Vortex Nucleation and the Levitation of Charged Helium II Drops

Intrinsic nucleation of quantized vortices in Helium II can be studied by means of rotating freely suspended superfluid drops at angular velocities above some critical value. The motivation for doing so is described, as well as recent progress in the electrostatic levitation of Helium II drops charged with positive ions. To date, stable levitation has been achieved for drops of order 100–150 micrometers in diameter, with a surface charge density about a factor of ten smaller than Rayleigh limit, and a diameter a similar factor less than the maximum allowed in normal gravity. We discuss the possibility of rotating these drops via the surface charge density and discuss the advantages of a microgravity environment, including the attainment of significantly larger suspended drops. Recent efforts to find optical seed particles for angular velocity measurements are discussed.     

Controlling charge on levitating drops

Levitation technologies are used in containerless processing of materials, as microscale manipulators and reactors, and in the study of single drops and particles. Presented here is a method for controlling the amount and polarity of charge on a levitating drop. The method uses single-axis acoustic levitation to trap and levitate a single, initially neutral drop with a diameter between 400 microm and 2 mm. This drop is then charged in a controllable manner using discrete packets of charge in the form of charged drops produced by a piezoelectric drop-on-demand dispenser equipped with a charging electrode. The magnitude of the charge on the dispensed drops can be adjusted by varying the voltage applied to the charging electrode. The polarity of the charge on the added drops can be changed allowing removal of charge from the trapped drop (by neutralization) and polarity reversal. The maximum amount of added charge is limited by repulsion of like charges between the drops in the trap. This charging scheme can aid in micromanipulation and the study of charged drops and particles using levitation.     

Particle electrification and levitation in a continuous particle feed and dispersion system with vibration and external electric fields

Electrification and levitation of particles in a continuous particle feed and dispersion system have been studied both theoretically and experimentally. This system consisted of a vibrator and inclined parallel electrodes. A mesh and a vibrating plate were used for the upper and lower electrodes, respectively. A dc voltage was applied to one of the electrodes and the other electrode was grounded. Particles fed to the lower electrode were charged by induction and levitated upward by the Coulomb forces. When the applied voltage was high enough, the particles passed through the mesh electrode. The charge of the particles was measured with a Faraday cup, and the particle behavior was observed with a high-speed microscope camera. The particle charges were also analyzed from experimentally obtained particle trajectories and numerically calculated electric fields. Finally, the conditions for the effective levitation and dispersion of the charged particles and their mechanisms were studied and have been described in detail.     

Quantitative analysis of agglomerates levitated from particle layers in a strong electric field

The electrification, agglomeration, and levitation of particles in a strong electric field were analyzed experimentally and theoretically. Particle layers of glass, alumina, and ferrite were formed on a plate electrode and an external voltage was applied. Microscopic observations of the agglomerates levitated from the particle layers revealed that the number of primary particles constituting an agglomerate is affected by particle diameter and electrical resistance, but not by the applied electric field. The electric field distributions in the system were calculated by considering the charges and geometries of the agglomerates formed on the particle layers. The charges of the agglomerates were obtained experimentally. All forces acting on the agglomerates (i.e., gravitational forces, Coulomb forces, interaction forces between polarized particles, image forces, and gradient forces) were analyzed under different conditions, including various electric field distributions and charges of agglomerates. Furthermore, the critical conditions for the levitation of the agglomerates were evaluated using a force balance.     

Oscillations of Charged Helium II Drops

We report on an experimental study of the shape oscillations of charged helium drops levitated with a magnetic field. Shape oscillations are excited with an AC electric field. Many different modes of oscillation of the drop are observable. The resonant frequencies of the drops are found to be a function of amplitude. Quantitative measurements of the damping of shape oscillations are made by using a laser beam focused through the drop. The observed damping of shape oscillations is found to be greater than the damping due to the viscosities of the liquid and the surrounding vapor. Other mechanisms possibly responsible for this damping are discussed. We also report experiments on drops with angular momentum.     

A novel methodology to study shape and surface tension of drops in Electric Fields

A novel methodology is introduced that can be used to study the behavior of conducting drops in electrostatic fields, when gravity effects are negligible. This methodology, called Axisymmetric Drop Shape Analysis — Electric Field (ADSA-EF), generates numerical drop profiles in the electrostatic field, for a given surface tension. Then, it calculates the true value of the surface tension by matching the theoretical profiles with the shape of the experimental drops, with the surface tension as an adjustable parameter. ADSA-EF can be employed for simulating drop shapes in the electric field, detecting the effect of an electric field on liquid surface tensions, and measuring surface tensions in microgravity, where current drop-shape techniques are not applicable. The predicted drop shapes in the electric field were compared with experimental images, indicating good agreement. Preliminary experiments according to ADSA-EF methodology suggested that the surface tension of water increases by about one percent in the electric field.     

Impurity ions in liquid helium

A technique for introducing a wide variety of positive ions into liquid helium is described and used to investigate positive atomic ions of K, Rb, Cs, Ca, Sr, and Ba. The zero-field mobilities of the impurity ions were measured in He II in the temperature range 1.2 < T < 1.4 K. The dependence of mobility on ion mass alone was found directly for Ca⁺ ions by investigating two isotopes. Measurements were made of the ion drift velocities in high electric fields, where nucleation of quantized vortex rings occurs. All the data and its analysis are consistent with the assumptions that all the impurity ions are singly charged, that the size of the cluster surrounding the ion is significantly different for the different ions, and that the ion-roton cross section is approximately geometric. A possible alternative model for the structure of the ion complex is discussed and a mechanism is suggested which may account for the observed variations in ionic radii.     

Molecular dynamics simulation of polarizable carbon nanotubes

This paper is aimed to develop a modified force field for molecular dynamics (MD) simulations of polarizable carbon nanotubes (CNTs). The effects of electrical polarization and the associated electronic degrees of freedom are represented by a network of negative charged shell particles which move relative to the surrounding positively charged carbon atoms in response to an applied electric field. In this setting, the negative and positive charges are exactly balanced so that the total system remains electrically neutral, and the motion of the shell particles relative to their equilibrium positions leads to polarization within the nanotube. Potential applications of the proposed model include simulations of controlled translocation of ions, water and polymers through solid-state CNT membranes.     

Magnetic Levitation of liquid helium

We report on the stable levitation of liquid helium drops of up to 2 cm diameter in a magnetic trap at temperatures down to 1.5 K in the earth's gravitational field. The production and properties of a magnetic trap for diamagnetic materials is discussed. The behavior of liquids in such a trap is analyzed, including the deformation of a liquid drop by the trap forces. We frequently observe two drops in the magnetic trap which are held in apparent contact for up to 3 minutes without coalescing. This non-coalescence effect was only seen above the superfluid transition temperature. We explain this effect in terms of the existence of a vapor layer between the drops caused by evaporation of the drops, much like the suspension of a liquid drop above a hot surface known as the Leidenfrost effect.     

Rapid preparation of crystalline colloidal arrays using a strong electric field dialysis

We present a simple method for rapid preparation of crystalline colloidal arrays (CCAs) by a strong electric field dialysis (SEFD). This method is based on rapid removing of ionic impurities in colloidal suspensions by applying a strong electric field. In a SEFD process, the negatively charged ions in colloidal suspensions are rapidly driven to the anode, the positively charged ones are rapidly driven to the cathode, and the colloidal particles are withheld in the dialysis tube. It was shown that the colloidal particles aggregated on the wall of the dialysis tube could block the SEFD process, which could be overcome by reversing the direction of the electric field. The purified colloidal particles can self-assemble into a crystalline colloidal array, which has an electrostatically stabilized three-dimensional periodic array of colloidal particles with a characteristic lattice spacing that can be varied by dilution. The reflection spectra show distinct peaks due to diffraction from CCAs. Atomic force microscopy (AFM) image illustrates the non-contacted ordering of the colloidal particles in the CCAs embedded in gels. This indicates the formation of high-quality single CCAs. Using a SEFD method, the preparation time of CCAs can be reduced. This new technique will greatly speed up the process of preparing polymerized crystalline colloidal arrays (PCCAs) into real-world application in the analytical field.     

Shape Oscillations in Levitated He II Drops

We describe experiments to study the shape oscillations of levitated He II drops. Drops of approximately 0.5 cm diameter are levitated magnetically with a superconducting solenoid, and shape oscillations are induced with an ac electric field. We have measured the damping of shape oscillations as a function of temperature. The damping rate is compared to that predicted by a two fluid, hydrodynamic model, which takes account the effect of the motion in the vapor. The effects of condensation and evaporation on the motion of the drop are also considered.     

The interaction between charged dust particles calculated in Cassini coordinates

The Poisson-Boltzmann equation for two charged dust particles surrounded by a cloud of charged plasma was numerically solved using the Cassini coordinates. The force of the electric field acting on each particle was determined by integrating the field pressure over the particle surface. It is shown that the dust particles can exhibit attraction at distances on the order of the Debye screening radius and the average spacing between particles. The attraction drops sharply if the particle charge is significantly smaller than the charge of surrounding plasma species.     

Single nanodimensional emitting protrusion on the surface of a tungsten carbide field emitter

Variations in the shape of a tungsten carbide emitter under the simultaneous action of strong electric fields and high temperatures have been studied by field emission microscopy techniques. Using controlled decrease in the applied electric field strength at a certain temperature of the emitter, it is possible to grow a single nanodimensional surface protrusion capable of emitting charged particles with stability comparable to that of carbon-based materials. The values of emission currents, current densities, emission angles, and reduced brightnesses are comparable to those known for emitters based on carbon nanotubes. Advantages of single nanoprotrusions as emitters are their complete reproducibility and the ability to emit both electrons and ions     

Charged Surface of Liquid Hydrogen at Near Zero Gravitation

The results of investigations of the evolution of the shape of the equipotentially charged surface of a liquid hydrogen layer in the external electric field in the case when the effective gravitation acceleration is near to zero are presented. The reconstruction phenomenon—the formation of a solitary wave at the flat charged surface of liquid hydrogen at some critical value of the external electric field—has been studied under the conditions of a total screening of the electric field in the bulk of a liquid by a surface charge. From the results obtained it follows that the transition of the flat surface into the reconstructed state is close to a second order phase transition. A voltage-temperature U–T phase diagram of the charged surface has been determined. The statement of a soft character of the transition has been confirmed also by the results of studies of the oscillations spectrum of the charged surface in electric fields lower and higher the critical value.     

乳状液电磁场破乳法的研究

提出了一种利用电磁感应原理产生涡旋电场进行破乳的新方法。将螺线管、磁芯构成的电感器与电容器串联组成Ｌ－Ｃ谐振电路。在此电路中通过高频电流，使其在破乳器内产生高频磁场，从而感生出高频涡旋电场。在此电场的作用下，乳状液滴极化并加速其运动与碰撞，达到破乳的目的。该法在输入信号电压约３００Ｖ、电流几＋ｍＡ、频率２０ｋＨｚ的条件下，可感应出强度为几十＋ｋＶ／ｃｍ的涡旋电场，５ｍｉｎ内静态破乳率大于９８％。在     

Experimental and numerical modeling of particle levitation and movement behavior on traveling-wave electric curtain for particle removal

Traveling-wave electric curtain (EC) has been developed for potential application in particle removal/shield on solar panels and other surfaces. Levitation and transport of a particle in a traveling-wave electric field were simulated. Results show that levitation directions/angles and levitation trajectories differ because of the difference in starting positions and starting times. The particles in the two positive acceleration regions are levitated in opposite directions, and the particles distributed on the dielectric surface are levitated and transported successively rather than simultaneously. Movement trajectories are complex and affected by various factors. In the current paper, movement trajectories are modeled to analyze which motion modes are advantageous or disadvantageous to particle removal. This process is beneficial to elucidate the mechanism of particle removal and provide a guidance for movement control by designing appropriate operating parameters.     

Affinity two-phase partitioning in acoustically levitated drops

Miniaturized (<1 microL) biospecific affinity two-phase partitioning in an acoustically levitated drop is described. Miniaturization commonly gives unfavorable surface/volume ratios, but in the levitation approach adsorption problems are minimized since the only surrounding wall is the liquid/air interface of the drop. Biotinylated liposomes were partitioned in aqueous poly(ethylene glycol)/dextran two-phase drops with NeutrAvidin-dextran as the affinity ligand. A two-phase drop was trapped and manipulated in a node of a standing ultrasonic wave. Alternatively, a two-phase system was formed by levitation and evaporation of a polymer one-phase drop. Phase mixing was achieved by adjusting the ultrasonic field and phase separation by readjusting the field. NeutrAvidin-dextran brought about the redistribution of biotinylated liposomes from the poly(ethylene glycol)-rich phase into the dextran-rich phase. Thus, an entire affinity two-phase separation procedure, including mixing of the phases and incubation to allow affinity interactions to develop under constant volume, followed by phase separation under controlled evaporation, can be performed in a single levitated drop. This miniaturized technique would allow the separation of biologically active membranes or organelles from individual cells for analysis.     

Field-amplified sample injection combined with water removal by electroosmotic flow pump in acidic buffer for analysis of phenoxy acid herbicides by capillary electrophoresis

A procedure that combines two common stacking techniques, field-amplified sample injection and water removal, with an electroosmotic flow pump, is used to separate phenoxy acid herbicides by capillary zone electrophoresis. Before sample loading, a long plug of water was hydrodynamically injected into the capillary both to serve as the medium to permit a high electric field strength and to contain sample anions. Because of this long length of water, the number of ions injected into the capillary was greatly increased. Electrokinetic injection at reversed voltage was then used for introducing negatively charged ions from the diluted sample into the column. The water was removed from the capillary using the electroosmotic flow (EOF) pump when the EOF of the background electrolyte was suppressed. This method afforded a sensitivity enhancement of greater than 3,000 times. Combined with solid-phase extraction, detection limits for the phenoxy acid herbicides as low as 0.01 ng/mL could be achieved.     

Protein separation by electrophoretic-electroosmotic focusing on supported lipid bilayers

An electrophoretic-electroosmotic focusing (EEF) method was developed and used to separate membrane-bound proteins and charged lipids based on their charge-to-size ratio from an initially homogeneous mixture. EEF uses opposing electrophoretic and electroosmotic forces to focus and separate proteins and lipids into narrow bands on supported lipid bilayers (SLBs). Membrane-associated species were focused into specific positions within the SLB in a highly repeatable fashion. The steady-state focusing positions of the proteins could be predicted and controlled by tuning experimental conditions, such as buffer pH, ionic strength, electric field, and temperature. Careful tuning of the variables should enable one to separate mixtures of membrane proteins with only subtle differences. The EEF technique was found to be an effective way to separate protein mixtures with low initial concentrations, and it overcame diffusive peak broadening to allow four bands to be separated simultaneously within a 380 μm wide isolated supported membrane patch.     

Iodine Vacancy Redistribution in Organic–Inorganic Halide Perovskite Films and Resistive Switching Effects

Organic–inorganic halide perovskite (OHP) materials, for example, CH₃NH₃PbI₃ (MAPbI₃), have attracted significant interest for applications such as solar cells, photodectors, light‐emitting diodes, and lasers. Previous studies have shown that charged defects can migrate in perovskites under an electric field and/or light illumination, potentially preventing these devices from practical applications. Understanding and control of the defect generation and movement will not only lead to more stable devices but also new device concepts. Here, it is shown that the formation/annihilation of iodine vacancies (V_I's) in MAPbI₃ films, driven by electric fields and light illumination, can induce pronounced resistive switching effects. Due to a low diffusion energy barrier (≈0.17 eV), the V_I's can readily drift under an electric field, and spontaneously diffuse with a concentration gradient. It is shown that the V_I diffusion process can be suppressed by controlling the affinity of the contact electrode material to I^? ions, or by light illumination. An electrical‐write and optical‐erase memory element is further demonstrated by coupling ion migration with electric fields and light illumination. These results provide guidance toward improved stability and performance of perovskite‐based optoelectronic systems, and can lead to the development of solid‐state devices that couple ionics, electronics, and optics.