Electrohydrodynamic Modes Research Articles

Analysis of beam currents under an oscillating cone-jet mode for developing high-precision electrospray thrusters

As an application of electrospray, electrospray thrusters are preferred to work in a steady electrohydrodynamic mode to output high-precision and adjustable thrust through controlling voltages and flow rates. However, voltages and flow rates can only be modified in a small range to avoid mode transiting, thereby limiting the thrust range. To increase the thrust, adding the number of emitters or more grids for beam acceleration is considered, but it inevitably increases mechanical complexities and risks of component failure. In this paper, electrosprays are investigated by analyzing the beam current over the time and frequency domains. Results suggest that the steady cone-jet transfers into an oscillating cone-jet with a frequency of several kHz as the flow rate increases. In the oscillating cone-jet mode, beam currents and flow rates still follow the power-law relation, which is derived for the steady cone-jet. The finding suggests that setting a desired thrust over the oscillating cone-jet mode is feasible, whose maximum thrust allowed is 2.3 times the one in the steady cone-jet mode. The calculated thrust noises of the oscillating cone-jet mode are lower than 0.1 μN/Hz0.5 in the millihertz band, which meets the requirement of most drag-free satellites. The oscillation frequency is also adjustable by changing voltages or flow rates, offering another control parameter for electrospray devices. Compared to other instable modes, the oscillating cone-jet mode demonstrates superior low-noise output and controllability, thereby making it another suitable operational mode for high-precision electrospray thrusters in addition to the steady cone-jet mode.

Linear stability of electrocapillary convection in an infinite liquid layer

Electrocapillary convection is the flow driven by the interfacial shear stress induced by a surface-charge density and electric fields. It has become an important topic in electrohydrodynamics recently. The present work examine the convective instability for electrocapillary liquid layers by the modal and non-modal approaches. The modal analysis finds two kinds of modes, namely purely hydrodynamic and electrohydrodynamic modes, while both of them are stable. In the non-modal analysis, the amplifications of initial and external disturbances are measured by the transient growth and the response function, respectively. Both the perturbation velocity and surface-charge density can be amplified significantly.

Emission Modes in Electrospray Thrusters Operating with High Conductivity Ionic Liquids

Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15°. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 μs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.

Experimental investigation of electrohydrodynamic modes in electrospraying of viscoelastic polymeric solutions

The main aim of this study is to explore the role of viscoelastic properties of polymeric solutions on mode transitions in the electrospray process. By adjusting the applied electric potential between the nozzle and the collecting substrate, various electrohydrodynamic (EHD) modes were photographed by using a high-speed camera. Then, the effect of operating parameters on the droplet size in the dripping mode and the jet profile in the jet mode was investigated. By categorizing the EHD modes of each viscoelastic solution into dimensionless operating maps, it can be seen that by increasing the solution concentration, the extents of dripping mode and beads-on-a-string structure dwindle, while the jet stabilizes in a wider range of electric capillary numbers. Furthermore, contrary to deionized water, when the applied voltage escalates, the stick jet mode is observed where the jet sticks to the outer surface of the nozzle and the asymptotic thickness of the jet falls.

Preparation of monodisperse charged droplets via electrohydrodynamic device for the removal of fine dust particles smaller than 10 μm

Electrohydrodynamic (EHD) device is utilized in various applications and could be useful for the suppression of particulate matter in ambient conditions. In this study, we focused on the ejection of charged droplets containing electrolytes in a microdripping mode and with high charge density. Several different electrolytes with different physical and electrical properties were tested for our EHD process in order to produce the charged droplets stably. Results from series images by high-speed camera represented that droplet size and frequency were dependent on the applied voltage and flow rate, and showed different behaviors in various EHD modes such as dripping, microdripping, mixed dripping, and unstable dripping. Consecutive experimental data for charge density showed that 15 wt% KCl solution was proper to obtain highly charged droplets with the size from 50 to 100 μm. For this solution, the suppression of the fine dust particle was tested for the removal of PM10, PM2.5, and PM1 at various applied voltages. The droplet formation in microdripping mode was effective for the removal of smaller dust particles and could be applicable for the air remediation in indoor or domestic environment.

Electrolyte-Plasma Modification of Surface of Ti-Based Alloy during Electrohydrodynamic Mode of Anodic Process

This article briefly reviews modern methods for oxidizing coating of valve metals (Al, Ti, Mg, etc.). These methods include microarc oxidation, plasma-electrolytic-arc oxidation, anodic - spark oxidation. Their common characteristics as well as differences between these methods and electrolyte-plasma processing by electrohydrodynamic mode of anodic process are described. The composition, microstructure and surface morphology of the coating formed on the surface of titanium alloys depending on the electrolyte-plasma processing parameters are investigated.

Propagating electrohydrodynamic mode in a nematic liquid crystal

Propagating electrohydrodynamic mode in a nematic liquid crystal

Order Parameter Distribution for the Electrophydrodynamic Mode of a Nematic Liquid Crystal

Abstract AC electric fields are known to induce a visible domain pattern in certain nematic liquid crystals. We have measured the focal length of the liquid crystal lens system responsible for this visibility. We have also measured the resistance change associated with this electrohydrodynamic mode. These results can be quantitatively interpreted in terms of the director's spatial distribution which is predicted by Helfrich's theory of conduction induced alignment.