Charged Droplets Research Articles

Translation-deformation coupling effects on the Rayleigh instability of an electrodynamically levitated charged droplet.

The breakup pathway of the Rayleigh fission process observed in the past experiments carried out using high-speed imaging of a charged drop levitated in an AC quadrupole trap has shown to exhibit several cycles of shape and center-of-mass oscillations followed by asymmetric breakup by ejecting a jet in the upward direction (i.e., opposite to the direction of gravity). We recently attempted to explain this using boundary integral simulations in the Stokes flow limit, wherein the position of the droplet and the polarity of the end cap electrodes were assigned using physical arguments, and the center-of-mass motion was not estimated consistently invoking quasi-static conditions. In this work, we explain the experimental observation of upward breakup of charged droplets in a quadrupolar field, using numerical calculations based on the boundary element method considering inviscid droplets levitated electrodynamically using quadrupole electric fields. The center-of-mass motion and the end cap are consistently calculated in the numerical scheme. The simulations show that the gravity-induced downward shift in the equilibrium position of the drop in the trap causes significant, large-amplitude shape oscillations superimposed over the center-of-mass oscillations of the drop. An important observation here is that the shape oscillations due to the applied quadrupole fields result in sufficient deformations that act as triggers for the onset of the instability below the Rayleigh limit, thereby admitting a subcritical instability. The center-of-mass oscillations of the droplet within the trap, which follow the applied frequency, are out of phase with the applied AC signal. Thus the combined effect of shape deformations and dynamic position of the drop leads to an asymmetric breakup such that the Rayleigh fission occurs upward via the ejection of a jet at the north pole of the deformed drop.

A novel formulation of simvastatin nanoemulsion gel for infected wound therapy: In vitro and in vivo assessment

Simvastatin, a well-known antihyperlipidemic drug, has antibacterial activity against a broad range of bacteria, especially Staphylococcus aureus. In present study, a nanoemulsion gel-based formulation containing Simvastatin was developed for infected wound therapy. Therefore, different formulations of Simvastatin nanoemulsion were prepared. Based on droplet size, polydispersity index and zeta potential, the best nanoemulsion formulation containing Simvastatin was selected for development of nanoemulsion gel formulation of drug using carbomer 934 as gelling agent. Thermodynamic stability of Simvastatin nanoemulsion was assessed at different conditions. The in vitro antibacterial activity against S. aureus as well as in vivo wound healing efficacy of both Simvastatin nanoemulsion and its nanoemulsion gel formulation were assessed.The droplet size, PDI, and zeta potential of selected Simvastatin nanoemulsion formulation were 75 nm, 0.3, and −29.4, respectively. The physical characteristics of nanoemulsion such as droplet size, charge and PDI did not change significantly in nanoemulsion gel formulation with viscosity of 11.12 Pas. Selected formulations of nanoemulsion and nanoemulsion gel of simvastatin were stable at 4 and 25 °C for 72 days and Minimum Inhibitory Concentration values of both nanoemulsion and nanoemulsion gel of drug against pathogenic strains S. aureus was 15.52 μg/ml which is two-fold lower than drug solution (31.25 μg/ml). Histological findings in mouse animal model of wound healing demonstrated the superiority of nanoemulsion gel belong other formulations.

The first micro thrust measurement of the ultrasonic electric propulsion system

The Ultrasonic Electric Propulsion system is mainly used in space missions within the limits of 50 kg, and employs capillary standing waves as emitters to extract charged droplets in electric field. Due to multi-physics field such as ultrasonic vibration, liquid mechanics and electrostatic field in the Ultrasonic Electric Propulsion system, its thrust prediction always faces a big challenge. In this work, the thrust of Ultrasonic Electric Propulsion system is first measured by a measurement platform, and the test results are 565.23 μN, 715.65 μN and 842.73 μN with an accuracy in the order of 10% at 5 kV, 7 kV and 9 kV, and a flow rate of 5 ml/h and ultrasonic vibration frequency of 120 kHz. The test results also indicate that the thrust value increases with extraction voltage in the same flow rate with propellant and vibration frequency. Besides, the vacuum emission plume of Ultrasonic Electric Propulsion system is first investigated by a high-speed imaging technique, which reveals different emission conditions at different time such as parabolic emission trajectory and varied plume angles.

Characterization of Spray Modes and Factors Affecting the Ionization Efficiency of Paper Spray Ionization.

In this study, we systematically evaluated the factors affecting the ionization efficiency of paper spray ionization (PSI), such as electric field, solvent supply rate, and paper thickness and hydrophobicity. The observed paper spray plume was classified into three modes: single cone-jet, multi-jet, and rim-jet modes. With the increase in the spraying voltage, the spray plume appeared in order of single cone-jet, multi-jet, and rim-jet modes. The rim-jet mode exhibited the lowest standard deviation and high ionization efficiency among the three spray modes. The main parameter determining the spray mode was the charge density of the droplets generated by paper spray, which depends on the electric field and solvent supply rate. A thicker paper reduced the electric repulsion between the jets and lowered the threshold voltage to reach the rim-jet mode. Lowering the solvent supply rate caused mode transitions from the single cone-jet to the rim-jet, possibly due to the increased droplet charge density. The hydrophobic modification on a paper substrate led to a different ionization mechanism or electrostatic spray ionization at low applied voltages.

Collisions of micron-sized charged water droplets in still air

We show that fixed points and their stable manifolds determine collision outcomes of oppositely charged water droplets settling in still air. For droplets with large enough opposite charges, the stable manifold of a saddle forms a separatrix that divides colliding trajectories from those that do not collide. A consequence of our findings is that for large charges, collision outcomes are not sensitive to the breakdown of hydrodynamics at small distances.

Diffusiophoresis of a highly charged dielectric fluid droplet induced by diffusion potential

Diffusiophoresis of a dielectric fluid droplet in electrolyte solutions is investigated theoretically, focusing on the electrophoresis component resulting from the induced diffusion potential in the electrolyte solution when the diffusivities of cations and anions there are different. The resultant electrokinetic equations are solved with a pseudo-spectral method based on the Chebyshev polynomials. We found, among other things, that the electrophoresis component dominates at a larger Debye length, whereas the chemiphoresis component at a smaller Debye length for a dielectric droplet of a constant surface charge density. The two components are of comparable magnitudes in the NaCl solution. The dual between the spinning electric driving force tangent to the droplet surface and the hydrodynamic drag force reinforced by the motion-deterring electrokinetic Maxwell traction from the surrounding exterior osmosis flow is crucial in the determination of the ultimate droplet motion. Unlike the chemiphoresis component, which is independent of the sign of charges carried by the droplet, the droplet moving direction as well as its magnitude in the electrophoresis component depends on the sign of charges carried by the droplet as well as the direction of the electric field induced by the diffusion potential. This gives the electrophoresis component excellent maneuverability in practical applications like drug delivery and enhanced oil recovery, where migration of droplets toward regions of higher solute concentrations is often desired.

The influence of charge characteristics of suspension droplets on the ion flow field in different temperatures and humidity

In order to clarify the charging characteristics of suspension droplets in ion flow field under different temperatures and humidity, the effective charging factor used to characterize the charging characteristics of suspension droplets is introduced in this paper, and a calculation method of charging factor is proposed based on the upstream finite element method (FEM). Then, the charging factor under different temperatures and humidity is calculated, and the analytic expression of the charging factor considering the influence of temperature and humidity is obtained by fitting the calculation results. The influence of suspension droplets on the ion flow field is analyzed. The results show that the charging factor is small and increases little with the relative humidity when the relative humidity is less than 60%, and the charging factor is large and increases rapidly with the relative humidity when the relative humidity is more than 60%. At the same relative humidity, the charging factor increases linearly with the temperature. The influence of charged suspension droplets on the ion flow field can be ignored when the relative humidity is less than 60% and must be considered under high temperature and humidity. The calculation method and analytic expression of the charging factor proposed in this paper can be used to model of ion flow field considering the influence of temperature and humidity and provide technical support for the construction of HVDC transmission lines across high temperature and humidity.

Electrostatic Charges Regulate Chemiluminescence by Electron Transfer at the Liquid–Solid Interface

The role of the electrostatic environment in chemical reactions has long been an important research field, but most studies have focused on the influence of external electric fields on chemical processes, while the effect from the intrinsic electrostatic charges on the solution itself has been ignored. How an electrostatic field generated by contact electrification affects the solvent environment in a chemical reaction and then the chemical reactivity is still ambiguous. Here, based on the inspiration of the droplet triboelectric nanogenerator, electrostatic interactions between a statically charged luminol droplet and the surrounding directional electrostatic field were analyzed, and we demonstrate a relationship between the sign of the luminol sample (negatively or positively charged) and its effect on the reaction reactivity. Our results show that the increased reaction activity and the enhanced chemiluminescence (CL) only occurred when the luminol droplet yields positive charges, while a negatively charged luminol, on the contrary, tends to inhibit the CL, which brings direct evidence of the charge carriers of triboelectricity being electrons at the liquid-solid interface. This work provides a strategy for electrostatically regulating CL by simply statically charging a reaction solution with a dielectric solid and also carries a cautionary message on what to consider when preparing a sample for a chemical reaction.

On Electrostatic Spraying of Salty Tap-Water on a Flat Target

Electrospraying is a process that uses electrostatic force to break up a liquid into droplets by using a strong electric field. There are different modes of electrospraying depending on the electric field strength and the liquid flow rate for a specified liquid. The simple-jet mode is among these modes, which can produce monodisperse droplets. This article is aimed at analyzing electrospraying in simple-jet mode of a pesticide—solution issued from the nozzle of a capillary of the spray—system. The analysis includes the formation of a jet charged by conduction at low applied voltages (below corona onset) and by corona discharge at voltages higher than the corona onset value. The electric field distribution in the space between the charged jet and the target is calculated at voltages below and above the corona onset value. The disintegration of the jet into droplets is assessed to determine the jet length and radius as well as the charge and radius of droplets. The current–voltage characteristic of the spray system is calculated and checked experimentally. The agreement between the calculated and measured currents in the spray system at the same applied voltage is satisfactory.

Coherent, time-correlated tunneling of density wave electrons

A growing body of evidence reveals that charge density wave (CDW) transport is a high-temperature cooperative quantum phenomenon. According to the time-correlated soliton tunneling (ST) model, quantum solitons, or electron-phonon correlates within the CDW condensate, act much like electrons tunneling through a Coulomb-blockade tunnel junction. Pair creation of charged fluidic soliton droplets is prevented by their electrostatic energy below a Coulomb-blockade threshold electric field. Above threshold, the quantum fluid flows in a periodic fashion, via a hybrid between Zener-like and coherent Josephson-like tunneling. We summarize the time-correlated ST model and compare model simulations with experiment. The ST model shows excellent agreement with coherent voltage oscillations, and with CDW current-voltage characteristics. Finally, we discuss implications for physics and potential applications.

Coalescence and counterflow of droplets on needle electrode with negative corona discharge

The coalescence of droplets on the discharge electrode surface in high humidity environments has rarely been studied, which may affect discharge characteristics. Meanwhile, directional transport of droplets is of great significance for many applications ranging from fluidic processing to thermal management. Here, corona discharge in a needle-plate electrode is adopted to explore the coalescence rule of droplets attached on the discharge electrode surface in a high-humidity environment, and to realize the counterflow of droplets. The experimental results show that the number of coalesced droplets on the needle electrode surface reaches the maximum under −7.5 kV at relative humidity ∼94% and ambient temperature ∼20 °C. When the applied voltage increases from −6 kV to −11 kV, the droplet moves up 2.76 mm in 5 s. The size of the attached droplet depends on the balance of coalescence and evaporation. The coalescence is mainly attributed to the dielectrophoretic force caused by the high electric field gradient. The evaporation is related to the ionic wind generated by the corona discharge. With regard to the counterflow phenomenon of the droplet, we speculate that the high concentration gradient of positive ions near the needle electrode provides a driving force for the negatively charged droplets. Meanwhile, the electrons and negative ions below the needle tip offer a repulsive force to the droplet. The shape and moving direction of the droplet attached to the needle surface can be manipulated by changing the voltage applied to the needle electrode, which shows the potential application value in realizing the self-cleaning of electrodes, liquid lenses and so on.

The best sensitive single-coordinate interference jet-drop measurement of electric field strength

The article discusses the features of the implementation of highly sensitive single-coordinate measurements of electric field strength (EFS) by using of jet-drop optical measuring systems (JDOMS). The use of moving monodisperse charged droplets of a jet-drop coherent flow, which acquire a displacement depending on the directions of movement, the level of the EFS and of the droplet charge, is discussed. The issues of implementation of stroboscopic interference measurements of droplet displacements under impulsed illumination of their surfaces, used as curved reflectors, are considered in JDOMS. It is shown that measuring the displacements of the reflected oh drop of the laser beam makes it possible to determine the displacements of the reflecting drop and the value of the EFS at the selected coordinate, respectively. The features of the use of liquids with pigments and additives in the form of nanopowders of highly reflective metals, liquid metals (or their alloys) with a low melting point are considered in the JDOMS.

Dynamic coalescence of two charged droplets with deflected angles in the presence of electric fields

The electrocoalescence dynamics of two charged droplets with different deflected angles are studied via molecular dynamics simulations. The simulated results show that under a weak electric field, these adjacent droplets can realize the complete coalescence at the deflected angles of 0° and 50°, but cannot realize that at the deflected angle of 90°. As the electric field strength increases to a critical value, owing to escaped ions, a secondary droplet is ejected from coalescing droplet, namely the partial coalescence. However, as the field strength far exceeds the critical field strength, charge transfer appears between these adjacent droplets by the liquid bridge. These two droplets with enough opposite charges move towards the opposite directions, respectively, such that the formed liquid bridge breaks at the interface again, leading to the appearance of non-coalescence. As a result, the complete-to-non coalescence transition is triggered by the strong electric field.

Generation of Ions from Aqueous Taylor Cones near the Minimum Flow Rate: "True Nanoelectrospray" without Narrow Capillary.

Generating ultrafine charged droplets using electrospray is crucial for attaining high ionization efficiency for mass spectrometry. The size of the precursor charged droplets depends on the spray flow rate, and conventional wisdom holds that an electrospray of a nL/min flow rate (nanoelectrospray) is only possible using narrow capillaries with an inner diameter of ∼1 μm or smaller. Here, the electrospray of aqueous solutions with high electric conductivities generated from a large off-line capillary of 0.4 mm i.d. has been performed using a high-pressure ion source. The electric discharge is avoided by operating the ion source at 2.5 bar gauge pressure. The highly stable Taylor cone can be tuned to a near-hydrostatic state that exhibits the "true nanoelectrospray" properties, i.e., high salt tolerance and minimal ion suppression. The Q1/2 scaling law describing the electrospray current I and flow rate Q is found to be valid down to the nanoflow regime under a condition that is free of electric discharge. For a given solution, the flow rate and the size of the initial droplets and ionization species can be controlled with the spray current as the indicator for the instantaneous flow rate without changing the emitter capillary of different sizes. In regard to the application, the nanoelectrospray with a large micropipette tip is easy to use, free of clogging when dealing with viscous and high-salt buffer solutions, and with reduced surface interaction with the emitter inner surface. An acquisition of very clean mass spectra of proteins from concentrated solutions of nonvolatile salts such as phosphate-buffered saline is demonstrated.

Enhanced Sunscreen Effects via Layer-By-Layer Self-Assembly of Chitosan/Sodium Alginate/Calcium Chloride/EHA.

The sunscreen nanocapsules were successfully synthesized by the way of layer-by-layer self-assembly using charged droplets (prepared by emulsification of LAD-30, Tween-80 and EHA (2-Ethylhexyl-4-dimethylaminobenzoate)) as templates. Chitosan/sodium alginate/calcium chloride were selected as wall materials to wrap EHA. The emulsions with the ratio of Tween-80 to EHA (1:1) were stable. A stable NEI negative emulsion can be obtained when the ratio of Tween-80 and LAD-30 was 9:1. Chitosan solutions (50 kDa, 0.25 mg/mL) and sodium alginate solutions (0.5 mg/mL) were selected to prepare nanocapsules. The nanocapsules were characterized via some physico-chemical methods. Based on the synergistic effects of the electrostatic interaction between wall materials and emulsifiers, EHA was effectively encapsulated. DLS and TEM showed that the sunscreen nanocapsules were dispersed in a spherical shape with nano-size, with the increasing number of assembly layers, the size increased from 155 nm (NEI) to 189 nm (NEII) to 201 nm (NEIII) and 205 nm after solidification. The release studies in vitro showed sustained release behavior of the nanocapsules were observed with the increase of the number of deposition layers, implying a good coating effect. The sunscreen nanocapsules could control less than 50% the release of EHA after crosslinking of calcium chloride and sodium alginate, which also could effectively avoid the stimulation of the sun protection agent on the skin.

On Influence of Liquids Viscosity on Drops Formation Regularities During Realization of Electrohydrodynamic Instability

In a theoretical analytical way, the peculiarities of the influence of the viscosity of a liquid on its electro-dispersion, from the end of the capillary, through which the liquid is supplied to the discharge system, or during the decay of a strongly charged drop, were investigated. It was shown that when a low-viscosity liquid was electrodispersed, it emited highly charged small droplets, initially unstable with respect to its own charge, breaking up into a couple of a hundred even smaller and highly charged ones, around which a corona discharge was ignited. As a result, a fan-shaped glow appeared at the top of the meniscus of the liquid at the end of the capillary or at the top of the decaying charged drop. For a viscous fluid, a series of successive decays of charged daughter droplets were immediately interrupted due to the damping effect of viscosity after a single split in half and the formation of self-charge-resistant droplets, and no corona discharge flare was formed.

Electro-osmotic and viscous effects upon pressure to drive a droplet through a capillary.

A charged oil droplet advancing into a charged capillary is considered, assuming the special case in which charges are opposite and equal. The droplet is surrounded by an aqueous phase that wets the capillary wall, such that a thin film adjacent to the wall is laid down as the droplet advances. Electro-osmotic conjoining pressures contrive to make the film even thinner than in an uncharged case. The pressure drop needed to drive the droplet along is examined. The pressure drop is dominated by capillarity but contains electro-osmotic and viscous corrections. The viscous correction is shown to be remarkably insensitive to the presence of electro-osmotic effects. The electro-osmotic pressure correction is negative, reflecting work done by the electro-osmotic conjoining pressure as film is laid down. The negative electro-osmotic correction to pressure drop can far exceed the positive viscous correction. As a result, in the presence of conjoining pressures, a droplet can be driven along a capillary channel with even less pressure drop than is seen for a static uncharged droplet.

On the Percolation Regime of the Bulk Transport of the Bi1.08Sn0.02Sb0.9Te2S Topological Insulator

The formation of nanosized charge droplets in the bulk of a three-dimensional topological insulator Bi1.08Sn0.02Sb0.9Te2S was evidenced using electron spin resonance. Since electrons and holes are “locked” in these droplets far apart from each other, their participation in bulk conductivity is possible only due to thermal activation or to variable range hopping. Our transport measurements give evidence that at relatively high temperatures conduction follows the activation behavior with the activation energies which due to percolation may be much smaller than the half of the band gap. We also demonstrate that the variable range hoping replaces the activated transport at sufficiently low temperatures.

Improved Continuous Inkjet for Selective Area Coating Using High‐Viscosity Insulating Inks

While drop‐on‐demand (DOD) inkjet has been broadening its applications, especially in manufacturing sectors such as printed electronics, continuous inkjet (CIJ) has been limited to use in conventional graphics and marking areas. So far, there have been few attempts to use functional materials in CIJ printing despite the advantages of its high‐speed printing capability and relatively low implementation costs. Herein, the performance of CIJ in terms of printable ink viscosity, print area, and printing speed is significantly improved. For demonstration, selective area printing methods using insulation inks are presented. The selective area coating is very difficult to implement since charged droplets becoming closer together can increase undesirable charge repulsion effects among them. To reduce the charge interaction among the charged droplets and maximize the printing efficiency, a new voltage driving scheme for selective area coating applications is proposed.

Electrospray propulsion: Modeling of the beams of droplets and ions of highly conducting propellants

This article presents a model for the beams of charged droplets and ions typical of electrospray thrusters. The model is applied to a typical highly conducting propellant operating in the mixed droplet-ion emission regime but can be extended to simulate other emission regimes. A key feature of the model is the separation of the beam in two zones: a small inner region including the droplet emission area, where the equations of motion of all droplets are integrated simultaneously while retaining direct droplet-on-droplet Coulomb interactions and a much larger outer region where the beam is treated as a continuum. In addition to the geometry of the electrodes, inputs to the model include the distribution of diameters and charge-to-mass ratios of the droplets and their initial positions and velocities. Most of this information is determined from experiments. The analysis of the numerical solution confirms that lateral oscillations of the slender jet producing the droplets is an important factor in the expansion of electrospray beams, and that droplets in flight are the main sources of ion emission in these propellants. In addition to improving the knowledge of the physics of electrospray beams, the model is a significant tool for designing the extractor and accelerator electrodes of electrospray thrusters and for minimizing the deposition of beam particles on the surfaces of spacecrafts.