Low Droplet Research Articles

Droplets on Lubricant-Infused Surfaces: Combination of Constant Mean Curvature Interfaces with Neumann Triangle Boundary Conditions.

Superior mobility of droplets on lubricant-infused surfaces (LIS) has recently attracted significant attention for designing liquid-repellent surfaces. Unlike sessile droplets on flat surfaces wherein the contact line is easily visible in experiments, the contact line on LIS is masked by the lubricant meniscus, and special imaging techniques are required to visualize the hidden droplet-lubricant interface. Moreover, the overall shape deviates significantly from the spherical cap geometry even at very low droplet volumes. These difficulties necessitate the need to model interfaces in order to assess the effect of surface and fluid properties on LIS. In this work, we first numerically simulate the droplet shapes to show that at very small volumes, droplet-air and droplet-lubricant interfaces are constant mean curvature (CMC) interfaces. Moreover, we elucidate that these mean curvatures are related by the ratio of interfacial tensions of the droplet-air and the droplet-lubricant interfaces. These insights reduce the modeling of LIS interfacial profiles to a simplified geometric problem, which is solved using the parametric equations of CMC surfaces along with the angles of the Neumann triangle as the boundary conditions. Predicted profiles of the droplet-air interface as a spherical cap, the droplet-lubricant interface as a nodoid, and the lubricant-air interface as a catenoid/nodoid show good agreement with experimental results in the literature. Importantly, we for the first time provide a framework, which accurately predicts the true contact angle at the hidden solid contact line by just using the information of the top spherical cap portion visible in experiments.

The Effect of Nanocoatings Enriched with Essential Oils on 'Rocha' Pear Long Storage.

The effect of coating ‘Rocha’ pears with alginate-based nanoemulsions enriched with lemongrass essential oil (LG) or citral (Cit) was investigated. Fruit were treated with the nanoemulsions: sodium alginate 2% (w/w) + citral 1% (w/w) (Cit1%); sodium alginate 2% (w/w) + citral 2% (w/w) (Cit2%); sodium alginate 2% (w/w) + lemongrass 1.25% (w/w) (LG1.25%); sodium alginate 2% (w/w) + lemongrass 2.5% (w/w) (LG2.5%). Then, fruit were stored at 0 °C and at 95% relative humidity, for six months. Fruit samples were taken after two, four and six months, and then placed at 22 °C. Upon removal and after 7 d shelf-life, fruit were evaluated for colour CIE (L*, h◦), firmness, soluble solids content (SSC), titratable acidity (TA), weight loss, electrolytic leakage, microbial growth, symptoms of superficial scald and internal browning. All nanoemulsions had droplets in the nano range <500 nm, showed uniformity of particle size and stable dispersion. Cit-nanoemulsions had lower droplet size and higher stability than LG. No nanoemulsion showed cytotoxicity. Coatings reduced fruit colour evolution and preserved better firmness than control. After shelf-life, better firmness was found in LG-coated fruit. Coatings did not affect SSC and TA. Microbial growth was below the safety limits in all treatments. Fruit treated with LG-nanoemulsions did not show scald symptoms and panelists preferred LG1.25% coated fruit. Cit2% treated fruit showed the highest scald and internal browning symptoms, while LG1.25% did not show any disorders. This study suggests that LG-nanocoatings have the potential for preserving the quality of ‘Rocha’ pear.

Anti-Microbial Potential of Nano-Emulsion form of Essential Oil Obtained from Aerial Parts of Origanum Vulgare L. as Food Additive.

Purpose: Foodborne diseases are still a serious problem in public health and natural compounds are being widely considered for their potential industrial protective additive in food products. Origanum vulgare L. has been known as an antimicrobial effective herb. This present study was carried out to examine the antimicrobial effect of O. vulgare essential oil nanoemulsion in comparison with conventional emulsion. Methods: The essential oil was obtained by hydrodistillation, analyzed by GC-Mass and formulated as a nanoemulsion to improve water dispersion by high-energy emulsification method. The antimicrobial activity of the prepared formulation was assessed by measuring the minimum inhibitory concentration (MIC), minimum bactericidal/fungicidal concentration (MBC/MFC) and zone of inhibition against some main foodborne pathogen microorganisms. Results: The main component of the oregano essential oil was carvacrol (78%) and the selected nanoemulsion formulation demonstrated low polydispersity (0.11) and mean droplet (72.26 nm) and it was stable even after 30 days of storage. The nanoemulsion form displayed significant activity against the Staphylococcus aureus, Candida albicans and Aspergillus niger with inhibition zones ranging from 8.7–22.3 mm. The MIC of nanoemulsion against the tested bacteria was within the range of 0.156 to 0.312 (mg/mL) and against the tested fungi were in the range of 0.078 to 0.156 (mg/mL). The MBC/MFC of nanoemulsion against the tested microorganisms were in the range of 0.312 to 5 (mg/mL). Conclusion: The study’s results demonstrated the possibility of using the nanoemulsion form of oregano essential oil as a food additive to inhibit the growth of some foodborne microorganisms and extending the shelf life of food products.

Techno-biofunctionality of mangostin extract-loaded virgin coconut oil nanoemulsion and nanoemulgel.

Techno-biofunctional characteristics of nanoemulsion and (nano)emulgel loaded with mangostin extracts were elucidated. Crude mangostins from mangosteen peels recovered by virgin coconut oil (VCO), mixed VCO and propylene glycol (PG), and pure PG were used. The extracts were loaded in the dispersed phase in the presence of mixed surfactants (Tween20/Span20) with a varying hydrophilic-lipophilic balance (HLB) from 10.2 to 15.1. Results showed that globular and uniformly distributed droplets of the nanoemulsion were observed. The small particle sizes (typically 18-62 nm) with the zeta potential of -39 to -54.5 mV were obtained when mixed emulsifiers with HLB values of 12.6 and 15.1 were employed. With HLB values of 12.6 and 15.1, nanoemulsions loaded with mangostin extracts prepared with mixed VCO-PG and pure PG-based extracts showed approximately a 2 to 3-fold lower droplet size diameter when compared with the VCO-based extract. For the stability test, all nanoemulsions were stable over three freeze-thaw cycles with some changes in pH, zeta potential, and droplet size. The DPPH● scavenging activity, H2O2 scavenging activity, reducing power and antibacterial activities (E. coli and S. aureus) of the nanoemulsions were greater than their corresponding bulk extracts. Nanoemulgels produced by embedding the nanoemulsions in a hydrogel matrix was homogeneous and creamy yellow-white in appearance. The nanoemulgels had a higher mangostin release (87-92%) than their normal emulgels (74-78%). Therefore, this study presented the feasibility of nanoemulsions and nanoemulgels loaded with mangostin extracts as a promising delivery system for bioactive polyphenol in food supplements, pharmaceuticals and cosmetics.

A digital microfluidic system with 3D microstructures for single-cell culture

Despite the precise controllability of droplet samples in digital microfluidic (DMF) systems, their capability in isolating single cells for long-time culture is still limited: typically, only a few cells can be captured on an electrode. Although fabricating small-sized hydrophilic micropatches on an electrode aids single-cell capture, the actuation voltage for droplet transportation has to be significantly raised, resulting in a shorter lifetime for the DMF chip and a larger risk of damaging the cells. In this work, a DMF system with 3D microstructures engineered on-chip is proposed to form semi-closed micro-wells for efficient single-cell isolation and long-time culture. Our optimum results showed that approximately 20% of the micro-wells over a 30 × 30 array were occupied by isolated single cells. In addition, low-evaporation-temperature oil and surfactant aided the system in achieving a low droplet actuation voltage of 36V, which was 4 times lower than the typical 150 V, minimizing the potential damage to the cells in the droplets and to the DMF chip. To exemplify the technological advances, drug sensitivity tests were run in our DMF system to investigate the cell response of breast cancer cells (MDA-MB-231) and breast normal cells (MCF-10A) to a widely used chemotherapeutic drug, Cisplatin (Cis). The results on-chip were consistent with those screened in conventional 96-well plates. This novel, simple and robust single-cell trapping method has great potential in biological research at the single cell level.

Low Exhaled Breath Droplet Formation May Explain Why Children are Poor SARS-CoV-2 Transmitters

A characteristic of the novel coronavirus disease 2019 (COVID-19) is the few cases of severely affected children. They not only rarely get symptoms but also seem to be poor transmitters. We propose that this may be explained by the mechanism of breath droplet formation, which occur during the re-opening of collapsed terminal bronchioles. Children's lungs are still developing. Compared to adults they have few alveoli and terminal bronchioles, thus less sites of aerosol formation where virus-loaded lung lining liquid can get airborne. In addition, children have a lower respiratory minute volume and tend to have a lower viral load. These points, together with the fact that asymptomatic carriers release less aerosols than symptomatic carriers support the idea that children are indeed poor transmitters of the COVID-19 virus.

Numerical Prediction of Feasibility to Lower Droplet Temperature in Duplex Current Feeding MIG Welding Process

Numerical Prediction of Feasibility to Lower Droplet Temperature in Duplex Current Feeding MIG Welding Process

Evaluation of the Performance of the Drag Force Model in Predicting Droplet Evaporation for R134a Single Droplet and Spray Characteristics for R134a Flashing Spray

Drag force plays an important role in determining the momentum, heat and mass transfer of droplets in a flashing spray. This paper conducts a comparative study to examine the performance of drag force models in predicting the evolution of droplet evaporation for R134a single droplet and spray characteristics for its flashing spray. The study starts from single moving R134a droplet vaporizing in atomispheric environment, to a fully turbulent, flashing spray caused by an accidental release of high-pressure R134a liquid in the form of a straight-tube nozzle, using in-house developed code and a modified sprayFoam solver in OpenFOAM, respectively. The effect of the nozzle diameter on the spray characteristics of R134a two-phase flashing spray is also examined. The results indicate that most of the drag force models have little effect on droplet evporation in both single isolated droplet modelling and fully two-phase flashing spray simulation. However, the Khan–Richardson model contributes to different results. In particular, it predicts a much different profile of the droplet diameter distribution and a much lower droplet temperature in the radial distance. The nozzle diameter has a significant impact on the flashing spray. A smaller diameter nozzle leads to more internse explosive atomization, shorter penetration distance, lower droplet diameter and velocity, and a faster temperature decrease.

Properties of orb weaving spider glycoprotein glue change during Argiope trifasciata web construction

An orb web’s prey capture thread relies on its glue droplets to retain insects until a spider can subdue them. Each droplet’s viscoelastic glycoprotein adhesive core extends to dissipate the forces of prey struggle as it transfers force to stiffer, support line flagelliform fibers. In large orb webs, switchback capture thread turns are placed at the bottom of the web before a continuous capture spiral progresses from the web’s periphery to its interior. To determine if the properties of capture thread droplets change during web spinning, we characterized droplet and glycoprotein volumes and material properties from the bottom, top, middle, and inner regions of webs. Both droplet and glycoprotein volume decreased during web construction, but there was a progressive increase in the glycoprotein’s Young’s modulus and toughness. Increases in the percentage of droplet aqueous material indicated that these increases in material properties are not due to reduced glycoprotein viscosity resulting from lower droplet hygroscopicity. Instead, they may result from changes in aqueous layer compounds that condition the glycoprotein. A 6-fold difference in glycoprotein toughness and a 70-fold difference in Young’s modulus across a web documents the phenotypic plasticity of this natural adhesive and its potential to inspire new materials.

The composition and oxidative stability of vegetarian omega-3 algal oil nanoemulsions suitable for functional food enrichment.

Long chain omega-3 polyunsaturated fatty acid (LCn3PUFA) nanoemulsion enriched foods offer the potential to address habitually low oily fish intakes. Nanoemulsions increase LCn3PUFA bioavailability, although they may cause lipid oxidation. The present study examined the oxidative stability of LCn3PUFA algal oil-in-water nanoemulsions created by ultrasound using natural and synthetic emulsifiers during 5 weeks of storage at 4, 20 and 40 °C. Fatty acid composition, droplet size ranges and volatile compounds were analysed. No significant differences were found for fatty acid composition at various temperatures and storage times. Lecithin nanoemulsions had significantly larger droplet size ranges at baseline and during storage, regardless of temperatures. Although combined Tween 40 and lecithin nanoemulsions had low initial droplet size ranges, there were significant increases at 40 °C after 5 weeks of storage. Gas chromatograms identified hexanal and propanal as predominant volatile compounds, along with 2-ethylfuran, propan-3-ol and valeraldehyde. The Tween 40 only nanoemulsion sample showed the formation of lower concentrations of volatiles compared to lecithin samples. The formation of hexanal and propanal remained stable at lower temperatures, although higher concentrations were found in nanoemulsions than in bulk oil. The lecithin only sample had formation of higher concentrations of volatiles at increased temperatures, despite having significantly larger droplet size ranges than the other samples. Propanal and hexanal were the most prevalent of five volatile compounds detected in bulk oil and lecithin and/or Tween 40 nanoemulsions. Oxidation compounds remained more stable at lower temperatures, indicating suitability for the enrichment of refrigerated foods. Further research aiming to evaluate the oxidation stability of these systems within food matrices is warranted. © 2019 Society of Chemical Industry.

Predicting droplet velocity in a Hele-Shaw cell

We study the motion of low viscous non-wetting droplet in a Hele-Shaw cell, while it is pushed by an external phase of imposed flow rate, at low capillary numbers. In this regime, the droplet's mobility, defined as the ratio between the droplet velocity and the external phase mean velocity, evolves non linearly with the capillary number, a signature of the different dissipation mechanisms at play. Experiments are performed with surfactant free air bubbles in fluorinated oil, and with surfactant laden fluorinated oil droplets in water. We propose a model based on a power balance which takes into account the dissipation in the thin wetting film trapped between the bubble (or the drop) and the channel wall. The full topography of this thin film is obtained theoretically for the bubble case. By contrast, the presence of surfactants in the drop case induces uncontrolled boundary conditions at the interface, thus imposing to use the experimental topography measured in the previous paper [Reichert et al., J. Fluid. Mech., 850 p.708 (2018)]. Remarkably, the model reproduces the experimental velocities and shows that the velocity can be strongly affected by a stagnant cap effect at the rear of the drop, even if localized in less than a few percents of the total film area.

Production and technological characteristics of avocado oil emulsions stabilized with cellulose nanofibrils isolated from agroindustrial residues

The present work provided the valorization of agroindustrial residues from the palm peach (Bactris gasipaes) industry to produce cellulose nanofibrils (CNFs) and considered their application as a stabilizer agent for avocado oil emulsions. The treatments applied generated CNFs bundles with high aspect ratio, stability and mobility of (-) 19.82 ± 7.7 (μ/s)/(V/cm) at water suspensions; crystallinity index of 31.5 % and superficial functional groups (FT-IR spectra) that remarked them as able to interact with polar and non-polar molecules. The little amount of 1.0 % of CNF was able to stabilize emulsions with high oil content (50 %) that remained steady after 30 days of storage at different temperatures, probably by CNF’s Pickering-mechanism. Emulsions with sorbitan monostearate (3.5 %) were also produced and the replacement or interaction of this agent with CNF (0, 50 and 1.0 %) was evaluated by creaming behavior, stability index and morphological characterization. It was not observed a synergistic effect between the components; emulsions that contained only CNF (CNF 100) showed superior characteristics, with no creaming or coalescence at 02 and 25 °C for 30 days; no coalescence after thermal treatments (02 and 80 °C) associated with extreme pH variations (2 and 11) and lower droplets size. Besides address a practical application for nanocelluloses obtained from discarded lignocellulosic material, the results suggests the possibility of produce a potentially edible emulsion from an oil with high oleic acid content and bioactive molecules without the addition of any surfactant or other chemical additive, that could be used for new food, cosmetic or pharmaceutical applications.

Gelatin molecular structures affect behaviors of fish oil-loaded traditional and Pickering emulsions

As the differences of traditional and Pickering emulsions might have resulted from stabilizer structures, this study analyzes the effects of gelatin molecular structures (uncrosslinked molecules vs. crosslinked molecules) on the preparation, long-term storage, and dilution of fish oil-loaded traditional and Pickering emulsions. Both traditional and Pickering emulsions have three types of droplets with different sizes, and all the droplet sizes were exponentially decreased with the increase of stabilizer concentration. Pickering emulsions have slightly lower droplet sizes compared with traditional emulsions. Traditional emulsions have three different emulsion forms (liquid, redispersible emulsion gel, and unredispersible emulsion gel), whereas Pickering emulsions only have the liquid form. Emulsion creaming stability was dependent on stabilizer molecular structures and stabilizer concentrations. The two emulsions have similar and good dilution stability. This work demonstrates that gelatin molecular structures affect droplet size, emulsion forms, and creaming stability, but not droplet size types and emulsion dilution stability.

Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

The present work examines numerically the aerodynamic breakup of a cluster of Diesel droplets moving in parallel with respect to the gas flow. Two- and three-dimensional simulations of the incompressible Navier–Stokes equations together with the VOF method are performed for Weber (We) numbers in the range of 5 up to 60 and non-dimensional distance between the droplets (H/D0) ranging from 1.25 to 20. The numerical results indicate that the proximity of droplets affects their breakup for distances H/D0 ≤ 5. For low droplet proximity distances (H/D0 ≤ 2.5), the droplets experience the so-called shuttlecock breakup mode, which has been also identified for droplets in tandem formations in a previous authors’ work and is characterized by an oblique peripheral stretching of the droplet. With decreasing H/D0 the breakup initiation time decreases, while the drag coefficient increases relative to that of isolated droplets. When the distance between the droplets is low enough (H/D0 < 1.5), this can result in critical We number, i.e. minimum We number leading to breakup, lower than that of an isolated droplet at the same conditions.

Bioprinting of high cell-density constructs leads to controlled lumen formation with self-assembly of endothelial cells.

Active nutrient supply and waste product removal are key requirements for the fabrication of long-term viable and functional tissue constructs of considerable size. This work aims to contribute to the fabrication of artificial perfusable networks with a bioprinting process, based on drop-on-demand (DoD) printing of primary endothelial cell (EC) suspension bioink (25 × 106 ± 3 × 106 cells/ml). The process results in prescribed lumen between two hydrogel layers, allowing its integration in common layering based bioprinting processes. Low volume bioink droplets (appr. 10 nl) as building blocks were deposited between two fibrin or Collagen I layers to realize shapeable, cell-rich aggregates. Unattainable with manual positioning, DoD printing allowed precise fabrication of various designs, such as spheroidal-, line-shaped, and Y-branch cellular structures, with a mean lateral extension of 285 ± 81 μm. For basic characterization, the cell suspension building blocks were systematically compared with preformed spheroids of the same cell type, passage, and number. Post printing investigations of initially loose cell arrangements showed self-assembly and formation of central lumen with a mean cross-sectional area of Ølumen = 6,400 μm2 at Day 3, lined by a single layer of CD31 positive ECs, as evaluated by confocal microscopy. Originating from this main lumen smaller, undirected side branches (Øbranches = 740 μm2 ) were formed by sprouting cells, inducing a first step towards a simplistic hierarchically organized network. These lumen could prospectively help for tissue construct perfusion in vitro or, potentially, as niche for angiogenesis of host vascularization in implants.

Impact of secondary droplet activation on the contrasting cloud microphysical relationships during the wet and dry seasons in the Amazon

Cloud microphysical data obtained from aircraft measurements during the GoAmazon2014/5 campaign were analyzed to examine the differences in cloud microphysical properties and relationships between the wet and dry seasons and their implications on cloud microphysical processes. Basically, the distinct differences in cloud microphysical properties between the wet and dry seasons were considered to be due to higher concentration of aerosols and their larger sizes in the dry season, leading to higher droplet concentration. Analyses of cloud microphysical relationships and mixing diagrams strongly suggest homogeneous mixing for most cloud segments in both the wet and dry seasons: diluted cloud parcels with smaller liquid water content (L) and lower droplet concentration (N) generally had smaller mean volume of cloud droplets (V). However, in the dry season some cloud segments included cloud parcels that had high N of small cloud droplets, which led to a low correlation between N and V and also between N and L. These features are speculated to be due to secondary activation of cloud droplets from the cloud condensation nuclei in the entrained air, which seemed more likely to occur in the dry season due to more favorable conditions for such a process, including larger sizes of entrained aerosols, higher fluctuation of vertical velocity and lager turbulent dissipation rate.

Optimization of control parameters of droplet density in citrus trees using UAVs and the Taguchi method

The control parameters of the unmanned aerial vehicle (UAV) should be carefully designed to improve UAV spraying performance on citrus trees. The present study investigated the optimal droplet distribution control parameters in citrus trees using a UAV and the Taguchi method, of which optimal results were observed with an inverted triangle citrus tree canopy shape, a spraying height of 1.40 m, and a flight speed of 1.0 m/s. Among the discussed control parameters, the flight speed presented the most significant effect with a contribution percentage of 74.0%. The established multiple regression model predicted an optimal spraying height of 1.27 m and a maximum droplet density of 35.39 droplets/cm2. In addition, the effects of individual control parameter on the droplet density of the lower layer of citrus trees were systematically analyzed, of which inverted triangle shape more significantly affected the droplet density of the lower layer and presented an 82.0% increase in droplet density as compared to the triangle shape. An improvement of 59.6% in the lower layer droplet density was observed at a spraying height of 1.40 m. In addition, the other spraying heights did not present significant differences in their coefficient of variation (CV) values. Keywords: droplet density, citrus, Taguchi method, aerial spraying, unmanned aerial vehicle (UAV), plant protection DOI: 10.25165/j.ijabe.20191204.4139 Citation: Hou C J, Tang Y, Luo S M, Lin J T, He Y, Zhuang J J, et al. Optimization of control parameters of droplet density in citrus trees using UAVs and the Taguchi method. Int J Agric & Biol Eng, 2019; 12(4): 1–9.

Separation of Floating Oil Drops Based on Drop-Liquid Substrate Interfacial Tension.

Though various strategies exist for the transport of oil drops suspended on a liquid substrate, selective manipulation of different kinds of drops based on their respective characteristics remains a challenge. In practice, it is possible to have multiple drops having different wetting states with the liquid substrate, whose separation is desired. In this work, we exploit curvature-induced capillary forces for the selective manipulation (transport as well as separation) of oil droplets based on their interfacial tension (IFT) with the underlying liquid substrate. To demonstrate this, we have selected two oils having different IFTs with the aqueous liquid substrate and tuned their curvature-induced capillary interaction (inward or outward from the source) by controlled addition of the surfactant. We experimentally realize three droplet manipulation regimes: repulsion, attraction, and separation regime. In the repulsion and attraction regimes, both the drops behave in a similar manner. Strikingly, in the separation regime, drops can be effectively separated based on their IFT; low IFT droplets are attracted toward the source, while high IFT droplets do the reverse.

Quantitative research of spray cooling effects on thermo-flow performance of the large-scale dry cooling tower with an integrated numerical model

An integrated model with a coupling procedure was developed to combine the dynamics of spray injections and natural draft of the dry cooling tower. By the integrated model, the interaction was clearly revealed that pre-cooling slightly reduces the buoyancy and ventilation of the tower while enhancing heat transfer, but velocity decrease of inflow air in turn promotes droplet evaporation and enhances spray cooling effects. Under an example pre-cooling system, the calculated inflow air temperature drop is about 6.6% larger than that obtained by the traditional model which neglected this interaction. Moreover, spray cooling effects on tower performance were directly quantified by the integrated model, and the tower achieved 6.94% heat dissipation improvement and 2.75% ventilation decrease. According to streamlines, effective air inflow area of the tower gradually shrinks, which promotes the formations of interferences among the injections and severely impedes the evaporation process. Especially near the inlet of the radiator, evaporation rate hardly grows, and it gets worse under the case of more nozzles. The larger droplet size and the lower droplet velocity under a smaller spray pressure are adverse to droplet evaporation, and spray cooling makes little sense at low air temperature and especially high air humidity. The proposed integrated model is a powerful tool for spray cooling research and optimization for the dry cooling tower and some other large-scale objects.

The Wide Fuel and its Characteristics

Water in diesel emulsion is an optional fuel, has a potential to reduce the formation of both nitrogen oxides, carbon dioxides and particulate matters in diesel engine. Thus the performance of this emulsified fuel strongly depends on the type of emulsion, stability of emulsion, amount of surfactant and the physio-chemical properties. In this study, water in diesel emulsion fuels of 5%, 10%, 20%, 25% water by volume was prepared by a mechanical stirrer. Physical and chemical properties of emulsion were examined as this properties could influence the spray characteristic of the emulsion which can affect the ignition delay and flame propagation, the carbon contents for emulsified fuel with 10% and 20% were low droplet size of the emulsion was found to be less than 2 µm. The water content in the emulsified fuel also include the amount of surfactant density and viscosity was found to be higher for all of the water in diesel emulsion the pure diesel.