Stability Of Droplets Research Articles

Transient study of droplet oscillation characteristics driven by an electric field

Electrowetting technology, a microfluidic technology, has attracted more and more attention in recent years and has broad prospects in terms of microdroplet drive. In this paper, the dynamic contact angle theory is used to develop a numerical model to predict the droplet dynamic contact behavior and internal flow field under electrowetting. In particular, based on the established computational model of droplet force balance, the dynamic process of a droplet under electrowetting is analyzed, including the perspective of pressure variation and force balance inside the droplet. The results show that when the alternating current frequency increases from 50 Hz to 500 Hz, the amplitude of the oscillation waveform after droplet stabilization is 0.036 mm, 0.016 mm, 0.013 mm and 0.002 mm, while the relevant droplet oscillation period T is 11 ms, 4 ms, 2 ms and 1 ms, respectively. It is also found that the initial phase angle does not affect the droplet oscillation amplitude. In addition, the pressure on the droplet surface under alternating current electrowetting increases rapidly to the maximum value with resonant waveform oscillation, and the droplet will present different resonance modes under voltage stimulation. The higher the resonance mode is, the smaller the droplet oscillation amplitude is and the streamline at the interface will present an eddy current, in which the number of vortices matches the resonance mode. A high resonance mode corresponds to a small droplet amplitude, while there are more vortices with a smaller size.

Influence of heat source arrangement on coupling characteristics of low-power pulsed laser-MAG hybrid welding

Low-power pulsed laser-MAG hybrid welding was carried out by different heat source arrangement methods (lead mode, distance between laser spot and welding wire tip (DLA)). The coupling effect and welding characteristics of hybrid heat sources under different modes were studied by using high-speed image (HSI) and real-time electrical signals. HSI observation and electronic signals showed that the lead mode and DLA significantly affected the coupling state of the heat source and welding characteristics (weld morphology, process stability, and droplet transfer). The influence of lead mode on weld forming was more significant than that of DLA. In laser-lead mode, when DLA = 1 ~ 2 mm, the laser and arc generated a perfect coupling effect, resulting in a good weld formation, less spatter, and greater penetration. In arc-lead mode, the weld formation was poor and the undercut defect was common, while the penetration was greater than that in laser-lead mode. In terms of welding process stability, the arc-lead mode was better than the laser-lead mode. When DLA = 1 ~ 2 mm, stable droplet transfer could be realized under both modes.

Experimental Investigation on the Droplet Stability of Superhydrophobic Mesh

Superhydrophobic surfaces could repel water due to the capillary force associated with surface roughness, which has a large range of applications, such as underwater drag reduction, heat transfer enhancement, oil/water separation, and so on. However, the engineering applications of superhydrophobic surfaces rely on the stability of the superhydrophobic surfaces. In this study, a hydrophilic metal mesh was modified to be superhydrophobic. The resulting superhydrophobic mesh was designed as a bowl capable of holding water without leaking and as a boat floating on top of water without sinking. The stability of an impacting droplet on a superhydrophobic mesh was investigated using both experiments and theoretical analysis. It was demonstrated that the capillary force is able to prevent water from passing through the mesh and maintain the stability of the air–water interface under dynamic pressure. Furthermore, a theoretical model was developed to diagnose the stability of the air–water interface on the superhydrophobic mesh when in contact with water, and the results are consistent with the experimental findings. The results of this work can be utilized to design robust superhydrophobic meshes and advance the field of droplet manipulation.

Effect of shock wave on nucleation and droplet growth of CO2 in flue gas in supersonic separators

To reduce carbon emissions caused by fossil fuel combustion, this work evaluates a potential carbon capture method. Based on heat transfer theory, aerodynamics and condensation dynamics, a mathematical model is proposed to depict the nonequilibrium condensation of CO2 in flue gas (N2–CO2). The interaction between shock wave and non-equilibrium condensation is clarified. The results show that when the shock wave enters nucleation area, the nucleation process is rapidly terminated and droplet growth process disappears. After the shock wave, the supersaturation decreases rapidly from 2.26 to 0.19, the gas changes from supersaturated state to unsaturated state, and the system returns to the thermodynamic equilibrium state. When the shock wave appears in the droplet growth area, there is a complete nucleation process. The gas molecules in front of the shock wave form stable droplets and release latent heat, leading to the fluctuation of temperature and pressure. After the shock wave, the droplet growth environment is destroyed, resulting in droplet radius and liquid content drop to 0. Increasing CO2 content can not only reduce the impact of shock wave on flows, but also strengthen the heat and mass transfer, and obtain larger droplet radius and higher liquid content.

Effect of high‐content ultrasonically emulsified lard on the physicochemical properties of surimi gels from silver carp enhanced by microbial transglutaminase

SummaryThe objective of this study was to investigate the effect of ultrasonically emulsified lard combined with microbial transglutaminase (MTGase) on the physicochemical characteristics of surimi gel from silver carp. Surimi gel with ultrasonically emulsified lard had higher whiteness, gel strength and water‐holding capacity. Storage modulus and loss modulus decreased with the addition of lard or ultrasonically emulsified lard to the surimi. However, the gel strength and WHC presented a decreasing tendency in surimi gels with more addition of ultrasonically emulsified lard, because the gel network could not overcome the damaging effect of higher ultrasonically emulsified lard (>6 g 100 g−1). MTGase increased the cross‐linking degree of gels (P < 0.05) regardless of the presence or absence of lard or ultrasonically emulsified lard. MTGase provided a denser and stronger gel network to ensure the stability of oil droplets. In general, the combination of ultrasonically emulsified lard and MTGase could effectively improve the color, textural and rheological characteristics of surimi‐based products.

A microfluidic method to systematically study droplet stability in highly concentrated emulsions

Preparation of high internal phase emulsions comes with many challenges. Droplet stability is hardly explored in these systems because they are not transparent, and highly dynamic. There is a clear need for advanced methods that allow elucidation of the mechanism of droplet coalescence in high internal phase emulsions. For this reason, we developed microfluidic chip that allow ultimate control over droplet size and high volume fraction. The chip concentrates emulsions in a concentration section, and subsequently allows coalescence investigation at volume fractions larger than 0.74. To illustrate the effect of surface active components, a systematic study was successfully carried out with β-lactoglobulin and sodium dodecyl sulfate, thus highlighting the versatility of the approach.

Suppression of crystallization in saline drop evaporation on pinning-free surfaces.

For sessile droplets of pure liquid on a surface, evaporation depends on surface wettability, the surrounding environment, contact angle hysteresis, and surface roughness. For non-pure liquids, the evaporation characteristics are further complicated by the constituents and impurities within the droplet. For saline solutions, this complication takes the form of a modified partial vapor pressure/water activity caused by the increasing salt concentration as the aqueous solvent evaporates. It is generally thought that droplets on surfaces will crystallize when the saturation concentration is reached, i.e., 26.3% for NaCl in water. This crystallization is initiated by contact with the surface and is thus due to surface roughness and heterogeneities. Recently, smooth, low contact angle hysteresis surfaces have been created by molecular grafting of polymer chains. In this work, we hypothesize that by using these very smooth surfaces to evaporate saline droplets, we can suppress the crystallization caused by the surface interactions and thus achieve constant volume droplets above the saturation concentration. In our experiments, we used several different surfaces to examine the possibility of crystallization suppression. We show that on polymer grafted surfaces, i.e., Slippery Omniphobic Covalently Attached Liquid-like (SOCAL) and polyethyleneglycol(PEGylated) surfaces, we can achieve stable droplets as low as 55% relative humidity at 25 °C with high reproducibility using NaCl in water solutions. We also show that it is possible to achieve stable droplets above the saturation concentration on other surfaces, including superhydrophobic surfaces. We present an analytical model, based on water activity, which accurately describes the final stable volume as a function of the initial salt concentration. These findings are important for heat and mass transfer in relatively low humidity environments.

Microscopic stabilization mechanism of nanoparticle-laden droplets under external electric fields: A molecular dynamics study

Electric dehydration process of crude oil is frequently accompanied by water droplet breakup, resulting in efficiency reduction of oil-water separation. To reveal the microscopic stabilization mechanism of nanoparticle-laden droplet, molecular dynamics (MD) simulations were adopted to investigate the electro-deformation and breakup dynamics. Good qualitative and quantitative agreements were observed between the present simulations and the literature experiments. The effects of electric fields (electrocapillary numbers and frequencies) and nanoparticles (concentrations and types) on the droplet stability were analyzed in terms of intermolecular total interactions. The analysis of quantum chemical calculations and weak interactions shows that stable hydrogen bonds can be formed between the silicon dioxide (SiO2) and water (H2O) molecules, enhancing droplet stability. The droplet stability under alternating current (AC) electric fields are higher than that under direct current (DC) electric fields, and the stability increases with increasing AC frequencies. At the total interactions of the nanoparticle-laden droplet system after switch-off electric field stronger than that after switch-on electric field, fast droplet recovered process was observed. The modified SiO2 NPs can efficiently enhance droplet stability, and high nanoparticle (NP) concentrations lead to high droplet stabilities. Eventually, the ranking of NP-laden droplet stabilities is roughly consistent with that of droplet pair electrocoalescence efficiencies, i.e., CAC (SiO2 modified with 6 carboxylic acid chains) > CAAL (SiO2 modified with 3 carboxylic acid chains and 3 alkane chains) > ALC (SiO2 modified with 6 alkane chains) > SiO2. The results of this work will be potentially valuable for optimizing the design of compact and efficient oil-water separators.

Spontaneous symmetry breaking, stability and adiabatic changes of 2D quantum droplets in amended Gross–Pitaevskii equation with multi-well potential

In this paper, we report the spontaneous symmetry breaking (SSB) of two-dimensional (2D) quantum droplets (QDs) for the amended Gross–Pitaevskii equation with Lee–Huang–Yang corrections and a Gaussian quadruple-well potential. The complete pitchfork symmetry breaking bifurcation diagrams are presented for the possible stationary states with four modes, which involve twelve different real solution branches and one complex solution branch, where the continuous asymmetric QDs branches bifurcate from the fundamental symmetric or antisymmetric one as the strength of nonlinearity exceeds a threshold value. Meanwhile the stability of the 2D QDs is explored by means of linear stability analysis and direct numerical simulations. Especially, for the asymmetric QDs, the SSB effect and the trend for getting the droplet asymmetry are discussed by changing the chemical potential. Finally, the adiabatic changes of stable stationary QDs are explored by modulating the position, depth, and width of the potential well as functions of time, and the strong robustness of QDs is found. Our results provide some theoretical basis for the SSB phenomena and related physical experiments on 2D asymmetric QDs.

The lineage-specific evolution of the oleosin family in Theaceae

Oleosins play essential roles in stabilization of lipid droplets (LDs) and seed oil production. However, evolution of this gene family has not been reported in Theaceae, a large plant family that contains many important tea and oil tea species. In this study, a total of 65 oleosin genes were identified in nine genome-sequenced Theaceae species. Among these genomes, the gene number of oleosin showed significant difference, with Camellia sinensis var. sinensis cv. Shuchazao and Camellia lanceoleosa displayed more oleosin numbers than other species. Phylogenetic analyses revealed that Theaceae oleosin genes were classified into three clades (U, SL, SH) respectively. Proteins within the same clade had similar gene structure and motif composition. Segmental duplication was the primary driving force for the evolution of oleosin genes in Shuchazao (SCZ), Huangdan (HD), C.lanceoleosa (Cla), and wild tea (DASZ). Synteny analysis showed that most oleosin genes displayed inter-species synteny among tea and oil tea species. Expression analysis demonstrated that oleosin genes were specifically expressed in seed and kernel of Huangdan (HD) and C.lanceoleosa. Moreover, expression divergence was observed in paralogous pairs and ∼1–2 oleosin genes in each clade have become activate. This study leads to a comprehensive understanding of evolution of oleosin family in Theaceae, and provides a rich resource to further address the functions of oleosin in tea and oil tea species.

Stabilization of water-in-water emulsion upon solvent shifting

We report the stabilization of all-aqueous droplets upon addition of ethanol, which were further transformed into robust capsules. An all-aqueous emulsion consisting of dextran (Dex)-rich droplets in a polyethylene glycol (PEG)-rich continuous phase was used as templates. Addition of ethanol induced gelation of dextran, forming aggregated pellets of poor interest. However, this feature was prevented by initially adding bovine serum albumin (BSA) or positively charged silica particles so that in this case, stable droplets formed upon solvent addition, which no longer coalesced with time. An alternative method consisting of pouring concentrated Dex solution in a mixture of PEG, ethanol and BSA or particles, also afforded stable droplets. These stable droplets were further converted into robust capsules, using carbodiimide chemistry or silylation. We expect this method for preparing capsules to be of interest for various applications in the field of microencapsulation.

Effect of Oil-Soluble/Water-Soluble Surfactants on the Stability of Water-in-Oil Systems, an Atomic Force Microscopy Study.

The stabilization mechanism of water-in-oil (W/O) emulsions has been studied by measuring the interactions between two water droplets in n-tetradecane using atomic force microscopy. The effects of water-soluble surfactants (SDS/CTAB/Tween 80), an oil-soluble surfactant (Span 20), and the coexistence of the water and oil-soluble surfactants on the stability of water droplets in oil were investigated separately. It is found that the addition of oil-soluble surfactants (Span 20) prevents the coalescence of water droplets in oil. To discuss the role of an oil-soluble surfactant, we analyzed the force curve by applying the theoretical model. The results demonstrate that the oil-soluble surfactant (Span 20) stabilizes dispersed droplets by adsorbing onto the interface and forming a relatively tighter layer with the increase in surfactant concentration, which hinders film rupture. This behavior of the surfactant could also be properly characterized by steric hindrance. A further step was taken by introducing another water-soluble surfactant. It is found that the addition of either SDS or CTAB into the water phase is futile in inducing droplet coalescence in the presence of Span 20. In contrast, Tween 80 was found to be effective in destabilizing water droplets, which could be due to the competitive adsorption between Tween 80 and Span 20 at the interface. By characterizing the interfacial adsorption of Tween 80 and Span 20 with a theoretical adsorption isotherm model, the result indicates that interface replacement would result in a loose adsorption layer that is insufficient to hinder droplet coalescence. Our study provides an intriguing understanding of the role of surfactants in the stabilization and destabilization of water-in-oil emulsions.

Wetting characteristics of Zn on Fe surfaces and its influencing factors: Molecular dynamics simulation

The wetting process of Fe-Zn solid-liquid system was simulated by molecular dynamics. The wetting behavior of Zn droplets on the Fe and the factors influencing the wettability were described in detail. The wetting angle first decreases uniformly during the wetting process and then remains stable. The wetting Angle is 35° Precursor membranes are formed during wetting. After the stabilization of the Zn droplet, the precursor membrane will still widen slowly. The atomic arrangement of the precursor membrane is influenced by the Fe matrix. The Al content is an important factor affecting the wetting. Within 0–40 at%, the wetting angle varies as an exponential function with increasing Al content. Finally, simulations were performed to analyze the effect of Fe matrix surface on the wettability. Due to the difference in microscopic roughness, the wetting angle of Zn droplets on the Fe (111) surface is the smallest and the wettability performance is the best. The energy potential barriers for the diffusion of zinc atoms on the (100) and (110) surface were calculated. The energy potential barrier is smaller of zinc atoms on the plane (110), and the precursor membrane is wider.

Au (III) cross-linked hollow organosilica capsules from 3-aminopropyltriethoxysilane

Hollow organosilica capsules have received extensive interest due to their application potentials in catalyst, sensor, drug delivery etc. In this work, we demonstrate a novel strategy to fabricate hollow organosilica capsules based on coordination interaction, by using 3-aminopropyltriethoxysilane (APTES) as precursor and Au (III) as cross-linker. In this approach, stable APTES droplets are first formed in water with the presence of Au (III) due to the coordination effect between Au (III) and the amino groups of APTES located on the surface of the droplets. Subsequently, the self-catalyzed hydrolysis/condensation of APTES allows for the formation of hollow organosilica capsules, in which the droplets of APTES themselves act as soft template and the Au (III) as cross-linker. The formation mechanism of the capsules was investigated, and potential of the as-prepared Au (III) cross-linked hollow organosilica capsules as glutathione (GSH) sensitive drug carriers was evaluated. In addition, Au particle embedded hollow capsules are further obtained by in-situ reduction of the Au (III) in the shell, which showed excellent stability towards the cyclic catalytic reductions of p-nitroaniline.

A numerical study on the levitation system for droplet preparation in a fuel-coolant interaction experiment

The MISTEE facility at KTH was designed to investigate the process and phenomena of a molten droplet falling into a water pool that may be encountered in fuel-coolant interactions (FCI) during a severe accident of light water reactors. An aerodynamic levitation mechanism is proposed to hold the molten droplet during its preparation (melting and heating up to a prescribed temperature) in an induction furnace. The crucible is flushed with argon through an injection nozzle at the bottom to prevent the droplet from falling out of the crucible. A numerical simulation of the aerodynamic levitation system is performed in the present study with the objective of determining and optimizing the design. The problem was simplified as an isothermal two-phase flow in an axisymmetric geometry. The simulation is realized through ANSYS Fluent v17 platform, which employs the VOF method to track interfaces between two phases and the SST k-omega model to describe turbulence flow of argon gas. The numerical model is validated against tests performed in the MISTEE facility after mesh sensitivity study. It is then applied to investigate the impacts of various parameters on the facility levitation capability and the droplet stability. According to the simulation results, stable molten droplets can be obtained in the designed experimental setup. The simulation also provides the appropriate values of argon inlet velocity and sample mass at which a stable droplet can be obtained inside the crucible before its discharge. Either higher or lower inlet velocity will destabilize the formation of the droplet. Considering the temperature-dependent melt properties, both surface tension and viscosity affect the movement and deformation of the molten droplet. The wettability of melt on the crucible wall is critical to droplet formation, and it is found that a poor wettability can ensure the levitation of droplet.

Influence of Emulsification Characteristics on the Pressure Dynamics during Chemical Flooding for Oil Recovery

Chemical flooding agents, especially surfactant-based flooding oil agents, have been widely studied due to their excellent emulsification ability. Nevertheless, there are many evaluation methods for chemical flooding agents, and the focus of each test is different. How to combine routine testing with reservoir conditions is an urgent problem to be solved. In this study, different kinds of chemical flooding agents were evaluated for interfacial tension (IFT), minimum emulsification speed, emulsification state, droplet size distribution, viscoelasticity, apparent viscosity in porous media, stability, and flooding efficiency. The influence of their characteristics on the pressure dynamics during flooding is analyzed, and the mechanism in the different stages of chemical flooding is clarified. A comprehensive analysis of IFT and the minimum emulsification speed is needed to evaluate the emulsification ability. The apparent viscosity when W/O emulsion flows through porous media is more accurate to evaluate the viscosity reduction of the chemical flooding agents than the traditional viscosity test. The chemical flooding dynamics is mainly divided into three stages: pressure increase (emulsification ability), pressure decrease (fluidity of formed oil in water (O/W) emulsion), and pressure stabilization (flooding efficiency). The stronger the emulsification ability, the faster the pressure increase and the wider the main flow channel. The easier the O/W emulsion is formed, the smaller the droplet size, the stronger the elastic deformation ability, and the higher the oil flooding efficiency. The smaller the droplet size and the stronger the viscoelasticity of O/W emulsion, the lower the maximum pressure in the flooding process and the slower the pressure decrease in the middle stage. Therefore, the evaluation of chemical flooding agents should mainly consider their emulsification ability and the formed O/W emulsion droplet size, viscoelasticity, and stability. This study clarified the evaluation focus of chemical flooding, and laid a foundation for the research and development and screening of chemical flooding agents, and the dynamic analysis of chemical flooding.

DISJOINING PRESSURE IN THIN SPHERICAL LIQUID FILMS AND VAPOR LAYERS WITH MOLECULAR CORRELATIONS INCLUDED

Based on the expression for a grand thermodynamic potential as a molecular density functional, disjoining pressures in thin liquid films around nanosized wettable spherical particles and in thin vapor layers around nonwettable particles are calculated depending the degree of lyophilicity, film thickness and particle size. A characteristic feature of the approach is the full consideration of hard-sphere molecular correlations according to the fundamental measure theory in the density functional method and finding the complete dependence of the grand thermodynamic potential of the system on stable droplet or bubble size. Although the obtained results show a qualitative agreement between the new calculated disjoining pressure dependences and those obtained by us earlier in the framework of a simpler gradient method of the molecular density functional, the new results differ significantly quantitatively. It is confirmed that the disjoining pressure in the liquid film around nanosized lyophilic particle grows with the particle radius and lyophilicity.

Flow-induced transition of compound droplet to composite microfiber in a channel with sudden contraction

The deformation behavior and hydrodynamic stability of a three-dimensional Newtonian single-core compound droplet during flow in a channel with sudden contraction were studied by numerical modeling. This research was motivated by the quest for conditions of the steady transition of a compound droplet into a composite microfiber, whose core is stretched as much as the shell. With this aim, the dynamics and morphology evolution of the compound droplet were analyzed in detail as functions of capillary number, core-to-shell relative viscosities, interfacial tensions, and the relative initial core radius. It was found that the effective elongation of the core occurs either with a significant increase in the shell viscosity relative to the ambient fluid or with a decrease in the core viscosity with respect to the shell. In this case, as the composite droplet advances into the narrowing zone of the canal, it continues to stretch, becoming a bullet-shaped composite microfiber. A new mechanism of disintegration of the compound droplet was revealed, which is caused by the core destabilizing effect and manifests itself either with an increase in the relative core/shell interfacial tension or the relative core viscosity.

Hydrophobic interaction at the O/W interface: Impacts on the interfacial stability, encapsulation and bioaccessibility of polyphenols

The interfacial complexation of modified goose liver protein (M-GLP) and hydrophobic polyphenols (curcumin, resveratrol, rutin and quercetin) and its role on gastrointestinal fate of polyphenols have been investigated. Results of fluorescence quenching suggested that the type and structural specificity of polyphenols (within oil phase) influenced their capacity to form interfacial complexation with M-GLP (within aqueous phase), which was hydrophobic interaction dominated. Raman spectroscopy and interfacial dilatational analysis revealed that the diketone structure and high hydrophobicity of curcumin caused the pronounced M-GLP-curcumin hydrophobic interaction and then enhanced the protein conformational flexibility. This conduced to the improved steric stability of the emulsion through strengthened interfacial loading and lateral association. Via the mechanism, there were improved physical stability and higher photo and storage stability of curcumin than resveratrol, rutin and quercetin. Moreover, emulsion droplets size distribution and in vitro digestion results revealed that, compared with other polyphenols, the stronger interfacial complexation between M-GLP and curcumin caused higher droplet stability during gastric environment and more sufficient intestinal lipolysis with O/W interface wreckage and related micelllization, finally yielding higher bioaccessibility for curcumin. This study may put further insight into the active role of hydrophobic compounds encapsulated on the emulsion system with delivery potency.

Influence of different electrolytes and oils on the stability of W1/O/W2 double emulsion during storage and in vitro digestion.

The aim of this study is to formulate a stable water-in-oil-in-water (W1/O/W2) double emulsion using different types of oils and electrolytes. W1/O was formulated with different electrolyte solutions (W1) dispersed in various oils (O) using polyglycerol polyricinoleate as a stabilizer. External aqueous phase was Tween-80 (W2), and W1/O dispersed in W2 was used. The emulsion containing NaCl or MgCl2 exhibited high encapsulation efficiency (EE) and maintained particle size. Regarding the oil type, the emulsion with MCT oil showed a small droplet size and a high viscosity and EE, presenting a stable droplet distribution in optical observation. The stability of emulsion containing NaCl was maintained during the in vitro digestion experiments. MCT oil, NaCl and MgCl2 have the potential to produce stable double emulsions for storage stability and in vitro digestion studies. The findings would be useful for preparing stable double emulsions used in the food and cosmetic industries.