Droplet Flow Research Articles

Effect of fluid properties on intensification of heat transfer process in oil-water droplet flow in a T-shaped microchannel

Droplet-based microfluidics refers to the production and utilization of micrometer-sized droplets. This technology has found widespread applications in drug delivery, biotechnology, disease prevention, chemical analysis, cell research, and functional material synthesis. Cooling of high heat flux electronic systems and hot spots is another application of the droplet flow which is focused by the current work. In this paper, formation and hydrothermal performance of oil droplets in water flow in a T-junction microchannel is studied by a 3D numerical simulation. Effects of different parameters such as surface tension, wettability and heat flux on the droplet size, heat transfer rate and pressure drop are investigated. It is observed that the shear between the discrete and continuous phases results in recirculating zones in both phases which contribute to improved mixing and heat transfer rates. Liquid film thickness adjacent to the heated surface and the droplet velocity are two other factors that positively affect the heat transfer. Decreasing the surface tension is shown to enhance the heat transfer rate. Moreover, an optimum value is found for the contact angle as 135° at which a 23 % heat transfer enhancement is achieved. Results also indicate that droplet size decreases with increase of contact angle while it slightly increases with increase of the surface tension and negligibly varies with the applied heat flux.

Effect of flame characteristics on an isolated ethanol droplet evaporating through stagnation methane/air flames: An experimental and numerical study

A single ethanol droplet evaporation through laminar methane/air stagnation flame is investigated at lean, stoichiometric and rich conditions experimentally and numerically. For the droplets having initial diameters of 20-70μm, the particle Reynolds number is measured via PIV/PTV, resulting in the slip velocity between the droplet and gas flow being small and the surrounding gas being able to carry without any significant convective effects. Evaporation rates, computed from Abramzon–Sirignano for a moving droplet towards a stagnation flame field, satisfy the ones computed from empirically determined evaporation rates from ILIDS measurements as a function of flame temperature, flame speed and flame thickness. Nevertheless, Abramzon–Sirignano model slightly overestimates the vaporization constant for lean cases. The distance traveled by the droplet after leaving the flame zone determines the average critical diameter. Accordingly, droplets larger than 18μm can cross the flame, leading to local modifications in the flame region due to heat loss and vapor diffusion from the droplet. Novelty and Significance Energy conversion in combustion should be carried out by optimizing efficiency, minimizing pollution, and preventing further climate change. The multiphase nature of spray combustion particularly adds further complexity due to the strong coupling of atomization, evaporation, mixing, turbulence, and chemical reactions. Although spray combustion involves a cloud of polydispersed droplets, it is of fundamental importance to understand the dynamics of an isolated droplet and its interactions with the flame front, which can provide indispensable information for spray combustion models. In this study, an experimental and computational framework has been developed to investigate droplet-flame interactions. The dynamics and evaporation characteristics of an isolated droplets interacting with stagnation flames have been determined by coupling several laser diagnostics, while further enhanced with Eulerian–Lagrangian simulations under flame conditions. Thereupon, the outcomes help the design of injectors of spray combustion systems and enhancement of evaporation models.

Influence of collision conditions between aerosol flows of liquid droplets and solid particles typical for wet vortex dust collectors

Various industrial devices and systems make use of collisions of droplets with solid particles in aerosol flows. The characteristics of these collisions are usually predicted adopting the same approaches as for binary collisions of a liquid droplet with a solid particle. However, the conditions of such binary collisions are different from those in the spray composition. There are very few experimental findings for sprays. The purpose of this research was to experimentally determine the characteristics of interaction of droplets with particles of slurry fuel components in sprays. High-speed three-dimensional video recording made it possible to identify conditions when liquid droplets break up, and droplets and particles agglomerate to form slurry droplets. Droplet-particle interaction regimes were identified. Collective effects in a cascade of droplet-particle interactions were recognized. The frequency of collision regime occurrence was determined, considering the recorded collective effects in sprays. The characteristics of droplet-particle collisions in sprays were compared with those of binary interactions. Conditions were determined when the findings on binary collisions can be adapted to different systems with multi-phase sprays. The findings obtained promote research and development in the field of secondary atomization of composite fuel droplets.

The role of reactant contact modes in droplet-based microfluidics

Undesired side reactions can significantly impact the efficiency and economic viability of chemical processes. Droplet-based microfluidics(DBM) offers precise control over reaction conditions by isolating reactants in distinct droplets. Each droplet acts as microreactor with high surface-to-volume ratio. This paper comprehensively investigates the role of two reactant contact modes that give rise to droplet flow. The first contact-mode, Conjugate Mass Transfer Mode(CMTM), is characterized by mass transfer of a reactant from Continuous phase(CP) to Dispersed phase(DP). In second mode, Single-Sphere Mode(SSM), both reactants coexist within the dispersed phase, and there is no effect of external mass transfer on performance. The Hadamard-Rybczynski flow field is used to obtain insights into the system behavior. Two reaction networks, Parallel and Series-Parallel reactions, are considered, which arise from side reactions that accompany the primary reaction.This work highlights the importance of choosing an appropriate reactant contact-mode when side reactions occur. The role of two experimentally controllable parameters, diffusivity ratio(Dr) and feed concentration ratio(M), is analyzed to identify the desirable contact mode. Notably, our study showed that, for Parallel reactions, CMTM has a superior performance compared to SSM under some conditions. Whereas for Series-Parallel reactions, SSM exhibits better performance when equal concentrations of reactants are employed.

Flavonoid- and limonoid-rich extracts from lemon pomace by-products: Technological properties for the formulation of o/w emulsions

Lemon pomace, the by-product derived from the industrial processing of citrus fruits for juice production, is a matrix rich in flavonoids and limonoids, bioactive compounds with interesting functional properties. The aim of the present work was to investigate the technological properties of flavonoids and limonoids-rich extracts recovered from lemon pomace by-products in model oil-in-water (o/w) emulsions. Two extracts, obtained with different extraction procedures, were characterized in terms of surface properties, either as pure extract or in combination with pea protein isolate. Then, the most surface-active extract was used to enrich, at increasing concentrations (0.5%, 1% and 2%), o/w emulsions (20% w/w) stabilized by pea proteins. Droplet size distributions, microstructure, flow behavior and oxidative stability were investigated. The extracts were able to lower the surface tension of the protein interfacial layer, with a concentration dependent behavior. In emulsions, depending on the amount added, the extract significantly altered the dispersion degree, by increasing oil droplet size and inducing flocculation at the highest extract to pea protein ratios. The emulsions showed a Newtonian behavior, with the exception of the sample with the highest amount of extract (2%) which also presented a yield stress. In accelerated oxidation tests, the extracts were shown to improve the chemical stability of the emulsions; however, the physical stability was impaired in the systems with the highest extract amounts (1%, 2%). Limonoids- and flavonoids-rich extracts recovered from lemon pomace can be considered promising as multifunctional ingredients in o/w emulsions, provided that their effect on the colloidal properties is properly addressed.

Pore-Scale Modeling of Pressure-Driven Flow and Spontaneous Imbibition in Fracturing-Shut-In-Flowback Process of Tight Oil Reservoirs

Tight oil reservoirs are characterized by multiple pore spaces where nano-micropores and multiscale fractures coexist, and each type of medium varies in scale, implying a tight coupling of multiscale fractures with matrix and giving rise to extremely complicated flow patterns. To further investigate its flow mechanism, we first construct three two-dimensional (2-D) fracture-pore geometry models based on microfocus computed tomography (CT) imaging of a typical tight rock. A pore-scale modeling workflow is thereafter developed using the Shan-Chen lattice Boltzmann model (SC-LBM) to simulate the pressure-driven flow and spontaneous imbibition. The influence of fracture-pore geometry on the pore-scale fluid exchange dynamics in the fracturing-shut-in-flowback process is clearly clarified. Results show that, for the porous medium model without fracture, the fracturing fluid can displace and replace some crude oil by spontaneous imbibition while a large amount of crude oil droplets remains unexploited away from the oil/water contact line, resulting in a low oil imbibition recovery. The injected fracturing fluid migrates into the deep position along the fractures, only a few entering the matrix pore space near the fractures. Small pores are the main channel for fracturing fluid to imbibe into the matrix pores, and the replaced crude oil droplets flow into fractures through large pores as intermittent or continuous pipe flow. The complexity of fracture network typically exerts a significant impact on the fluid exchange dynamics during pressure-driven flow and spontaneous imbibition. The more complex the fracture network, the larger the volume of fracturing fluid injected, the easier for oil droplets replaced from the matrix pores, and the more difficult for the flowback of fracturing fluid in the fracture-pore geometry model. Our understanding will provide a basis for explaining the underlying mechanisms of oil replacement by pressure-driven flow and spontaneous imbibition in the fracturing-shut-in-flowback process.

Study on the influence of collision conditions on the surface morphology of compound droplets

In this study, a three-dimensional compound droplet collision numerical model is established by using volume of fluid. The morphological evolution of compound hollow droplets affected by high-speed solid droplet was studied in detail. Parameterized analysis is conducted on the velocity VS, center distance ϕ, and diameter ζ of high-speed small droplets. Through the analysis of the compound droplets flow field, it is found that the broken mode of compound droplets is caused by the increase in Pn (dimensionless pressure) and θ (velocity angle). The results show that the surface Pn of compound droplets is positively correlated with the velocity VS of high-speed small droplets, while there is a more complex relationship with the dimensionless center distance ϕ and dimensionless diameter ζ. When the values of ϕ and ζ are appropriate, Pn can reach its maximum value. The broken mode of compound droplets can be divided into three categories: shear deformation, shear crushing, and violent crushing.

Experimental study on the effect of process parameters on postcombustion CO2 capture using rotating packed bed technology

AbstractEnvironmental issues are the most discussed in today's industrialized world, and this has always been associated with the emission of greenhouse gases. CO2 emissions, especially from power plants, are considered the major greenhouse gas source. Capturing CO2 from large emission sources is necessary to address the worst impacts of climate change, including more frequent and severe droughts, heat waves, and intermittent rainfall. Conventional post‐combustion CO2 capture has always been questioned because of its high impact on energy costs. Rotating packed bed technology has shown the potential to reduce the cost of CO2 capture. This paper presents an experimental study on an in‐house designed rotating packed bed absorber for the capture of CO2 from a flue gas stream. Effects of process parameters such as rotating speed, liquid‐to‐gas ratio, and monoethanolamine (MEA) concentration on CO2 capture efficiency and overall volumetric mass transfer coefficient were studied. The study showed that as rotor speed increased, the CO2 capture efficiency increased, this can be attributed to higher rotor speed, resulting in more droplet flow, indicating an increase in the area of contact between the flue gas and the MEA solvent. Also, as the concentration of MEA increased CO2 capture efficiency increased, since an increase in concentration results in an increase in the rate of reaction. Finally, the study showed that a rotating packed bed absorber is an efficient unit for CO2 capture since it enhances mass transfer performance.

Pattern identification of kerosene-water two-phase flow in Y-shaped microchannels using terahertz spectroscopy

In this paper, the terahertz time-domain spectroscopy (THz-TDS) is proposed to investigate the behavior underlying kerosene-water two-phase flow in Y-shaped rectangular microchannels. The microchannel is initially saturated with water and water forms an external continuous phase at low kerosene flow velocities (vk), while a dispersed phase is formed at high vk. The flow process of kerosene-water two-phase flow in microchannel was in situ monitored using a THz-TDS peak intensity, and four flow patterns such as droplet flow, plug flow, slug flow and deformed interface flow are identified over a wide range of vk from 2.08 to 43.13 cm/s at selected water flow velocity (vw) of 13.96 cm/s and 27.71 cm/s. Due to the strong dependence of flow patterns on the vw, only droplet, plug and slug flows are observed at high vw. In addition, with the vw increasing from13.96 cm/s to 27.71 cm/s the ranges of vk for droplet flow and plug flow patterns are expended by ∼100%. A parameter β = k/vw, where k is the slop of the peak intensity of THz-TDS against vk, was proposed to characterize the kerosene-water two-phase flow patterns, and each β only corresponded to one phase for various vk at different vw. These results render THz-TDS a feasible technology for investigating the behavior of oil-water two-phase flow inside microfluidic devices and lay the foundation for experimental and numerical investigations on multiphase flow in microchannels for further study.

Effect of Nasal Inhalation on Drug Particle Deposition and Size Distribution in the Upper Airway: With Soft Mist Inhalers.

Delivery of drugs to the lungs is commonly achieved using nasal and/or oral breathing-assisted techniques. The route of inhalation can substantially change the fate of inhaled droplets. The Respimat® Soft Mist™ Inhaler (SMI) is a commercially available efficient inhaler with 40-60% effectiveness. In the present study, we used computational fluid dynamics (CFD) with a custom setup to investigate the effect of a combined oral/nasal inhalation route on the SMI's regional droplet deposition, size distribution, and flow field. Our setup used a modified induction port (MIP) to mimic nasal inhalation inside the human respiratory tract. Six different oral/nasal flow rate ratios inside the MIP were applied (total flow rate of 30l/min). An overall good agreement was achieved between simulation outcomes and in vitro results. Our results confirmed that the combined inhalation route affects the flow field, altering the MIP's droplet deposition and size distribution. The lowest depositional loss, mainly in the mouth area, was observed at oral/nasal flow rate ratios of O/N = 1 and O/N = 2 with 3% and 7.7% values, respectively. Droplets with a 2-5µm diameter range showed the highest droplet mass inside the MIP at all combined flow rates. We observed less intense vortexes followed by a lower level of turbulent kinetic energy at the oral/nasal ratio of 1. Increasing the relative humidity (RH) at oral/nasal flow rate ratios of 0.07, 1, and 14 led to an increase in droplet deposition at the outlet of the MIP.

Fuel effects on the electrostatic control of charged droplet trajectories

The use of electrostatic fields to control the trajectory of charged droplets is investigated as a new technology to enhance mixing in liquid-fuelled combustors. The canonical configuration of a spray in crossflow is numerically studied with a focus on the effects of the fuel type on the onset of charge-induced breakup and droplet trajectories for a range of bulk flow velocities and electrostatic field strengths at conditions relevant to gas turbine applications. The investigated fuels are ethanol, n-heptane, and n-decane. Operational maps for each fuel are provided to assist the selection of the external electrostatic field required to achieve a balance between the drag and electrostatic forces, and enable a system design that considers fuel flexibility. The results demonstrate that the fuel type has an important impact on the diameter at which the charge-induced breakup is achieved, which mainly depends on the droplet equilibrium temperature. It is also shown that, for cases where the droplet net charge is fixed to a given fraction of the maximum possible charge (based on the Rayleigh limit), the temperature of the droplet at injection could be used as a parameter to control the onset of secondary breakup. Analysis of the strength of the electrostatic field necessary to achieve droplet stabilisation in a bulk flow shows that a balance between the electrostatic and drag forces can only be achieved for relatively low values of the bulk flow velocity, if the strength of the electrostatic field is kept below the breakdown limit of the carrier phase. This balance mainly depends on the droplet net charge and flow conditions, whereas the effect of the type of fuel on the drag force is less important. When the charge is imposed as a fraction of the maximum possible value at injection, for low values of the bulk flow velocity, the strength of the electrostatic field that balances the drag tends to become independent of the droplet diameter for a wide range of droplet sizes. Further investigation of the trajectories of evaporating droplets demonstrates that, although affected by the type of fuel through the evaporation rate, with the same settings of the electrostatic field, it is possible to achieve evaporation in a confined region for all the fuels and ambient conditions studied in this work once the initial droplet charge and initial droplet diameter are fixed. The present findings offer new insights for the development of future technologies for fuel preparation with enhanced mixing and fuel flexibility.

Polymeric droplet formation and flow pattern evolution in capillary microchannels: Effect of fluid elasticity

Polymeric droplets are widely employed in fields such as chemical, biomedical, and materials engineering. However, the study of polymeric droplet formation is still insufficient due to the complex elasticity. In this work, the effect of fluid elasticity on the flow patterns for polymeric droplet formation in cross-junction microchannels is investigated by means of finite-volume direct numerical simulations. The volume of fluid method with cell-based adaptive mesh refinement technique is used to capture the interface. Additionally, the rheological behavior of polymeric fluids is described using the exponential Phan-Thien–Tanner constitutive model. The simulated flow behaviors are highly consistent with the experimental observations. The results indicate that three typical flow patterns of dripping, jetting, and threading flows are obtained at different fluid elasticities (denoted by the Weissenberg number Wi) and viscosities (denoted by the Capillary number Ca). Meanwhile, the elastic effect is found to be greater in the dripping flow, significantly reducing the axial tensile stress. It is demonstrated that changes in the stretched state of polymer macromolecules with the same Wi at different Ca lead to variations in the strength of elastic action, which, in turn, affects the extension length and the pinch-off time of droplets. Finally, a relationship equation between the extension length and time of the polymer fluid is established. This present study aims to provide important insight into the preparation of polymeric droplets in microchannels.

Design and investigation of novel gradient flow fields for proton exchange membrane fuel cell

Ensuring sufficient gas supply and high distribution uniformity are crucial issues in the flow field design to improve the performance and durability of proton exchange membrane fuel cell (PEMFC). However, previous studies often ignore the gas-water distribution variations along the flow direction and adopt identical design for the entire flow field, resulting in unsatisfactory performance. To fill in this gap, this paper innovatively proposes two types of gradient flow field (gradient narrowing and gradient droplet) considering the gas-water distribution characteristics. The channel of the gradient narrowing flow field gradually decreases in all directions along the flow direction, and the gradient droplet flow field is composed of staggered droplet blocks, and the distribution density gradually decreases along the flow direction. The Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method is used for the comprehensive evaluation of flow fields. The results demonstrate that the gradient flow field can significantly improve the performance and physical field distribution. The gradient narrowing flow field has the best performance, with 11.39 % improvement in peak power density, 77.96 % increment in average oxygen mass fraction, and 43.11 % improvement in oxygen uniformity compared to parallel flow field. Further, the key geometric parameters sensitivity analysis of gradient narrowing flow field is carried out. This study contributes to understanding the underlying mechanism of mass transfer properties in the novel gradient flow field, and exploring its potential for high-performance flow fields design.

Research on the Methods for Obtaining Droplet Impingement Characteristics in the Lagrangian Framework

The impact of supercooled water droplets is the cause of aircraft icing, and the acquisition of impingement characteristics is the key to icing prediction and the design of ice protection systems. The introduction of water droplet collection efficiency is required to obtain the characteristics for the Lagrangian method. In this work, a traditional flow tube method, a local flow tube method, and a statistical method are established to calculate the local collection efficiency, based on Lagrangian droplet trajectories. Through the numerical simulations of the air–droplet flow field around an NACA 0012 airfoil, the accuracies of the three methods in regard to collection efficiency are verified. Then, these three methods are applied to obtain the results for water droplet trajectories and the collection efficiency of an S-shaped duct, a 2D engine cone section and an icing wind tunnel. The results show that the distributions of water droplet collection efficiency obtained by the three methods are consistent and the three methods are all feasible when the water droplets do not overlap or cross before hitting the aircraft surfaces. When the water droplets are shadowed by upstream surfaces or blown by air injection, the droplet trajectories might overlap or even cross, and the local collection efficiencies obtained by the traditional flow tube method, local flow tube method, and statistical method might differ. The statistical method is relatively accurate. However, not all the droplet impingement characteristics obtained by the three methods are different due to these effects, and the non-crossing of the droplet trajectories is not a necessary condition for the use of the flow tube method. The effects of trajectory crossings are analyzed and discussed in detail in different situations for the three methods. This work is helpful for understanding and accurately calculating the droplet impingement characteristics and is of great significance for simulations of the aircraft icing process and anti/de-icing range.

Miniaturized droplets flow reactor for one-step highly controlled synthesis of SnO2 quantum dots at room temperature

In recent years, the conventional methods of synthesizing nanomaterials have been surpassed by the emergence of the microfluidics route, which has brought forth numerous advantages and transformed the domain of nanomaterial synthesis. However, the synthesis of semiconducting oxide nanomaterials, specifically Tin oxide (SnO2), remains a crucial area of research due to its remarkable advantages as a viable alternative to toxic and costly materials. Additionally, SnO2 quantum dots (QDs) exhibit immense potential across a diverse range of applications due to their exceptional optical and electrical properties. The existing synthesis methods for SnO2 QDs are either time-consuming or involve high-temperature conditions. To address these challenges, the droplet-based microfluidic technique has emerged as a promising approach for the controlled synthesis of various semiconducting nanomaterials. This article highlights the synthesis of SnO2 QDs with tunable size through the utilization of a droplet-based microfluidic technique, providing precise control over droplet volumes. X-ray diffraction analysis verified the presence of rutile-type tetragonal structure in SnO2 QDs. From the transmission electron microscopy analysis, the average particle size was calculated to be 1.90 nm, 2.09 nm and 2.63 nm for the volume of droplet corresponding to 33.25 μl, 27.84 μl and 18.25 μl respectively. Furthermore, with a decrease in particle size a tunabilty in optical bandgap, from 4.60 eV to 4.00 eV was observed. This work provides insights into the influence of droplet volume on the particle size which in turn affects the associated properties of SnO2 QDs in a droplet-based microfluidic synthesis system.

Numerical simulation of droplet dispersion within meso-porous membranes

Analysis of membrane processes in fluid processing, and their main influencing operating conditions are relevant in a variety of industrial applications. Increasing regulatory scrutiny and environmental considerations are forcing industries across all sectors, from food and pharma to oil and gas, to further understand and optimise the handling and formulation of liquid systems for efficient process design. In a generic setup for emulsification and liquid formulation the flow and dispersion behaviour of a liquid oil droplet on its way through a porous water filled membrane is analysed. A set of high-resolution numerical simulations of a single oil droplet dispersed in water through a porous membrane structure with varying contact angles is performed. In this work cluster analysis of volume-of-fluid simulation results to obtain statistical droplet size distributions is conducted and further analysed to highlight the effect of the contact angle as well as pressure drop on the dynamics of the system. It is observed that based on the membrane surface activity the droplet behaviour changes from filtration with coalescence when the membrane is lipophilic to emulsification with droplet break-up when the membrane is lipophobic. Furthermore, the pressure drop is identified as a key factor for the dynamics of the droplet process and the frame in which it occurs. These results highlight that the membrane wettability is a determining factor for the emulsification or filtration effectiveness of a membrane for various applications.

Electric manipulation on deformation of ionic ferrofluid sessile droplets.

Active electric-driven droplet manipulation in digital microfluidics constitutes a promising domain owing to the unique and programmable wettability inherent in sessile ionic droplets. The coupling between the electric field and flow field enables precise control over wetting characteristics and droplet morphology. This study delves into the deformation phenomena of ionic sessile ferrofluid droplets in ambient air induced by uniform electric fields. Under the assumption of a pinned mode throughout the process, the deformation is characterized by variations in droplet height and contact angle in response to the applied electric field intensity. A numerical model is formulated to simulate the deformation dynamics of ferrofluid droplets, employing the phase field method for tracking droplet deformation. The fidelity of the numerical outcomes is assessed through the validation process, involving a comparison of droplet geometric deformations with corresponding experimental results. The impact of the electric field on the deformation of dielectric droplets is modulated by parameters such as electric field strength and droplet size. Through meticulously designed experiments, the substantial influence of both field strength and droplet size is empirically verified, elucidating the behavior of ionic sessile droplets. Considering the interplay of electric force, viscous force, and interfacial tension, the heightened field intensity is observed to effectively reduce the contact angle, augment droplet height, and intensify internal droplet flow. Under varying electric field conditions, droplets assume diverse shapes, presenting a versatile approach for microfluidic operations. The outcomes of this research hold significant guiding implications for microfluidic manipulation, droplet handling, and sensing applications.

Study on the self-adaptive process of droplet traffic to a perturbation of flow resistance at the arms of a micro-T-junction

The response of the water–oil droplet flow to a perturbation of flow resistance added at the arms of a bypassed micro-T-junction is studied through a numerical method. The capillary number of the continuous flow varies between 0.007 and 0.034. Once an additional flow resistance is introduced at the arms of the T-junction, the flow self-adapts to the perturbation and droplets show complex dynamics at the junction, including splitting, merging, and reshaping. During the self-adaptive process, the instantaneous splitting ratios of the dispersed flow show positive correlations with that of the continuous flow. The correlation coefficient reduces as the increase in the amplitude of the perturbation. The ensemble splitting ratios of them, however, are independent. It shows a rather simple law that the ensemble splitting ratios of oil anchor at around a constant value while the corresponding parameter of water varies from 0 to 1. It illustrates that the droplet flow splitting at a T-junction resembles a resilient system, the stiffness of which depends on the capillary number of the flow. The energy loss of the system induced by a perturbation of the flow resistance is self-reduced by just redistributing of droplets into the two arms.

Photothermal-driven flow with water droplets for effective removal of indoor fine particulate matters

Airborne particulate matter (PM) causes hazardous problems in human health, and water droplets have been utilized for PM removal in the air. However, since conventional wet scrubbing techniques mostly rely on mechanically generated water droplets, they have intrinsically limited in collection efficiencies of a single droplet. In addition, PM capturing strategies for removal of fine PM need to be established to enhance PM removal performance based on flow dynamics. Here, we propose a steam flow of water droplets generated by a photothermal membrane for air purification. The generated flow captures PM efficiently by the aid of the coupled effects of water droplets and hydrodynamic flow. The flow achieves high and sustainable deposition constants of 0.458 and 0.472 h−1 for PM1.0 and PM2.5 within 1 h at 4 solar intensity, which is 2.20 and 2.24 times higher than those of without solar irradiation, respectively. The deposition constants of PM using steam flow were higher than that of previous wet scrubbing methods despite a low volume of water usage. Our photothermal-driven flow with water droplets could be utilized as effective indoor air purification system that is low cost, energy-efficient, and environmentally-friendly.

In-flight imaging of aerosol jet printer droplets to enable spatially resolved flow rate measurements

The effect of process drift in aerosol jet (AJ) printing has been a hindering factor in its large-scale adoption in printed electronics. In this study, we developed an in situ droplet imaging system using a double-pulse laser to quantify the distribution and velocity of sparse droplets between the AJ printer nozzle and the substrate. The droplet velocity and distribution were then used to evaluate a spatially resolved droplet flow rate that was capable of detecting and quantifying process drift over time. Using the droplet imaging system, the effect of different focus ratios was analyzed. The droplet flow rate was shown to be indicative of the deposited line morphology for low to moderate focus ratios. At a high focus ratio, the tool was able to identify non-idealities in the fluid properties which are explained in a detailed analysis. AJ printing demonstrates significant promise for the additive manufacturing of electronics, and this work represents a step toward non-invasive, in-flight monitoring in real time.