Charged Droplets Research Articles

Microscale Templating of Functional Particles Using Self‐Limiting Electrospray Deposition

AbstractElectrospray deposition (ESD) uses strong electric fields applied to solutions and dispersions exiting a capillary to produce charged monodisperse droplets driven toward grounded targets. Self‐limiting electrospray deposition (SLED) is a phenomenon in which highly directed, uniform, and even 3D coatings can be achieved by trapping charge in the deposited film, redirecting the field lines to uncoated regions of the target. However, when inorganic particles are added to SLED sprays, the buildup of charge required to repel incoming material is disrupted as particle loading increases. Due to its fibril gelling behavior, methylcellulose (MC) SLED can form nanowire morphologies. These wires, when used as a binder, can separate particles and prevent percolation. In this work, a variety of conductive and insulating particles are explored using patterned and un‐patterned substrates. This exploration allows us to maximally load particles for high‐concentration and highly controlled self‐limiting functional sprays. This is demonstrated using Ti3C2Tx MXene to functionalize an interdigitated electrode for use as a supercapacitor.

High‐Precision Drop‐on‐Demand Printing of Charged Droplets on Nonplanar Surfaces with Machine Learning

Direct printing methods are widely recognized as efficient techniques for manufacturing printed electronics. However, several challenges arise when printing on nonplanar surfaces, especially using the drop‐on‐demand (DoD) approach. These challenges include ink flow due to gravity, precise ink deposition, and reproducibility. This study introduces an innovative method for highly accurate DoD material jetting on nonplanar 3D conductive surfaces, enabling precise production and trajectory control of charged droplets. The technique involves using a grounded 3D substrate as the target, where in‐flight droplets are subjected to an external electric field generated by gate electrode installed on a piezo activated droplet dispenser. Individual droplets are generated and controlled using a complex trigger system that relays variable‐voltage signals to the gate electrode. Moreover, a predictive model for droplet deposition, exhibiting an accuracy of 87%, is developed utilizing supervised machine learning (ML). This approach significantly improves the accuracy and repeatability of droplet deposition. Overall, this study presents an effective method of integrating piezoelectric and electrohydrodynamic printing technologies, complemented by ML. It addresses the challenges associated with printing on nonplanar surfaces using the DoD material jetting technique and shows considerable promise for enhancing efficiency, accuracy, and repeatability in the manufacturing of printed electronics.

Hydrogen affection on softening and melting behaviours of high alumina sinter in gas-injection blast furnace

As the Al2O3 content increasing in iron ore, some problems presented in blast furnace (BF) processing, such as worsened burden permeability and slag viscosity. Injection of H2 may improve the permeability and reduce the CO2 emission of BF, because of its less density and higher reduction potential when temperature is higher than 820 °C. Therefore, the influence of H2 concentration on the softening and melting behaviour of high alumina sinter is a worth investigate topic. In this article, the high alumina sintering samples with Al2O3 content of 2.11 wt% are investigated in a specially designed experimental apparatus capable of continuously changing pressure on the burden. The results show that the softening temperature range of the samples is expanded and their melting temperature range is reduced with the increasing of H2 concentration from 0 vol% to 20 vol% in reduction gas. But the overall change on softening-melting temperature range is just from 280 °C to 310 °C, which is relatively small comparing with other references reported results. The addition of H2 promotes the reduction of FeO and is conducive to separate between the iron phase and slag phase before the charge droplet formation, which facilitates the aggregation and precipitation of metallic iron in soften state. Simultaneously, higher H2 concentration leads to more pronounced improvements in the permeability of the high alumina burden.

Factors Promoting Lipopolysaccharide Uptake by Synthetic Lipid Droplets.

Lipoproteins are essential in removing lipopolysaccharide (LPS) from blood during bacterial inflammation. The physicochemical properties of lipoproteins and environmental factors can impact LPS uptake. In this work, synthetic lipid droplets containing triglycerides, cholesterols, and phospholipids, were prepared to mimic lipoproteins. The physicochemical properties of these lipid droplets, such as charges, sizes, and lipid compositions, were altered to understand the underlying factors affecting LPS uptake. The amphiphilic LPS could spontaneously adsorb on the surface of lipid droplets without lipopolysaccharide binding protein (LBP); however, the presence of LBP can increase LPS uptake. The positively charged lipid droplets also enhance the uptake of negatively charged LPS. Most interestingly, the LPS uptake highly depends on the concentrations of Ca 2+ near the physiological conditions, but the impact of Mg 2+ ions was not significant. The increase of Ca 2+ ions can improve LPS uptake by lipid droplets; this result suggested that Ca 2+ may play an essential role in LPS clearance. Since septic shock patients typically suffer from hypocalcemia and low levels of lipoproteins, the supplementation of Ca 2+ ions along with synthetic lipoproteins may be a potential treatment for severe sepsis.

Effect of droplet charge density on stabilization of oil‐in‐dispersion emulsions co‐stabilized by binary mixed surfactants and nanoparticles

AbstractIonic surfactant and similarly charged nanoparticles can co‐stabilize oil‐in‐dispersion (OID) emulsions at extremely low concentrations (0.001 cmc/0.001 wt%), in which particles do not adsorb at the oil/water interface but distribute in the aqueous phase forming a dispersion. In this paper, the effect of droplet charge density on stabilization of the n‐decane‐in‐water OID emulsion was examined by using a cetyl trimethyl ammonium bromide (CTAB)/C12B (dodecyl dimethyl carboxyl betaine) binary mixture at a low fixed total concentration (0.01 mM) with varying molar fractions of CTAB. A model based on the Derjaguin‐Landau‐Verwey‐Overbeek (DLVO) theory is proposed to calculate interaction energies between droplets and between droplets and particles. It is found that the droplet charge density can be well compensated by particle concentration along the stabilization boundary, and the OID emulsion still follows the DLVO stabilization. Particles tend to surround droplets at large distances but may form a monolayer between approaching droplets at shorter distances, which significantly reduces the van der Waals attraction between droplets. In addition, the induced auxiliary droplet–particle repulsion is proportional to the number of particles per unit area of droplet surfaces, which together with the droplet–droplet repulsion ensures a large total repulsion preventing droplets from flocculation and coalescence. This work explains quantitatively the stabilization of OID emulsions, which have potential applications in emulsion products such as foods, cosmetics, pesticides, and various industrial emulsion systems. Moreover, the development of the OID emulsions represents an important advancement in green chemistry as it substantially reduces the required amounts of emulsifiers and their environmental impact after use.

A multiphase flow model of water droplets dielectrophoretic-induced air dehumidification phenomena

Air humidity in indoor spaces plays a critical role in human comfort and health. Dehumidification systems are used for building humidity controls, but they can take significant energy consumption, especially in geographic locations with high outdoor humidity and warm climates. Consequently, there is a growing demand for innovative dehumidification processes that consume minimal energy. Dielectrophoretic air dehumidification represents one such promising approach. However, it has not garnered significant attention due to the absence of engineering models and simulation tools capable of evaluating its performance and limitations at large-scale airflows. A new numerical multiphase CFD model, which is also experimentally validated, is developed in a customized Reacting Foam solver based on OpenFOAM® version 9. The newly developed model seeks to decrease substantial energy consumption and lower costs by leveraging the dielectrophoretic phenomenon to regulate moisture levels in the air. The solver integrates a hybrid Eulerian-Lagrangian framework to track the droplet's trajectory and growth rate while solving the continuum equations for the moist air. An electrospray produces electrically charged droplets, which grow during their in-flight trajectories as water vapor condenses onto their surfaces. The role of electrostatic forces in promoting vapor condensation within a high-gradient electrical field is investigated, and the dielectrophoretic vapor nucleation process on charged water droplets is discussed. The CFD model was validated against results from the literature and from proof-of-concept experiments conducted by the authors, which showed a 2 % air dehumidification with a single electrospray and airflow rate of 5 cubic feet per minute. The simulation results indicated that augmenting the number of electrically charged spray droplets increased the dehumidification of the air to 25 %. The initial mean droplet diameter, the orientation of the injector and relative humidity significantly influence the assessment of dehumidification. Scaling up this approach to larger airflow volumes is identified as a potential future research direction.

High-Performance Membranes Based on Spherical-Beaded Nanofibers and Nanoarchitectured Networks for Water-in-Oil Emulsion Separation.

High-performance separation materials for oil-water emulsions are crucial to environmental protection and resource recovery; however, most existing fibrous separation materials are subject to large pore size and low porosity, resulting in limited separation performance. Herein, we create high-performance membranes consisting of spherical-beaded nanofibers and nanoarchitectured networks (nano-nets) using electrostatic spinning/netting technology, for water-in-oil emulsion separation. By manipulating the nonequilibrium stretching of jets, spherical-beaded nanofibers capable of generating a robust microelectric field are fabricated as scaffolds, on which charged droplets are induced to eject and phase separate to self-assemble nano-nets with small pores. Benefiting from 3D undulating networks with cavities originating from 2D nano-nets supported by 1D spherical-beaded nanofibers, the membranes exhibit under-oil superhydrophobicity (>152°), a striking separation performance with an efficiency of >99.2% and a flux of 5775 L m-2 h-1, together with wide pressure applicability, antifouling, and reusability. This work may open up new horizons in developing fibrous materials for separation and purification.

Influence of constant and alternating electric fields on the deformation and destruction of a liquid droplet

Examining the impact of electromagnetic field, which provides thrust in contemporary rocket engines, is turning into a significant issue. Its resolution is required to guarantee the safety of space flight by enhancing engine performance and lowering fuel consumption. Spacecraft propulsion is achieved by low-thrust rocket engines. The principle of operation for ion colloid engines is the electrostatic acceleration of charged droplets. A static electric field accelerates charged liquid droplets created by electrospraying them in a low-thrust colloid engine. A promising technology in many industries is the active control of droplet motion and deformation with electric field. An electromagnetic field impact on droplet deformation and destruction in a viscous liquid is examined. The effect of electromagnetic field on individual droplets and emulsions is examined in terms of their physical mechanisms. Constant and alternating electric field effects on liquid droplet are represented numerically. The effects of droplet electric capillary number and viscosity ratio on droplet unsteady deformations is explored. The parameters that correspond to the droplet destruction are found. The results obtained have potential applications in enhancing the efficiency of current industrial electric dehydrators and in advancing the development of new electromagnetic field-based demulsification technologies.

Mitigation of fine hydrophobic liquid aerosols by polydispersed uncharged and charged water droplets

Mitigation of fine hydrophobic liquid aerosols by polydispersed uncharged and charged water droplets

Model for atomization droplet size and energy distribution ratio at the distal end of an electrostatic nozzle

Electrostatic atomization minimum quantity lubrication (EMQL) employs the synergistic effect of multiple physical fields to atomize minute quantities of lubricant. This innovative methodology is distinguished by its capacity to ameliorate the atomization attributes of the lubricant substantially, which subsequently augments the migratory and infiltration proficiency of the droplets within the complex and demanding milieu of the cutting zone. Compared with the traditional minimum quantity lubrication (MQL), the EMQL process is further complicated by the multiphysical field influences. The presence of multiple physical fields not only increases the complexity of the forces acting on the liquid film but also induces changes in the physical properties of the lubricant itself, thus making the analysis of atomization characteristics and energy distribution particularly challenging. To address this objective reality, the current study has conducted a meticulous measurement of the volume average diameter, size distribution span, and the percentage concentration of inhalable particles of the charged droplets at various intercept positions of the EMQL nozzle. A predictive model for the volume-averaged droplet size at the far end of the EMQL nozzle was established with the observed statistical value F of 825.2125, which indicates a high regression accuracy of the model. Furthermore, based on the changes in the potential energy of surface tension, the loss of kinetic energy of gas, and the electric field work at different nozzle orifice positions in the EMQL system, an energy distribution ratio model for EMQL was developed. The energy distribution ratio coefficients under operating conditions of 0.1 MPa air pressure and 0 to 40 kV voltage on the 20 mm cross-section ranged from 3.094‰ to 3.458‰, while all other operating conditions and cross-sections had energy distribution ratios below 2.06‰. This research is expected to act as a catalyst for the progression of EMQL by stimulating innovation in the sphere of precision manufacturing, providing theoretical foundations, and offering practical guidance for the further development of EMQL technology.

Knapsack air assisted electrostatic sprayer for agricultural formulations

An experimental prototype of a backpack-type rechargeable battery-powered air-assisted electrostatic sprayer was designed and tested for performance while preserving techno-commercial competence and affordability for India's marginal and small farmer communities. The designed prototype electrostatic sprayer achieved good levels of charge induction on the spray particles at various electrode potentials (1 to 12 kV). At a charging electrode potential of 9 kV and a nozzle discharge rate of 2 mL s−1, electrostatically charged spray had a maximum charge to mass ratio (CMR) of 1.79 mC kg−1. The Electric Ducted Fan (EDF) used for high velocity air assistance was capable of transporting charged spray droplets onto distant targets such as orchard trees and field crops with a spray throw of up to 5 m. The high-pressure atomization method produced fine droplets within a Volume Median Diameter (VMD) range of 90 to 100 µm, resulting in better charge induction and wrap-around effect. As the prototype features fewer moving mechanical components, reduced total noise and vibrations, it promises operator comfort in the long run while requiring less system maintenance. Environmental contamination can be minimized as the large quantity of harmful chemicals be prevented from drifting into the soil and nearby waterbodies. The competitive performance and lower investment could encourage majority of the Indian famers to upgrade with the developed air assisted electrostatic spraying system contributing to agro-socio-economic welfare.

Dynamics of the Aspiration of Charged Droplets into a LC-ESI-MS System.

Electrospray ionization (ESI) enables coupling between liquid chromatography (LC) and mass spectrometry (MS). Since it is a gentle ionization method, it is frequently used for the analysis of large biomolecules. In recent years, several experimental setups have demonstrated that the use of ESI results in the formation of charged droplets that are aspirated into the vacuum systems of mass spectrometers. This results in a variety of consequences, such as instrument contamination, which can impede the analytical performance. We investigate the signatures of aspirated charged droplets with a commercial LC-ESI-MS system at analytical conditions. Previous observations without LC coupling are reproduced and show that significant droplet aspiration is probably taking place at analytical LC-ESI-MS conditions. This common phenomenon likely decreases the instrument sensitivity. Analyte can be released by isolation and fragmentation of droplet fragments; thus, aspirated droplets can mask analyte even in the mass analyzer region. The complex morphology of droplet MS/MS mass spectra is highly reproducible at the same experimental conditions. This implies the existence of distinct molecular reaction pathways of the droplet fragments. To assess the effect of droplet aspiration on analytical applications, relevant method and ion source parameters, which are commonly varied during method optimization, were investigated. Further variations of the solvent composition revealed that the aspirated droplets and their fragmentation are particularly sensitive to the solvent composition and thus also to the LC solvent gradient in an analytical experiment.

Are Hydroxyl Radicals Spontaneously Generated in Unactivated Water Droplets?

Spontaneous ionization/breakup of water at the surface of aqueous droplets has been reported with evidence ranging from formation of hydrogen peroxide and hydroxyl radicals, indicated by ions at m/z 36 attributed to OH•‐H3O+ or (H2O‐OH2)+• as well as oxidation products of radical scavengers in mass spectra of water droplets formed by pneumatic nebulization. Here, aqueous droplets are formed both by nanoelectrospray, which produces highly charged nanodrops with initial diameters ~100 nm, and a vibrating mesh nebulizer, which produces 2 – 20 µm droplets that are less highly charged. The lifetimes of these droplets range from 10s of µs to 560 ms and the surface‐to‐volume ratios span ~100‐fold range. No ions at m/z 36 are detected with pure water, nor are significant oxidation products for the two radical scavengers that were previously reported to be formed in high abundance. These and other results indicate that prior conclusions about spontaneous hydroxyl radical formation in unactivated water droplets are not supported by the evidence and that water appears to be stable at droplet surfaces over a wide range of droplet size, charge and lifetime.

Nanopore ion sources deliver individual ions of amino acids and peptides directly into high vacuum

Electrospray ionization is widely used to generate vapor phase ions for analysis by mass spectrometry in proteomics research. However, only a small fraction of the analyte enters the mass spectrometer due to losses that are fundamentally linked to the use of a background gas to stimulate the generation of ions from electrosprayed droplets. Here we report a nanopore ion source that delivers ions directly into high vacuum from aqueous solutions. The ion source comprises a pulled quartz pipette with a sub-100 nm opening. Ions escape an electrified meniscus by ion evaporation and travel along collisionless trajectories to the ion detector. We measure mass spectra of 16 different amino acid ions, post-translationally modified variants of glutathione, and the peptide angiotensin II, showing that these analytes can be emitted as desolvated ions. The emitted current is composed of ions rather than charged droplets, and more than 90% of the current can be recovered in a distant collector.

Multifunctional membranes with super-wetting interface and in-situ Fenton-like sites for efficient oil-in-water emulsion separation

The surge in oil-in-water (O/W) emulsions poses a threat to the ecological environment. Membrane separation technology can achieve complete O/W emulsion separation through size sieving; however, its efficiency is limited by membrane fouling. In this study, an efficient O/W separation membrane with passive and positive anti-fouling properties was fabricated using Fe3O4 functionalized attapulgite (ATP) nanofibers. The anti-fouling mechanism of the multifunctional membranes for the separation of various O/W emulsions was studied. During separation process, a hydration layer, which mitigates oil adhesion with binding energy as low as −169 kcal·mol−1, forms on the interface of Fe3O4-ATP. When a cationic surfactant is present, positively charged oil droplets are attracted to the membrane surface, where they compress against each other, leading to demulsification and escape. Synergistic effects including electrostatic shielding, interaction energy, and steric hindrance were demonstrated through characterization and simulation. Our findings showed that the membrane exhibited a high filtration efficiency with an oil rejection of over 99 % and a steady flux of over 73 % of its initial value. After filtration, oil fouling degraded into smaller sizes and escaped from the membrane structure through the Fenton-like catalytic function of Fe3O4, resulting in a flux recovery ratio of up to 90 %. This study provides a reference for the construction of multifunctional membranes for efficient separation processes and advancing anti-fouling research on membrane interfaces.

Microencapsulation of turmeric oleoresin: complex coacervation and crosslinking strategies for improving encapsulation efficiency and stability in gastrointestinal simulated fluids

Turmeric oleoresin microcapsules were prepared by complex coacervation of gum arabic (GA) and chitosan (Ch) using genipin as a crosslinker. For this propose, the effect of pH and the biopolymer mass ratio ( R GA / Ch pH ) on the zeta-potential and coacervate yield and their viscoelastic properties were evaluated. A liquid-like behavior was found for all the samples in the small- and large-amplitude oscillatory shear regions, with sample R 5 4.5 having the highest viscoelastic properties. Samples R 6 3 , R 5 4.5 and R 4 6 with the highest coacervate yield (∼98%) for each pH chosen to produce the microcapsules. Then, turmeric oleoresin-gum arabic emulsions were produced and characterized. The pH affected the droplet size and charge. At pH 3.0, the droplets were about 50 nm and had a negative charge. At pH 4.5 and 6.0, the droplets were larger but more negatively charged. These emulsions were coated with chitosan and crosslinked with genipin were lyophilized to make microcapsules, showing high encapsulation efficiency (>98%) and low surface oil content (4.5–9.0%). The crosslinked turmeric oleoresin microcapsules showed lower swelling and water uptake in gastrointestinal media than did the uncrosslinked powders. The results of this research will contribute to the knowledge of coacervate systems for nutraceutical applications.

Effect of polyelectrolyte (PAA) on the dynamics of weakly charged microemulsion droplets in acidic medium

We use the dynamic light scattering technique (DLS) to study the dynamic behavior of complex constituted by weakly charged microemulsion in the presence of polyelectrolyte. We investigated the impact of adding oppositely charged polyacid (PAA), which is a pH-dependent polyelectrolyte, on the dynamics of charged microemulsion nanodroplets in an acidic medium at pH = 4.5 when the polyacrylic acid are partially charged. In a diluted regime, two diffusive relaxation modes are seen when adding (upon addition) the polyacrylic acid PAA to the microemulsion nanodroplets due to collective diffusion and self-diffusion fluctuations. In the concentrated regime, we observed three diffusive relaxation modes when charged microemulsion nanodroplets complexed with oppositely charged polyacrylic acid. We attribute the two first relaxation modes to the collective diffusion (fluctuation of concentration) and self-diffusion of the microemulsion nanodroplets. The third mode which is a slower relaxation mode; we attribute this relaxation process to the motion of the network formed by the polyacid PAA and microemulsion nanodroplets, or “breathing mode” of the network.

Minimal Peptide Sequences That Undergo Liquid-Liquid Phase Separation via Self-Coacervation or Complex Coacervation with ATP.

The simple (self-)coacervation of the minimal tryptophan/arginine peptide sequences W2R2 and W3R3 was observed in salt-free aqueous solution. The phase diagrams were mapped using turbidimetry and optical microscopy, and the coacervate droplets were imaged using confocal microscopy complemented by cryo-TEM to image smaller droplets. The droplet size distribution and stability were probed using dynamic light scattering, and the droplet surface potential was obtained from zeta potential measurements. SAXS was used to elucidate the structure within the coacervate droplets, and circular dichroism spectroscopy was used to probe the conformation of the peptides, a characteristic signature for cation-π interactions being present under conditions of coacervation. These observations were rationalized using a simple model for the Rayleigh stability of charged coacervate droplets, along with atomistic molecular dynamics simulations which provide insight into stabilizing π-π stacking interactions of tryptophan as well as arginine-tryptophan cation-π interactions (which modulate the charge of the tryptophan π-electron system). Remarkably, the dipeptide WR did not show simple coacervation under the conditions examined, but complex coacervation was observed in mixtures with ATP (adenosine triphosphate). The electrostatically stabilized coacervation in this case provides a minimal model for peptide/nucleotide membraneless organelle formation. These are among the simplest model peptide systems observed to date able to undergo either simple or complex coacervation and are of future interest as protocell systems.

Statistical‐Mechanical Theory on the Probability Distribution Function for the Net Charge of an Electrolyte Droplet

AbstractDroplets of electrolyte solutions in an insulating medium are ubiquitous in nature. The net charges of these droplets are normally nonzero, and they fluctuate. However, a theory on the probability distribution function for the net charge of droplets is lacking, so far. Thus, a statistical‐mechanical theory of a charged droplet is developed including the effect of the electrostatic energy of charging as well as the random distribution of ions. Then, the probability distribution function for the net charge of an electrolyte droplet is calculated assuming that it is generated from a macroscopic solution with the different cation and anion concentrations. Using the Gaussian approximation and Stirling's formula, the analytic results for the average and variance of the net charge of a droplet are obtained.

Critical charges for droplet collisions

Two micron-sized water droplets approaching each other do not always coalesce due to the cushioning effect of the air between them. When the droplets do not carry any electrical charges, one needs to consider the breakdown of hydrodynamics at very small scales to decide whether the droplets collide and coalesce or not. In contrast, two approaching droplets that are oppositely charged always coalesce if the charges are large enough. To find the charge for which the transition to charge-dominated collisions occurs, we computed the collision efficiency of charged, hydrodynamically interacting droplets settling in quiescent air, including the noncontinuum regime at small interfacial distances. For oppositely charged droplets, we find that the transition occurs when a saddle point of the relative droplet dynamics exits the region within which the continuum hydrodynamics breaks down. For cloud droplets with radii 16 and 20µm, we observe that the transition occurs at ∼103 elementary charges e. For charges smaller than this, we predict that the coalescence rate depends primarily upon the Knudsen number (Kn, the ratio of the mean-free-path of air to the mean droplet radius), whereas coalescence for much larger charges does not depend upon Kn. For droplets charged with the same polarity, we find the critical charge to be substantially larger (∼104e for the above radii) for reasons that we discuss. Published by the American Physical Society 2024