Micro-sized Droplets Research Articles

Transmission interference fringe (TIF) technique for the dynamic visualization of evaporating droplet

The transmission interference fringe (TIF) technique was developed to visualize the dynamics of evaporating droplets based on the Reflection Interference Fringe (RIF) technique for micro-sized droplets. The geometric formulation was conducted to determine the contact angle (CA) and height of macro-sized droplets without the need for the prism used in RIF. The TIF characteristics were analyzed through experiments and simulations to demonstrate a wider range of contact angles from 0 to 90°, in contrast to RIF's limited range of 0–30°. TIF was utilized to visualize the dynamic evaporation of droplets in the constant contact radius (CCR) mode, observing the droplet profile change from convex-only to convex-concave at the end of dry-out from the interference fringe formation. The TIF also observed the contact angle increase from the fringe radius increase. This observation is uniquely reported as the interference fringe (IF) technique can detect the formation of interference fringe between the reflection from the center convex profile and the reflection from the edge concave profile on the far-field screen. Unlike general microscopy techniques, TIF can detect far-field interference fringes as it focuses beyond the droplet-substrate interface. The formation of the convex-concave profile during CCR evaporation is believed to be influenced by the non-uniform evaporative flux along the droplet surface.

Morphology and heat transfer of a train of microdroplets impinging on the heated surface for spray cooling

Owing to the superior cooling performance of the droplet impingement process, the spray cooling is widely used in various industries including aerospace, e-motors, data centres and so on. In the present study, the morphological evolution and heat transfer characteristics of vertical and oblique impacts on the heated surface were investigated experimentally and numerically. A droplet generation system and a substrate heating system were designed to observe continuous vertical and oblique droplets impinging on the hot surface by a high-speed camera. The coupled Level Set and VOF (CLS-VOF) model and adaptive mesh refinement technique were used to establish a mathematical model for a train of micro-sized droplets impinging on the heated surface, which was validated by experimental data. The film that spreads on the surface exhibits asymmetry due to the horizontal velocity of the droplets. When continuous droplets impinge vertically on the heated wall, the evolution of the crown and the wall temperature appear periodic. The effects of impingement angles on morphological patterns and heat transfer of a train of micro-sized monodispersed droplets with the same Weber numbers were analyzed further. The results indicated that with a smaller angle of impingement, the crown is more likely to splash, and the crown asymmetry is less obvious.

Solvent extraction of La, Nd, and Eu using a newly designed solutions spray system

This research covers a new continuous solvent extraction “solutions spray system” technique to extract lanthanum (La), neodymium (Nd), and europium (Eu) from chloride solution using DEHPA. This setup enhanced the extraction kinetics and mass transfer compared to the conventional mixer-settler by generating micro-sized droplets of the feed streams in the mixing chamber, causing speedy and better mass transfer, and the special design of the settling column boosted the separation process with improved quality products. Variables like pH, DEHPA concentration, ionic strength, and aqueous/organic phase ratio were studied to determine the performance of this setup. At the optimum values of these variables (2.0 pH, 0.303 M DEHPA, and 1.54 mol/L ionic strength), the percent extraction of La, Nd, and Eu was 95.03 %, 99.36 %, and 99.98 %, and the distribution ratio was 19.14, 155.25 and 4165 respectively, which showed the extraction in a single stage. Furthermore, the values of separation factors, SFEu/La = 6.69, SFEu/Nd = 5.33, and SFNd/La = 1.26 at 0.50 pH, showed the selectivity of these elements. Later on, McCabe Thiele diagrams were plotted for extraction and stripping sections to see the minimum number of theoretical stages for the complete recovery of REEs and their selective separation from each other.

Microfluidics-based condensation bioaerosol sampler for multipoint airborne virus monitoring

To facilitate rapid monitoring of airborne viruses, they must be collected with high efficiency and concentrated in a small volume of a liquid sample. In addition, the development of low-cost miniaturized samplers is essential for multipoint monitoring. Thus, in an attempt to fulfill these requirements, this study developed a microfluidic condensation bioaerosol sampler (MCBS). The developed sampler comprised two parts: a virus growth section and a virus droplet-to-liquid sample conversion section, each of which was fabricated on a chip using microfluidic technology. The condensation nucleus growth technique used in the virus growth section grew nanometer-sized airborne viruses into micro-sized droplets, making it possible to collection of viruses easier and with high efficiency. In addition, the virus droplet-to-liquid sample conversion section controlled the transport of droplets based on electrowetting technology. This enabled the collected airborne viruses to be concentrated in tens of microliters of the liquid sample. To evaluate the performance of both the sections, the virus dropletization, virus collection efficiency, and virus droplet-to-liquid sample conversion efficiency were evaluated through quantitative experiments. H1N1 and HCOV-229E viruses were used to conduct quantitative experiments on MCBS. We could obtain virus liquid samples with at 72.8- and 89.9-times higher concentration through 1:1 evaluation with a commercial sampler. Thus, the developed sampler facilitated efficient collection and concentration of airborne viruses in a compact, cost-effective manner. This is expected to facilitate rapid and accurate multipoint monitoring of viral aerosols.

Micro-sized droplet formation by interaction between dielectric barrier discharge and liquid

Liquid atomization technology is one of the applications in various fields of modern industry because it improves reactivity, diffusion, and permeability of liquids. However, existing atomization technologies are severely limited by the physical and chemical properties of the solution or the object to be treated, and there is a growing need to develop atomization technologies that solve these problems. We have developed a device that atomizes liquids to the nanoscale based on the interaction with a dielectric barrier discharge, which enables the atomization of various types of solutions, including water-based and oil-based solutions. Herein, we report the results of visualizing the dynamics of liquid atomization using a high-speed camera. The device atomizes solutions in three modes: instability of the solution jet; physical fragmentation of the solution droplets by the impact of the plasma streamer; and collapse of the droplet surface and generation of a smoke-like mist during the streamer ejection from the solution droplet. The combined and repeated action of these three modes on the produced microdroplets is expected to result in nano-sized mists of the solution.

An improved Coupled Level Set and Volume of Fluid (i-CLSVoF) framework for sessile droplet evaporation

Surface-tension-dominant droplet evaporation is ubiquitous and of importance to many applications. We present an improved Coupled Level Set and Volume of Fluid (i-CLSVoF) framework without explicit interface reconstruction for modelling micro-sized droplets with and without evaporation. In the i-CLSVoF framework, an improved surface tension force model with additional filtering steps to filter un-physical spurious velocities is developed and implemented. A simple, yet efficient, velocity-potential based approach is proposed to reconstruct a divergence-free velocity field for the advection of the free surface during droplet evaporation. This approach fixes the numerical issues resulting from the evaporation-induced velocity jump at the interface. The smeared mass source term approach incorporated in this work guarantees greater numerical stability than the non-smeared approach. Three different evaporation models (constant mass flux, thermally driven evaporation and droplet evaporation at room temperature) are implemented in the i-CLSVoF. Corresponding numerical benchmark cases (dam break, droplet relaxation and droplet evaporation subjected to different evaporation models) are conducted to validate the surface tension and the evaporation models. Good agreement between the numerical and corresponding analytical solutions is found. The model developed in this work shows convincing performance in modelling surface-tension-dominant flow with and without evaporation.

Optical detection of bovine serum albumin using charged cholesteric liquid crystal droplets functionalized with surfactant

Monodispersed micrometer-sized Cholesteric liquid crystal (CLC) droplets were prepared by using the flow-focusing microfluidic device, which was used for the detection of protein bovine serum albumin (BSA). The PVA/SC12S stabilized micro-sized CLC droplets (CLCPVA/SC12S) exhibited flashing spots with red reflection. These CLC droplets were effective in the detection of BSA in an aqueous solution through a configurational change from red reflection flashing spots (homeotropic configuration) to a central spot of red reflection (planar configuration) with a detection limit of 0.15 µM. We found that the electrostatic interaction caused by the adsorption of negatively charged BSA below isoelectric point on the positively charged CLC droplets can cause the arrangement of the CLC molecules in droplets to change from flashing spot to central spot reflection. Our results also highlight that the limit of detection and response times of CLCPVA/SC12S droplets are tunable as a function of the concentration and pH of protein. These CLCPVA/SC12S droplets are highly sensitive and are inexpensive and simple to make. With this novel approach, we can detect protein by observing optical reflection patterns with/without crossed polarizers in CLCPVA/SC12S droplets, which, on the contrary, are required in the case of the nematic biosensors.

A new approach to deposit homogeneous samples of asbestos fibres for toxicological tests in vitro.

In this paper we describe the results obtained with a novel method to prepare depositions of asbestos fibres for toxicological tests in vitro. The technique is based on a micro-dispenser, working as an inkjet printer, able to deposit micro-sized droplets from a suspension of fibres in a liquid medium; we used here a highly evaporating liquid (ethanol) to reduce the experimental time, however other solvents could be used. Both the amount and spatial distribution of fibres on the substrate can be controlled by adjusting the parameters of the micro-dispenser such as deposition area, deposition time, uniformity and volume of the deposited liquid. Statistical analysis of images obtained by optical and scanning electron microscopy shows that this technique produces an extremely homogeneous distribution of fibers. Specifically, the number of deposited single fibres is maximized (up to 20 times), a feature that is essential when performing viability tests where agglomerated or untangled fibrous particles need to be avoided.

Amphiphilic Bowl-Shaped Janus Particles Prepared via Thiol-Ene Click Reaction for Effective Oil-Water Separation.

Janus particles for oil-water separation have attracted widespread attention in recent years. Herein, we prepared a bowl-shaped Janus particle that could rapidly separate oil and water through a thiol-ene click reaction and selective etching. Firstly, snowman-like composite microspheres based on silica and mercaptopropyl polysilsesquioxane (SiO2@MPSQ) were prepared by a hydrolytic condensation reaction and phase separation, and the effects of the rotational speed and molar ratios on their microscopic morphologies were investigated. Subsequently, bowl-shaped Janus particles with convex hydrophilic and concave oleophilic surfaces were prepared via a thiol-ene click reaction followed by HF etching. Our amphiphilic bowl-shaped Janus particles could remarkably separate micro-sized oil droplets from an n-heptane-water emulsion with a separation efficiency of >98% within 300 s. Based on the experimental and theoretical results, we proposed the underlying mechanism for the coalescence of oil droplets upon the addition of the amphiphilic bowl-shaped Janus particles.

Experimental investigations on heat transfer enhancement of enhanced surfaces in spray cooling using HFE-649

The present study utilizes commercial HFE-649 fluid to analyze the effect of enhanced surfaces on spray cooling. Two pressure swirl nozzles, N1 and N2 with an orifice diameter of 0.51 mm and 1.19 mm respectively, have been used to generate micro-sized droplets and to diffuse these droplets on copper specimen of 10-mm2 cross-section. The volumetric flux range of the fluid varies from 1.00 to 21.67 cm3/cm2s. Three enhanced surfaces, namely, two micro-coated surfaces having coating thickness of 100 µm and 200 µm, and a micro-finned (1 mm fin height) surface, have been used in the present study. Heat transfer enhancement of these enhanced surfaces has been compared with a reference plain surface. With micro-coated surface having thickness 100 µm, the maximum heat flux and heat transfer coefficient of 115 W/cm2 and 9.87 W/cm2K with N1, and 143.48 W/cm2 and 10.31 W/cm2K with N2 has been achieved, respectively. When compared to plain surface, coating thickness of 100 µm resulted in a maximum enhancement in heat flux of 61.54% with N1 and 52.91% with N2, and enhancement in heat transfer coefficient of 352% with N1 and 326% with N2. Effect of subcooling at two fluid inlet temperatures, 25℃ and 49℃, on heat transfer performance of enhanced surfaces has also been investigated. The critical heat flux also increased by up 97.1% for the micro-coated surface having 100 µm thickness at near saturated condition when compared to the plain surface, depending upon the volumetric flux. A new correlation for calculating HTC and CHF has been proposed which is based on the earlier correlation proposed by Estes and Mudawar for plain surface. In addition, a correlation to estimate CHF for enhanced surfaces has also been proposed in terms of density ratio, Weber number and Jakob number.

Numerical investigation of air cushioning in the impact of micro-droplet under electrostatic fields

Air cushioning widely occurs when a droplet impacts onto a solid or fluid surface at low velocity, which is mediated by the lubrication pressure of a thin air layer. Such air cushioning phenomena for micro-sized droplets bear important implications for precision coating and inkjet printing. In this study, we investigate numerically the air cushioning in the micro-sized droplets of various sizes impacting on a solid surface based on the volume of fluid method as implemented in the OpenFOAM framework. We find that the critical impact speed for bouncing on the air cushion increases as the droplet radius decreases, while the Weber number remains in a narrow range from 1 to 4. The scaling law of the critical impact speed for bouncing is derived by balancing the lubrication pressure of the air cushion with the capillary pressure and droplet inertia. The impact mode transforms from bouncing to wetting with an electric field. A group of phase diagrams of the electric Bond number vs the Weber number is presented for various droplet sizes. The diagrams are consistent with the scaling law of the critical electric field for the wetting-without-bubble mode. The findings provide insights for applications based on micro-droplet deposition, such as inkjet/electrohydrodynamic printing and spray coating, to avoid the adverse effect of air cushioning or air entrapment.

Multiphasic lignocellulose-based suspension for oil-water interfacial stabilization: Synergistic adsorption and phase behavior

This study described the multiphasic and multi-sized lignocellulose-based suspension (LBS) prepared by green method and its adsorption and phase behavior at O/W interface. The LBS consisting of lignin containing microfibrils (LMFs), lignin containing cellulose nanofibers (LCNFs), and lignin nanoparticles (LNPs), was obtained by mechanical fibrillation and high-shear treatments. They had different functions in emulsion stabilization: (1) synergistic irreversible adsorption of LCNFs and LNPs limited the coalescence of droplets and formed micro-sized droplets; (2) droplets filled in the LMFs network creating a strong fiber-droplet network structure. The fluorescent micrographs confirmed the synergistic irreversible adsorption of LCNFs and LNPs on the surface of the droplets, which was conductive to the high interfacial stability. The droplets were deformed rather than being destroyed under the high flow speed. The increasing viscosity, improving gel-like behavior, decreasing creep compliance and increasing yield stress demonstrated that the internal droplets can support the fiber network to delay the destruction under shear force. And the fiber-droplet network can automatically regenerate in situ after completed destruction.

Formulation of Chitosan-Saponin Microbeads using Microfluidic Technology

Type II diabetes mellitus caused adverse impact on almost 3.5 million patients locally. Conventional treatment for such disorder includes oral administration of anti-diabetic drugs or herbs supplement. However, the performance of this drugs in regulating blood glucose level is not too reliable due to the occurrence of weight gain, hypoglycaemia and low tolerability as well as structural breakdown of herbal supplements during consumption. In this project, an alternative approach was attempted by immobilising saponin, a known insulin stimulant extracted from plants (e.g. bitter gourd, eggplant or soapbark) onto chitosan beads via microfluidic technology. This technology offers advantages of producing micro-sized droplet with high surface area to volume ratio and minimal coefficient of variation for more accurate dosage. The droplets undergo solidification via cross-linking with NaOH as solvent. These beads are approximately 550 microns with 5.12 % polydispersity in size. The solidified beads were used as carrier for the immobilisation of saponin. This was achieved through altering the surface charge of the microbeads with Phosphate Buffer Solution (PBS), a commonly used electrolyte for the surface modification of chitosan before coating saponin onto its surface. Successful adsorption is rectified through FTIR, FESEM and EDX analysis.

Numerical investigation on the characteristics of single droplet deformation in the airflow at different temperatures

Micro-sized droplets in air may impact aircraft wings and induce severe ice accretion. The deformation and acceleration of a single droplet in a continuous airflow are simulated using the multiphase lattice Boltzmann flux solver to compute the flow field, and the phase-field method is used to track the droplet–air interface. The effects of droplet size, airflow velocity, and ambient temperature on the morphological evolution, flow field structure, and droplet motion are analyzed. The results indicate that the deformation of the droplet increases with Weber number, which distinguishes different deformation modes. With the increase in the droplet size and airflow velocity, the deformation of the droplet becomes greater in less time, and the characteristic alternate compressions in the axial and radial directions disappear. Moreover, different subzero temperatures have little effect on the droplet acceleration despite a different deformation amplitude, while the droplet acceleration is attenuated at normal temperatures.

Functional graphene oxide coated diatomite for efficient and recyclable demulsification of crude oil-in-water emulsion

The separation of micro-sized oil droplets from crude oil-in-water (O/W) emulsion is urgently required by petroleum industry and the environmental protection. Herein, we demonstrate a magnetically recyclable diatomite-graphene oxide (M-DM-GO) composite relying on the superior adsorption performance of graphene oxide (GO) and cost-effectiveness of diatomite (DM). Combining the improved positive surface charge by chemical grafting polyethyleneimine (PEI) on the GO surface, the as-prepared M-DM-GO exhibited high demulsification efficiency (Ed) of 96.8% for crude O/W emulsion with pH 6.0 at optimal dosage of 350 mg/L. Importantly, M-DM-GO displayed favorable demulsification performances under broaden pH conditions, suggesting its wide application scopes. Meanwhile, due to the magnetic response capability, M-DM-GO ensured efficient demulsification performance with Ed> 90% after 5 cycles. The efficiency, recyclability and wide pH application conditions make the M-DM-GO have universal application prospects in the management of crude O/W emulsion.

Freezing dynamics of supercooled micro-sized water droplets

The freezing of mini-sized supercooled water droplet includes two successive stages, rapid porous dendritic ice formation in recalescence stage and gradual ice growth in solidification stage. However, the freezing of supercooled micro-sized water droplet is significantly different and remains ambiguous. By implementing crystallization kinetics into heat and mass transfer analysis in the present study, a 3-dimensional numerical model is built to depict the fast freezing dynamics of micro-sized water droplets with diameters of 10 and 1 μm. The temporal evolutions of temperature and solid mass fraction during freezing process are elaborated, revealing two specific freezing patterns of the micro-sized droplets. For the water droplet with d = 10 μm, the propagation of porous ice decelerates due to strong heat conduction within the droplet. While for the water droplet with d = 1 μm, the cold energy is abundant for ice freezing due to strong heat exchange with external cold air stream, leading to the consecutive propagation of compact ice with solid mass fraction of 1.0. Furthermore, the influences of the location and number of the nucleation site on the freezing process are discussed. Limited by the external heat transfer, the freezing time is independent of the number of nucleation site for the water droplet with d = 10 μm. For the water droplet with d = 1 μm with abundant cold energy, multiple nucleation sites can accelerate the freezing of the droplet.

Simulation investigation of the spontaneous motion behaviors of underwater oil droplets on a conical surface.

A conical surface can realize the spontaneous transportation of micro-sized oil droplets in an aqueous environment without energy input, exhibiting great potential for applications in microfluidics, chemical micro-reactors, water remediation, etc. However, the precise manipulation of an oil droplet on a cone is still very challenging because the dynamic behavior of a droplet on a cone is not fully understood. Herein, the dynamic behavior of oil droplets on a cone is quantitively studied via numerical simulations, and the effects of wettability, apex angle, and droplet size on the droplet's dynamic behavior are systematically analyzed. The results show that the moving velocity and transport distance of the droplet on the cone are highly related to the droplet shape on the cone. It was found that a clamshell-shaped droplet moves faster than a barrel-shaped droplet. Besides, the clamshell-shaped droplet with a larger size, on the cone with a smaller apex angle and smaller contact angle tends to obtain a faster moving speed and a longer transportation distance. The droplet shape adopted on the cone was determined by the cone wettability and the size of the droplet relative to the local curvature of the cone. It was found that the oil droplet tends to form a barrel shape on the cone with a highly oleophilic and small apex angle, and tends to form a clamshell shape on cones with a highly oleophobic and large apex angle. In addition, the droplet might transit from a barrel shape to a clamshell shape when it moves from the cone tip to the cone base, and the trigger time of the transit is negatively correlated with the contact angle and apex angle of the cone. This work provides a microscale understanding of the dynamic behavior of an underwater oil droplet on a cone, and also offers theoretical guidance for manipulating the behavior of a droplet on a cone and for the rational design of cone surfaces for spontaneous droplet transport and droplet collection.

Evaporation of levitating liquid microdroplets over a dry heated surface

The present work is devoted to the study of the evaporation process of micro-sized water droplets levitating over a heated dry substrate. The study of this process is relevant in connection with the development of spray cooling systems. Due to the extreme complexity of this phenomenon the mechanism of spray cooling is still not fully understood. In this work we studied the evaporation of micro-sized water droplets levitating over a dry substrate heated from below. The working area was open to the atmosphere. Evaporation was studied in the temperature substrate range from 23 to 95°C. During the experiment local values of the substrate temperature and geometric characteristics of the drop were determined. In the experiment a shadow method with high spatial resolution was used, shooting was performed with a high-speed camera.

Lab-on-printed circuit board based water harvesting condensation particle counter for ubiquitous monitoring of airborne ultrafine particles

We have developed a water harvesting condensation particle counter (WHCPC) using lab-on-printed circuit board (PCB) technology. Our method provides an inexpensive, compact, and highly accurate system for the ubiquitous monitoring of airborne ultrafine particles (UFPs). Lab-on-PCB-based WHCPC uses water as the working fluid to monitor harmless UFPs; it does not require water replenishment using a moderated water condensation method. Lab-on-PCB-based WHCPC consists of two main parts, a UFP growth section and a mini-OPC, through which UFPs are grown into micro-sized droplets and the grown droplets are single-counted. The UFP growth section consists of two PCBs and a 3D-printed channel. The heater, temperature sensor, and hydrophilic micropillar array wick, which are essential for the growth of UFPs, are integrated on the PCBs. Simulations were performed to verify the saturated-air generation and water collection in the UFP growth section. The dew points of the inlet and outlet air samples were measured to verify water collection using the moderated water condensation method. Through quantitative experiments using UFPs artificially generated in the laboratory, particles of 9.4 nm could be counted. It was possible to precisely measure the number concentration (2500–270000 N cm−3) over a wide range. In addition, the stability of the proposed system was confirmed through a long-term comparison with the reference CPC in an outdoor environment. The proposed system realized the sensorization of CPC because it could monitor harmless UFPs, with minimal maintenance; the proposed system is also inexpensive and compact.

Breath Figure Spot: a Recovery Concentration Manifestation

Directing a jet of humid air to impinge on a surface that is cooled below the dew point results in micro-sized water droplets. Lord Rayleigh discussed the phenomenon by contrasting clean to flame-exposed glass and called such behaviour Breath Figures (BF). Historically, utilizing dew as a water source was investigated by several scientists dating back to Aristotle. However, due to the degrading effects of air as a non-condensable gas (NCG) such efforts are limited to small scale water production systems and exhaled breath condensate (EBC) technology, to name a few. Recently, the concept of BF has been utilized extensively in the generation of micro-scale polymer patterns as a self-assembly process. However, the generation of BF on surfaces while being impinged by a humid air jet has not been quantified. In this work, we illustrate that a BF spot generated on a cooled surface is a manifestation of a recovery concentration. The concept is analogous to the concept of adiabatic-wall temperature defined for heat transfer applications. Upon closer examination of the vapor concentration distribution on a cooled impinged surface, we found that the distribution exhibits distinct regimes depending on the radial location from the center of the impingement region. The first regime is confined within the impingement region, whereas the second regime lies beyond this radial location including the wall jet region. Scaling analysis as well as numerical solution of the former regime shows that the maximum concentration on the surface is equivalent to its counterpart of a free unbounded jet with similar geometrical conditions. Additionally, the scaling analysis of the latter regime reveals that the jet speed and standoff distance are not important in determining the recovery concentration. However, the recovery concentration is found to vary monotonically with the radial location. Our conclusions are of great importance in optimizing jet impingement where condensation phase change is prevalent.