Separation Of Droplets Research Articles

Magnetically Actuated Surface Microstructures for Efficient Transport and Tunable Separation of Droplets and Solids

Efficient transportation of droplets (≈101–102 μL) and small solid objects (≈101–102 mm3) have important applications in many fields, such as microfluidics, lab‐on‐a‐chip devices, drug delivery, etc. A novel multifunctional surface consisting of a periodic array of micro‐lamellae from a soft magnetoactive elastomer on a plastic substrate is reported for these purposes. The physical origin of the propulsion is the bending of soft magnetic lamellae in nonuniform magnetic fields, which is also observed in uniform magnetic fields. The magnetoactive surface is fabricated using a facile and rapid method of laser ablation. The propulsion of items is realized using a four‐pole rotating magnet. This results in a cyclic lamellar fringe motion over the microstructured surface and brings an advantage of easy reciprocation of transport by rotation reversal. Two modes of object transportation are identified: “pushing” mode for precise control of droplet and solid positioning and “bouncing” mode for heavier solid objects transportation. A water droplet of 5 μL or a glass sphere with a 2.1 mm diameter can be moved at a maximum speed of 60 mm s−1. The multifunctionality of the proposed mechatronic platform is demonstrated on the examples of selective solid–liquid separation and droplet merging.

Discrete quantum droplets in one-dimensional binary Bose–Einstein condensates

We discuss the propagation and interaction of discrete quantum droplets in one-dimensional binary Bose mixtures by variationally and numerically solving the underlying generalized discrete Gross–Pitaevskii equation. Useful analytical expressions are obtained for the energy, the chemical potential, the density profile, and the droplet separation in the regime of a small droplet. We show that for weak lattice coupling, the stationary state of small droplets remains intact and does not display any special structures. An irregular dynamics is observed in both single and dual droplets with flat-top plateau in the limit of relatively strong coupling. Our results reveal also that two droplets interact weakly and cannot form a bound-state due to the intriguing interplay of the discreteness and quantum fluctuations.

Numerical investigation of highly viscous droplet generation based on level set method

Piezo-driven needle valves are widely used in electronic packaging due to their fast response, high resolution and good dispensing consistency. However, the stable generation of high-viscosity droplets is one of the key issues to its packaging quality. To investigate the formation mechanism of the high-viscosity droplet. In this paper, a 2D finite element model of the drop-on-demand injection process of the high-viscosity droplet is established based on the level set method, the droplet formation and separation processes are numerically simulated, and the reliability of the simulation results is checked by comparing the outcomes with published data. Specifically, the detailed evolution of the high-viscosity droplet formation and separation process is gained by coupling the two-phase flow-level set method and the dynamic grid technique, and the pressure distribution in the injection chamber is further discussed and the effects of operating parameters on the droplet formation volume are examined. The results of the study show that the needle motion is the main factor of pressure fluctuations in the injection chamber. Moreover, we also found that among the parameters of needle stroke, nozzle diameter, supply pressure, fluid viscosity, and surface tension, the nozzle diameter has the most significant effect on droplet volume, while surface tension has the least effect on droplet formation.

Oblique bouncing of a droplet from a non-slip boundary: computational realization and application of self-spin droplets

Recent studies have demonstrated the significance of droplet spinning motion on droplet collision outcomes and raised the question of generating spinning droplets in an experiment. The present work proposed the idea of making an initially non-spinning droplet self-spin by its oblique impact on a non-slip boundary. We computationally demonstrated the feasibility of the idea and exploited its application in the binary collision of spinning droplets. Specifically, a parametric study was conducted to investigate the effects of impact velocity, impact angle, and liquid viscosity on the spinning droplet. The results showed that a larger impact velocity or a larger liquid viscosity causes an increased angular speed of the spinning droplet, however increasing impact angle leads to a nonmonotonic variation of the angular speed. The oblique-impact-induced droplet spin is attributed to the asymmetric gas film flow, asymmetric lubrication pressure, and shear stress within the gas film region, leading to the earlier bouncing motion on one side to rotate the droplet. In addition, the synergetic effects of the droplet stretching length and the intensity of asymmetric lubrication pressure account for the variation of droplet spin angular speed and droplet separation. As a preliminary application of the proposed idea, the feasible technical approach of realizing the binary collision of spinning droplets was computationally realized.

Efficient agricultural drip irrigation inspired by fig leaf morphology

Irrigation is limited by water scarcity. Here, we show how a drip irrigation system inspired by the leaf of the fig tree Ficus religiosa (also known as the bodhi tree) can improve irrigation efficiency. The reverse curvature of the leaf regulates the convergence process of multiple water streams, while its long-tail apex allows for fast water drainage with the droplet separation centroid beyond the leaf apex. We explain why drip frequency increases after the break-up of contact line pinning at the apex tip by using scaling laws for drip volume and analyzing drainage dynamics. We build a drip irrigation emitter inspired by the bodhi leaf apex and compare the germination efficiency of wheat, cotton, and maize under different irrigation modes. These results show that the proposed bodhi-leaf-apex-mimetic (BLAM) drip irrigation can improve water saving while ensuring germination and seedling growth.

Performance evaluation of a novel electrostatic-vane compound demister using numerical simulation

A novel compound demister has been developed, incorporating an electrostatic field into the vane demister. This study aims to investigate the influence of various structural and operational parameters of the compound demister on droplet separation efficiency and pressure drop through numerical simulations. The results indicate that the compound demister, in comparison to the vane demister, significantly enhances the separation efficiency of fine droplets. At gas velocity of 6 m/s, the separation efficiency for 10 μm droplets reaches up to 90%, accompanied by a mere 46.8 Pa pressure drop. The separation efficiency demonstrates a negative correlation with vane spacing and electrode wire radius, while exhibiting a positive correlation with voltage and the number of electrode wires. However, the vane turning angle and the geometry of turning corner have a negligible effect on the separation efficiency. On the other hand, pressure drop increases with both increasing vane spacing and turning angle.

Bionic collection system for fog-dew harvesting inspired from desert beetle

Obtaining clean water resources from the air is an important method to solve the water shortage problem in arid and semi-arid areas. Improving the efficiency of material water collection from the air remains a huge challenge that needs to be overcome. Inspired by natural creatures, a new combined mist dew collector is proposed. The collecting part of the system has a gradient structure with alternating convexity and endows it with different chemical wettability. Janus sponge will be used as the storage part and mechanically assembled together to form a fog/dew collection system. This combination approach can improve collection efficiency by adjusting the liquid behavior throughout the entire fog collection process, including nucleation of droplets during the capture process, high-speed transmission on conical channels, and separation and storage of droplets from Janus sponge. The synergistic effect of conical capture and sponge separation collection improves the efficiency of fog collection. Compared with traditional fog collection structures, our proposed collection system significantly improves the efficiency of fog collection. Meanwhile, our collection system can effectively collect dew under conventional subcooling conditions. The readily available low-cost raw materials, easy to assemble and disassemble, and easy to carry, our proposed fog/dew collection system combines fog and dew collection as a promising solution to alleviate freshwater shortages in many underdeveloped, arid, and island areas.

Experimental Characterization and Evaluation of Crude Spiking Influence on Oil/Water Dispersed Flow in Pipe.

This study centers around examining the impact of introducing varying (small) quantities of crude oil into mineral oil (Exxsol D60) on the resultant properties of dispersions and emulsions in oil-salty-water mixture properties such as rheology, droplet size distribution, separation duration, and interfacial tension. The experimentation encompassed bottle tests and a compact flow loop configuration featuring a 2 m horizontal pipe segment. The findings indicate that blends of oil infused with crude oil, combined with salty water at water ratios of 25% and 50%, necessitate an extended duration for separation and for the establishment and stabilization of interfaces, in contrast to mixtures of unaltered oil and saline water. To illustrate, in samples with spiking concentrations ranging from 200 to 800 ppm within a 25% water fraction, the separation period escalates from 51 s to 2 min and 21 s. Interestingly, when the water fraction increased to 75 percent, the impact of crude oil spiking on separation time was minimal. The analysis revealed that the Pal and Rhodes emulsion viscosity model yielded the most accurate predictions for the viscosity of resulting emulsions. The introduction of crude oil spiking elevated emulsion viscosity while diminishing interfacial tension from 30.8 to 27.6 mN/m (800 ppm spiking). Lastly, a comparative assessment was performed between droplet size distributions in the devised dispersed pipe flow and observed in an actual emulsion system comprising crude and salty water.

Effect of transverse vortex on performance of wave plate demister

AbstractCorrugated plate mist eliminators are commonly employed in industrial processes to eliminate liquid droplets from gas flows. Engineering a mist eliminator with excellent separation performance including high removal efficiency and low pressure drop and is a primary consideration in industrial processes. This study focuses on the separation of tiny droplets reinforced by the generation of transverse vortices with the help of curved wings. The Euler–Lagrange model is developed to predict the behavior of droplets in gas streams, and velocity distributions and particle trajectories on the demisters were derived. And the prediction equations of the curved wing structure sizes on the efficiency and pressure drop of the mist eliminator are established. The results demonstrated that the grade separation efficiency of 10~20 μm droplet was significantly improved by 25.7%~66.51%. Moreover, for the total removal efficiency, it could be improved from .8 to .92 at 5.5 m s−1, and the accompanying resistance factor was as low as 2.7. Among the proposed arrangements, the double‐row single‐column array arrangement was characterized by higher demisting efficiency and lower pressure drop. Overall, compared to other optimization methods, this proposed optimization approach by means of transverse vortices yielded a higher demisting efficiency and less pressure drop.

The Influence of Physical Properties on the Membrane Morphology Formation during the Nonisothermal Thermally Induced Phase Separation Process.

The physical properties of a polymer solution that are composition- and/or temperature-dependent are among the most influential parameters to impact the dynamics and thermodynamics of the phase separation process and, as a result, the morphology formation. In this study, the impact of composition- and temperature-dependent density, heat capacity, and heat conductivity on the membrane structure formation during the thermally induced phase separation process of a high-viscosity polymer solution was investigated via coupling the Cahn-Hilliard equation for phase separation with the Fourier heat transfer equation. The variations of each physical property were also investigated in terms of different boundary conditions and initial solvent volume fractions. It was determined that the physical properties of the polymer solution have a noteworthy impact on the membrane morphology in terms of shorter phase separation time and droplet size. In addition, the influence of enthalpy of demixing in this case is critical because each physical property showed a nonhomogeneous pattern owing to the heat generation during phase separation, which in turn influenced the membrane morphology. Accordingly, it was determined that investigating spinodal decomposition without including heat transfer and the impact of physical properties on the morphology formation would lead to an inadequate understanding of the process, specifically in high-viscosity polymer solutions.

Research on water saving performance of a new type of demisting cooler for cooling towers

During the operation of the cooling tower, a large amount of white mist will be generated due to drift, evaporation, and other reasons. The droplets condensed from white mist in winter are easy to freeze and adsorb at the outlet of the cooling tower, affecting the service life of the equipment and increasing operating costs. In order to reduce the mist, a new type of mist removal cooler was developed. The influence of the structure of the mist removal cooler (baffle spacing, cooling pipe diameter, and baffle vertical height) on resistance characteristics, droplet separation characteristics, and heat transfer characteristics was studied through numerical simulation and obtain surface regression equations related to all factors. The results showed that the mist removal cooler can effectively recover water vapor from wet air. The optimal structure of the demister cooler was determined to be a combination structure with a baffle plate placed on one side of the cooling tube, a plate spacing of 0.1 m, a pipe diameter of 0.03 m, and a vertical height of 0.1 m. This structure can separate 48.29% of droplets while recovering 18.62% of water vapor, with significant water-saving benefits.

Fabrication of carboxymethyl starch/xanthan gum combinations Pickering emulsion for protection and sustained release of pterostilbene

Carboxymethyl starch (CMS)/xanthan gum (XG) combinations with different ratios (CMS/XG: 1/1, 3/1, 5/1, 7/1, 9/1, w/w) were used as Pickering emulsion delivery systems to encapsulate pterostilbene (PTS) to improve its stability. The results showed that the Pickering emulsion prepared using CMS/XG combinations could effectively encapsulate PTS. When the mass ratio of CMS to XG was 1:1, the encapsulation efficiency reached 91.20 %. The spherical particles in the PTS emulsion were dissociated and homogenous. The results of backscattered light experiments and storage stability studies showed that the PTS emulsion system prepared using CMS/XG was uniform and stable, with no obvious phase separation or emulsion droplet coalescence. With an increase in the mass ratio of XG, the water distribution in the emulsion became more evenly distributed, and the aggregation of droplets was reduced. The PTS emulsion prepared using CMS/XG improved the storage retention percentage of PTS. The cumulative release of PTS in the simulated gastric fluid was significantly lower than that in simulated intestinal fluid. The Pickering emulsion prepared using CMS/XG combinations can be used as a delivery system for functional foods and help to develop an efficient and reliable release system for hydrophobic bioactive substances.

Droplet microfluidic technologies for next-generation high-throughput screening

Droplet microfluidics has evolved into a promising platform for high-throughput screening (HTS), allowing for rapid and precise analysis of thousands of samples encapsulated within droplets. Droplet microfluidic platform offers versatility, high-throughput, and the ability to compartmentalize reactions for a wide range of applications including pharmaceutics, cell analysis, and combinatorial chemical analysis. While droplet-based microfluidics has made considerable advances in automating basic laboratory tasks, for instance, manipulation, storage, and analysis, there has been comparatively little advancement toward HTS applications. The complexity of the technology, the lack of standardization, and the challenges associated with screening large numbers of samples are all factors that have contributed to the limited adoption of droplet-based microfluidics in HTS. In this perspective, we provide a comprehensive overview of the progress of droplet microfluidics as a potential platform for next-generation HTS, specifically in the domain of droplet separation and library generation. We hope that this perspective will inspire further research in relevant academic fields and contribute to the development of innovative HTS strategies based on droplet microfluidic technologies.

Shear flow-driven droplet motion with smoothed dissipative particle dynamics

Droplet motion in shear flow is a typical phenomenon in nature that also plays an important role in the spread of COVID-19 when viruses like SARS-CoV-2 exist in exhalation droplets. In this paper, we adopted smoothed dissipative particle dynamics (SDPD) to simulate the Couette flow and Poiseuille flow. Then, the self-diffusion coefficient and the Schmidt number of the quiescent flow in SDPD are confirmed by comparison with those in DPD. In addition, the SDPD method with pacific coefficients is used to simulate the disintegration of droplets and the collisional aggregation and separation of droplets in shear flow. The applicability of the SDPD method is demonstrated through these simulations, which can be useful in the investigation of many complex fluid flows during the expansion of COVID-19.

Two-dimensional Euler grid approximation method for multi-droplet motions

Multi-droplet motion in gas flow is a significant two-phase flow in nuclear power plant, chemical industry and environmental engineering. In contrast to droplet generation and collision in microscopic view, multi-droplet motion presents both macroscopic interaction with gas flow and microscopic single droplet motion characteristic. To simulate the multi-droplet motion, we presented the Euler grid approximation method to update the multi-droplet imformation in all nodes under each calculation step, and constructed a criterion about droplet velocity and direction to ensure droplet motion limited on grid nodes without loss of accuracy. The novelty of this method lies in its description of the virtual streamlines of multiple particles in the same state, as well as the discontinuous flow field composed of these virtual streamlines, furthermore, this method is independent of the time step and can simultaneously calculate droplet information of the entire field. Meanwhile, this paper focuses on the two-phase gas-liquid flow in the gravity separation space of the rotary vane separator, and the motion behavior of multiple droplets in the flow field is simulated based on the parameters of the actual steam-water separator.The result shows that the droplets tend to gather towards the center of the main stream, which is the focus area for droplet separation, and the position with higher density may be the point where the droplets collide. The model can be adopted to calculate the separation efficiency in the process of multi-droplet motion in the gravity separation space, rotary vane separator and other steam separation devices, and the results will help guide the optimal design of the separation device structure.

Electrospun polymethyl methacrylate fibers-based membrane with heterogeneous structure achieving a full-particle size separation of oil-water emulsion

There is an urgent requirement of developing new technology with low energy consumption to separate oil-water emulsion, especially when the emulsion is stabilized using surfactants. Electrospun fiber membranes (EFM) may be favorable for oil/water emulsion separation due to its high permeability resulting from its high porosity. However, high porosity in EFM restricts the separation of water droplets under 200 nm from emulsion. Here, we prepared a heterogeneous EFM through self-assembly of Ethyl cellulose (EC) colloid and Polyfluortetraethylene (PTFE) nanoparticles and subsequent self-organization on polymethyl methacrylate (PMMA) EFM to attain the separation of small particle size water droplets from oil/water emulsion. The preparation mechanism, oil/water emulsion separation effect and mechanism were disclosed by the morphology, water absorption, particle size distribution, TG, BET, Karl Fischer testing and surface tension characterization. The results showed that the prepared heterogeneous structure EFM could remove full particle size water droplets especially the water droplets below 200 nm from oil/water emulsion during membrane separation process. Hydroxyl groups, hydrophobic PTFE nanoparticles and hydrophobic PMMA formed the heterogeneous structure which exerted different directions force on water droplets. The size separation and water absorption were the main action in oil/water emulsion separation.

Simulation and optimization of a conical type swirl-vane separator in nuclear SG

Swirl-vane separator is used as primary separator in nuclear SG for liquid droplet separation from steam flow. In order to improve the separator performance, in present paper, based on the commercial CFD software STARCCM+ and Euler-Euler two-fluid model, three-dimensional simulation and optimization of a conical swirl-vane separator are performed to study the flow filed and separation performance of this new designed separator. According to the results, a low-pressure and low-velocity wake region is formed at the swirler outlet and its shape narrows first and then enlarges along the flow direction due to the reduced flow channel of the conical barrel. The steam axial velocity exhibits a “M” type distribution while the tangential velocity displays a “V” type distribution in the radial direction. The steam radial velocity is about an order lower than the other two velocity components. The static pressure decreases sharply in the swirler, demiser and diffuser part, which is the main energy loss in the separator. On the whole, the structure of conical barrel and drain holes are very important for liquid separation. Within a certain range, a smaller hole diameter, a larger axial location of drain holes and a smaller conical angle can improve the separation efficiency of the swirl vanes, but also increases the pressure loss of the conical barrel section.

Noncontact liquid-solid nanogenerators as self-powered droplet sensors.

Liquid-solid triboelectric nanogenerators (L-S TENGs) can generate corresponding electrical signal responses through the contact separation of droplets and dielectrics and have a wide range of applications in energy harvesting and self-powered sensing. However, the contact between the droplet and the electret will cause the contact L-S TENG's performance degradation or even failure. Here we report a noncontact triboelectric nanogenerator (NCLS-TENG) that can effectively sense droplet stimuli without contact with droplets and convert them into electrical energy or corresponding electrical signals. Since there is no contact between the droplet and the dielectric, it can continuously and stably generate a signal output. To verify the feasibility of NCLS-TENG, we demonstrate the modified murphy's dropper as a smart infusion monitoring system. The smart infusion monitoring system can effectively identify information such as the type, concentration, and frequency of droplets. NCLS-TENG show great potential in smart medical, smart wearable and other fields.

Simple Method to Generate Droplets Spontaneously by a Superhydrophobic Double-Layer Split Nozzle.

Given the problems of traditional droplet generation devices, such as the complex structure and processing technology, difficulty in droplet separation, and low transfer accuracy, we propose a low-adhesion superhydrophobic double-layer split nozzle (SDSN). It realizes spontaneous droplet generation by using an interfacial tension force inside the micro-hole to drive the droplet snap-off. It successfully achieves stable and highly consistent droplets on the micrometer-scale circular micro-hole. Droplets with a volume in the range of 0.65-1.75 ± 0.007 μL can be precisely achieved by adjusting the hole size of the SDSN from 100 to 500 μm. The SDSN is prepared by conventional mechanical drilling, chemical etching, and low surface energy modification. Compared with traditional droplet generation devices, no photolithography process is required, and the cost is lower. Moreover, the droplets can be obtained directly without any post-processing, avoiding the problem of separating droplets from another solution. The stability of SDSN is good, and the droplet volume is not affected by the fluctuation of external conditions. The rate of droplet generation can be freely adjusted by adjusting the speed of the electronic microinjection pump without affecting the droplet volume. It enables efficient droplet transfer without liquid residue, which improves the transfer accuracy and helps to save the use of expensive reagents. This simple but effective structure will be of great help to make breakthroughs in next-generation spontaneous droplet generation, liquid transport, and digital microfluidic devices.

Observations and simulations for phase separation process of immiscible Fe50Sn50 alloy droplets placed on a chilling surface

Liquid phase separation and microstructure evolution mechanisms of immiscible Fe50Sn50 alloy droplets placed on a chilling surface have been investigated by both arc melting experiments and lattice-Boltzmann simulations. The pattern selection of phase-separated morphologies strongly depended on the sample diameter. With the reduction in sample size, phase separation time was shortened, two-layer macrosegregation morphologies first transformed into intermediate phase-separated morphologies and finally changed into a homogeneously dispersed microstructure. The farther away from the sample bottom the longer the phase separation time and the larger the Sn-rich phase. A heat transfer equation was proposed to analyze the temperature field characteristics of Fe50Sn50 alloy droplets placed on the surface of a water-cooled copper mold. Theoretical analyses revealed that the sample gradually cooled from the bottom to the top of the sample. There existed a relatively high temperature gradient of several hundred Kelvins per millimeter between the top and the bottom of the arc melted sample, which induced the occurrence of an extremely intense Marangoni convection. The smaller the sample size the larger the temperature gradient and the greater deviation-degree of Fe-rich zone in two-layer macrosegregation structure from the bottom of the sample. The simulated two-layer phase-separated morphology agreed well with the experimental observations. Numerical simulations demonstrated that the effect of Marangoni convection on the phase separation was significantly stronger than the Stokes motion, and Fe-rich globules with a larger density tended to move upward to form a Fe-rich zone deviating from the bottom of the sample.