Ethylene glycol and a commercially reachable thermic fluid therminol 62 are preferred as based fluids. Nanofluids are prepared by dispersing 0.1 %, 0.25 %, 0.5 % and 0.1 % weight fraction of surface modified Multi-walled carbon nanotubes. The nanomaterials are surface modified before dispersion in the thermic fluids and extreme care is taken to prevent property deterioration during surface modification. Thermal conductivity of test fluids is evaluated in the temperature range of 100 to 300°C while viscosity is evaluated in the range of 50 to 150°C s. A 15 to 20 % increase in the thermal conductivity is observed with the dispersion of carbon nanotubes. The viscosity is significantly increased in the low-temperature ranges while the change is minimal above 70. The data could be predicted successfully using artificial neural networks.

On the effect of coalescence on the rheology of emulsions

We present a numerical study of the rheology of a two-fluid emulsion in dilute and semidilute conditions. The analysis is performed for different capillary numbers, volume fractions and viscosity ratios under the assumption of negligible inertia and zero buoyancy force. The effective viscosity of the system increases for low values of the volume fraction and decreases for higher values, with a maximum for approximately 20 % concentration of the disperse phase. When the dispersed fluid has lower viscosity, the normalised effective viscosity becomes smaller than 1 for high enough volume fractions. To single out the effect of droplet coalescence on the rheology of the emulsion we introduce an Eulerian force which prevents merging, effectively modelling the presence of surfactants in the system. When the coalescence is inhibited the effective viscosity is always greater than 1 and the curvature of the function representing the emulsion effective viscosity versus the volume fraction becomes positive, resembling the behaviour of suspensions of deformable particles. The reduction of the effective viscosity in the presence of coalescence is associated with the reduction of the total surface of the disperse phase when the droplets merge, which leads to a reduction of the interface tension contribution to the total shear stress. The probability density function of the flow topology parameter shows that the flow is mostly a shear flow in the matrix phase, with regions of extensional flow when the coalescence is prohibited. The flow in the disperse phase, instead, always shows rotational components. The first normal stress difference is positive, except for the smallest viscosity ratio considered, whereas the second normal difference is negative, with their ratio being constant with the volume fraction. Our results clearly show that the coalescence efficiency strongly affects the system rheology and that neglecting droplet merging can lead to erroneous predictions.

Effect of aggregation on magnetic permeability of magnetic fluid at microwave and radio frequencies

The properties of kerosene based magnetic fluid with Fe3O4 nanoparticles are studied for the two samples, one is well dispersed fluid while the other has aggregates. The presence of aggregates are identified by optical characterization techniques. The frequency dependent complex magnetic permeability is determined and compared for both samples. The effect of aggregates on microwave properties of magnetic fluid is reported. The microwave absorption and heating in magnetic fluid is useful in applications like attenuator, electromagnetic shielding hyperthermia, etc.

Finite Element Study of Flow of Partially Ionized Fluid Containing Nanoparticles

This article investigates the impact of nanosized particles (Cu and Ag) on the thermal performance of partially ionized non-Newtonion liquid (Casson fluid) exposed to non-uniform magnetic field in the presence of thermal radiations. Mathematical models based on basic governing laws are complex, nonlinear and coupled which are solved by finite element method in order to investigate the underlying physics. The results are validated by comparing with already published benchmarks. Convergence, error and mesh-free analysis are done. The CPU times of present method and method used in published benchmark are noted. The present method has less CPU time than CPU time required by method used in published benchmark. The wall shear stress increases, whereas wall heat flux decreases as the intensity of the magnetic field is increased. This observation is noted for the both cases of Cu and Ag nanoparticles. However, the wall shear stress for the case of Ag nanoparticles is greater than the wall shear stress for the case of Cu nanofluid. The usage of Ag nanoparticles is recommended as their dispersion in the base fluid increases the effective thermal conductivity in comparison of Cu nanofluid. Hall and ion-slip currents have shown remarkable increase in velocity and a significant reduction.

Analytical and numerical studies of mass flux dispersed from tapered swirl atomizer with various nozzle cut profiles

Spraying dispersed fluid by swirl atomizer into a main gas stream in order to obtain homogeneous blended flow within a restricted distance from the atomizer position is a significant issue. The dispersal of sprayed fluid mass-flux from the atomizer should be correlated to mass flux distribution of the main flow gas. Consequently, it is essential to control the distribution of liquid mass flux from the injector to match the distribution of main gas flux to attain perfect mixing with identical liquid/gas mass flux ratio all over the pipe. The present study examines the liquid mass flux pattern injected from tapered swirl atomizer having various cut shapes. A mathematical approach to circumferentially evaluate the outlet mass fraction distribution from generalized tapered nozzle cut was analyzed. The derived mass fraction distribution equation is dependent on the flow-angle, the cut-angle, and the arc-angle. A CFD simulation was performed to evaluate the flow angle at the outlet from the nozzle cut. The finding of the present work delivers a guide for researchers and engineers to the preferred injector cut profile to give the best mass flux uniformity. The simulated spray pattern relied on circumferential mass flux at nozzle outlet has been in great coincidence with what depicted by imaging. More studies could be performed to have the most appropriate cut shape of the injector with various flow profiles of the main gas stream.

Modeling thermal conductivity of Ag/water nanofluid by applying a mathematical correlation and artificial neural network

Abstract Due to the significance importance of effective thermal conductivity of heat transfer fluids in various renewable energy system, such as geothermal and solar thermal plants, using naofluids can result in augment in the efficiency. Metallic nano particles dispersion in a pure fluid leads to considerable enhancement in the thermal conductivity. The improvement in the thermal conductivity is dependent on various factors. In the present research, two machine learning algorithms, a correlation and Group Method of Data Handling, are applied to predict thermal conductivity of silver/water nanofluid. Temperature, concentration and size of solid particles are considered as the input data. According to statistical comparison of the models, employing GMDH artificial neural network results in more precise and appropriate model. The coefficients of correlation, R-squared values, for the proposed correlation and ANN-based models are 0.948 and 0.99 respectively.

Characterization of Anionic–Nonionic Surfactant Mixtures for Enhanced Oil Recovery

Enhanced oil recovery (EOR) has been one of the favorable methods in improving oil recovery and extending the production time. One of the techniques used in enhanced oil recovery (EOR) is gas flooding, because of the gases sweeping ability to displace the oil into the production well. Unfortunately, the gas injection method causes severe gas fingering because of the higher mobility and lower viscosity and density of the gas, compared to oil, which creates uneven fluid dispersion (fingering). Thus, a stabilized surfactant foam is introduced to control the injected gas mobility. This paper aims to characterize how the blends of non-anionic and anionic surfactants could act as foam stabilizing agents and to provide a brief exploration on their ability to improve the gas flooding process. The foamability and foam stability of single and mixed surfactant were tested at ambient and elevated temperature; additionally, a salt tolerance study was performed to identify the capability of the foam to withstand real reservoir conditions. Two anionic surfactants: sodium dodecyl sulphate (SDS) and alpha olefin sulfonate (AOS) and one non-ionic surfactant, octyl phenol ethoxylate (TX-100), were used in the study. Each single surfactant was tested for its viscosity and surface tension at the air/water interface under ambient conditions and compared with the mixed surfactants. Data has shown that blending of anionic and non-ionic surfactants has contributed to lower IFT compared to the individual surfactants and to increase viscosity, which is an early indicator of good foam stabilizer for foam flooding, which may lead to oil recovery improvement. The mixture of non-ionic and anionic surfactants, particularly TX-100 and AOS, showed better foamability and foam stability at elevated temperature than the individual pure surfactants.

Сравнительные тепло- и массообменные испытания пакетных насадок с пленочным и капельным течением жидкости

The paper focuses on comparative studies of heat and mass transfer and hydraulic characteristics of two structured packings which provide heat and mass transfer of contacting media in the film flow of liquid Mellapak 250.X and with fluid dispersion in the gas volume PVN.22. The experiments were carried out in a heat-mass exchange column of 400 mm in diameter and of 1 m in height of a packed bed. The method of evaporative cooling of water in the air flow was adapted to obtain heat and mass exchange characteristics of packed contact devices. Findings of research show the effect of irrigation density and gas flow on heat and mass transfer and hydraulic characteristics of packings which provide heat and mass transfer of contacting media in film and droplet flows. A criterion for estimating the efficiency of contact devices is proposed.

Interaction of linear modulated waves and unsteady dispersive hydrodynamic states with application to shallow water waves

A new type of wave–mean flow interaction is identified and studied in which a small-amplitude, linear, dispersive modulated wave propagates through an evolving, nonlinear, large-scale fluid state such as an expansion (rarefaction) wave or a dispersive shock wave (undular bore). The Korteweg–de Vries (KdV) equation is considered as a prototypical example of dynamic wavepacket–mean flow interaction. Modulation equations are derived for the coupling between linear wave modulations and a nonlinear mean flow. These equations admit a particular class of solutions that describe the transmission or trapping of a linear wavepacket by an unsteady hydrodynamic state. Two adiabatic invariants of motion are identified that determine the transmission, trapping conditions and show that wavepackets incident upon smooth expansion waves or compressive, rapidly oscillating dispersive shock waves exhibit so-called hydrodynamic reciprocity recently described in Maiden et al. (Phys. Rev. Lett., vol. 120, 2018, 144101) in the context of hydrodynamic soliton tunnelling. The modulation theory results are in excellent agreement with direct numerical simulations of full KdV dynamics. The integrability of the KdV equation is not invoked so these results can be extended to other nonlinear dispersive fluid mechanic models.

In Vivo Intrathecal Tracer Dispersion in Cynomolgus Monkey Validates Wide Biodistribution Along Neuraxis.

It is commonly believed that in intrathecal (IT) drug delivery, agent distribution is confined to a narrow region close to the injection site, thereby undermining the efficacy of the method. To test the claim, multimodal in vivo imaging was used to experimentally observe the effects of IT infusion in cynomolgus monkey, looking at cerebrospinal fluid flow, anatomy, and dispersion of a radiolabeled tracer. At high infusion rates, the tracer reached the cervical region after only 2 h, demonstrating rapid and wide distribution. The same in vivo nonhuman primate imaging data also provided evidence in support of a computational fluid dynamic model for the prediction of drug distribution following IT injection. Tracer dispersion was predicted in two specimens matching the distribution acquired with positron emission tomography (PET). For the third specimen, tracer dispersion simulations were conducted as a blind study: predictions were made before in vivo biodistribution data was known. In all cases, the computational fluid dynamics (CFD) predictions of drug dispersion after IT administration showed close spatio-temporal agreement with tracer biodistribution in vivo. Validation by in vivo nonhuman primate data confirms our ability to predict the biodistribution of intrathecally administered agents in subject-specific models of the central nervous system from first principles. The experiments reinstate IT delivery as a viable administration method when targeting molecules to the whole spine or the brain. The proposed computational methodology enables rational design of novel therapies for neurological diseases that require reliable, efficient, and safe delivery of therapeutic agents to specific target sites in the central nervous system.

Micro instabilities and rotating spokes in the near-anode region of partially magnetized plasmas

Electron and ion transport in the near-anode region of a partially magnetized plasma under conditions typical of Hall thrusters or magnetron discharges is studied with fully kinetic, Particle-In-Cell Monte Carlo Collision (PIC-MCC) simulations assuming a uniform magnetic field and no ionization. We derive a simple relation that defines the magnetic field at the transition point between negative and positive sheaths. For magnetic fields around or above this transition point, PIC-MCC simulations show the development of short wavelength azimuthal instabilities that cascade to longer wavelengths (“rotating spokes”) as the magnetic field is increased. Both short-wavelength and large-wavelength fluctuations can coexist under some conditions. A detailed study of the fluid dispersion relation is used to analyze the PIC-MCC results. Small coherent structures can be associated with the destabilization of ion sound waves by density gradient and collisions. Longer wavelengths or rotating spokes are characteristic of the collisionless Simon-Hoh instability. The small structures are dominant for larger plasma density gradients, while the larger structures correspond to smaller density gradients and larger magnetic fields. Anomalous transport associated with these instabilities can be significant, with effective collision frequencies larger than 2×107 s−1 in xenon for magnetic fields above the transition point.

Enhanced aeration efficiency in non-Newtonian fluids using coaxial mixers: High-solidity ratio central impeller with an anchor

One of the major challenges in bioreactors is maintaining a critical level of dissolved oxygen concentration in highly viscous fluids. In this perspective, we investigated how the aeration efficiency (gas holdup per unit power) can be enhanced by operating the downward-pumping high-solidity ratio central impeller at a low critical speed (290 rpm) with a slowly-rotating anchor impeller (10–30 rpm). The non-Newtonian fluid used in this study was carboxymethyl cellulose (1 wt% CMC) which obeys the power law model. Electrical resistance tomography (ERT) and computational fluid dynamics (CFD) were employed to characterize the gas dispersion in non-Newtonian fluids. For the first time, we also explored the effect of anchor impeller hydrodynamics on gas dispersion in non-Newtonian fluids both numerically and experimentally in a co-rotating mode. At a fixed central impeller speed of 290 rpm, a significant drop in the local gas holdup was observed when the anchor impeller speed increased from 30 to 50 rpm. Through air velocity vector obtained via CFD, this study shed light on how the anchor impeller hydrodynamics affect gas dispersion. With an increasing solidity ratio (SR) of the central impeller, a superior aeration efficiency was observed at the speed ratio of 29 compared to that at 43. The research findings hold promise for enhancing the aeration efficiency in non-Newtonian fluids under laminar flow regime.

Developing O/O (oil-in-oil) droplet generators on a chip by using ECF (electro-conjugate fluid) micropumps

The previously reported O/O (Oil-in-oil) droplet generators are based on the usage of microfluidics and membrane emulsification techniques powered by external bulky syringe pumps. This configuration brings problems such as dead volume, air contamination, and difficult integration with other microfluidic components. The paper proposes an O/O droplet generator on a chip based on the integration of the MEMS (Micro Electro Mechanical Systems) technique and an electro-hydrodynamic method. In the generator, two ECF micropumps are used to pump the continuous and dispersed phase liquids, separately. By integrating our ECF micropumps, the droplet-generator-on-a-chip is easily fabricated, mainly using photolithography-based molding and electroforming. Among over 50 types of ECFs, we select HFE-7200 as a dispersed fluid and dibutyl decanedioate (DBD) with its surfactant of Span 80 at a concentration of 3% as a continuous fluid. We investigate the independent flow rates of two ECF micropumps, confirm the flow pattern diagrams of two-phase fluids, and examine the generated droplet diameter, the production rate, and the polydispersity index of O/O droplets. The results demonstrate that our device can generate O/O droplets with a low polydispersity index simply by adjusting the voltages applied to the ECF micropumps.

Particle-scale modeling of biomass gasification in the three-dimensional bubbling fluidized bed

Biomass gasification in a three-dimensional bubbling fluidized bed is numerically simulated via the computational fluid dynamics coupled with the discrete element method (CFD-DEM) incorporated with heat and mass transfer, and chemical reactions. The particle-scale investigation of the heterogeneous reaction rate of the biomass particles along with the essential aspects (including fluid force, collision force, dispersion) of the solid phase is presented. The results indicate that biomass particles behave with larger horizontal and vertical distance especially in the freeboard region than the sand particles. Strong backmixing of the solid material exists near the sidewall, and large fluid force on the particles appears in the lower bed. Similar trends of the particle-scale fluid force, velocity and dispersion can be observed for the biomass and sand particles, though the magnitudes are larger for the biomass. Chemical reactions result in the axial segregation of the biomass particles. Among all the heterogeneous chemical reactions, the char oxidation is fastest while the hydrogenating reaction is the slowest. Increasing the temperature, and steam-to-biomass ratio and equivalence ratio values all enhance the particle-scale behaviors of both the biomass and sand particles. Also, the temperature improves nearly all the reaction rates. The results obtained here provide essential understanding regarding the particle-scale behaviors of the biomass gasification process in the fluidized bed.

Microneedle-assisted microfluidic flow focusing for versatile and high throughput water-in-water droplet generation

Microdroplets have been utilized for a wide range of applications in biomedicine and biological studies. Despite the importance of such droplets, their fabrication is associated with difficulties in practice that emerge from the incompatible nature of chemicals, such as surfactants and organic solvents, with biological environments. Therefore, microfluidic methods have recently emerged that create biocompatible water-in-water droplets based on aqueous two-phase systems (ATPS), most commonly composed of water and incompatible polymers, dextran (DEX) and polyethylene glycol (PEG). However, so far, DEX- and PEG-based water-in-water droplet generation schemes have been plagued with low throughput, and most systems can only generate DEX-in-PEG droplets; PEG-in-DEX droplets have been elusive due to chemical interactions between the polymers and channel walls. Here, we describe a simple approach to generate water-in-water microdroplets passively at a high throughput of up to 850 Hz, and obtain both DEX-in-PEG and PEG-in-DEX droplets. Specifically, our method involves a simple modification to the conventional microfluidic flow focusing geometry, by the insertion of a microneedle to the flow focusing junction, which causes three-dimensional (3D) flow focusing of the dispersed phase fluid. We observe that the 3D flow focusing of the dispersed phase enables excellent control of droplet diameters, ranging from 5 to 65 µm, and achieves a high throughput. Moreover, we report the passive microfluidic generation of PEG-in-DEX droplets for the first time, because in our system the 3D flow focusing of the disperse phase separates the disperse PEG phase from the channel walls, negating the commonly observed wall wetting issues of the PEG phase. We expect this microfluidic approach to be useful in increasing the versatility and throughput of water-in-water droplet microfluidics, and help enable future biotechnological applications, such as microparticle-based drug delivery, cell encapsulation for single cell analysis, and immunoisolation for cell transplantation.

Influence of Shear‐Thinning Rheology on the Mixing Dynamics in Taylor‐Couette Flow

AbstractNon‐Newtonian rheology can have a significant effect on mixing efficiency, which remains poorly understood. The effect of shear‐thinning rheology in a Taylor‐Couette reactor is studied using a combination of particle image velocimetry and flow visualization. Shear‐thinning is found to alter the critical Reynolds numbers for the formation of Taylor vortices and the higher‐order wavy instability, and is associated with an increase in the axial wavelength. Strong shear‐thinning and weak viscoelasticity can also lead to sudden transitions in wavelength as the Reynolds number is varied. Finally, it is shown that shear‐thinning causes an increase in the mixing time within vortices, due to a reduction in their circulation, but enhances the axial dispersion of fluid in the reactor.

Squid‐Inspired Smart Window by Movement of Magnetic Nanoparticles in Asymmetric Confinement

AbstractThe camouflage used by cephalopods is an interesting topic in biomimetics. Squid, part of the cephalopod family, have transformable skin that can be made transparent or darkened through control of the light‐absorption area. This is achieved using a muscular structure. A smart‐window scheme inspired by this is developed. Magnetic nanopigments dispersed within an asymmetric pyramidal array are used and the light‐absorption area is manipulated through use of a magnetic field. Refractive‐index‐matched liquids are used as a dispersion media to avoid refraction at the interface. The transparency hysteresis (ON/OFF ratio) performance depends on the ratio of magnetic nanoparticles and the dispersive fluid and a model is derived to explain this. In addition, a 3D‐printed structure is proposed and demonstrated for better performance.

Effective removal of zinc from industrial plating wastewater using hydrolyzed olive cake: Scale-up and preparation of zinc-Based biochar

In this study, the removal of zinc from plating wastewaters using fixed bed columns packed with hydrolyzed olive cake was investigated. This work discusses the significance of the scale-up and provides a brief overview of the potential uses of exhausted biosorbent for the generation of zinc-based materials. First, biosorption studies were performed to analyze biosorption behavior of hydrolyzed olive cake in different sized columns. Influence of important design parameters such as contact time, linear velocity and bed height/diameter ratio on biosorption performance was studied. Results indicated a significant effect of contact time on scale-up. Higher bed depths increased the biosorption capacity due to a larger dose of absorbent in the packed columns. As the flow rate increased, the breakthrough time decreased and the exhaustion of the column was reached faster. In addition, operating at the same contact time and bed height/diameter ratio and at a linear velocity much higher in the pilot column, the zinc uptake capacity was found to be higher. Better biosorption results could be due to velocity influencing the mass transfer and hydraulics improving the fluid dispersion. Finally, exhausted biosorbent was subjected to different thermal treatments. Preliminary tests showed that zinc-loaded solid can be recycled for the generation of zinc-based materials.

Assembly of particle strings via isotropic potentials.

Assembly of spherical colloidal particles into extended structures, including linear strings, in the absence of directional interparticle bonding interactions or external perturbation could facilitate the design of new functional materials. Here, we use methods of inverse design to discover isotropic pair potentials that promote the formation of single-stranded, polydisperse strings of colloids "colloidomers" as well as size-specific, compact colloidal clusters. Based on the designed potentials, a simple model pair interaction with a short-range attraction and a longer-range repulsion is proposed which stabilizes a variety of different particle morphologies including (i) dispersed fluid of monomers, (ii) ergodic short particle chains as well as porous networks of percolated strings, (iii) compact clusters, and (iv) thick cylindrical structures including trihelical Bernal spirals.