Permeation by Electrowetting Actuation: Revealing the Prospect of a Micro-valve Based on Ionic Liquid.

The electrowetting behavior of ionic liquid significantly promotes microfluidic technology due to the advantage of manipulation of ionic liquid without additional mechanical parts. Recently, a novel micro-valve that shows good prospects was proposed by MacArthur et al. based on the permeation of ionic liquid under electric field. Inspired by their work, the permeation process of ionic liquid (EMIM-Im) droplets actuated by electrowetting was investigated in this work using molecular dynamics simulation. The wettability of substrate, electric field strength and electric field polarity were varied to investigate their influences. On the substrate side, results showed that the hydrophilic substrates tend to stretch and adsorb the droplet and hence hinder the permeation process, whereas the hydrophobic substrates facilitate permeation due to their low attraction for liquid. Particularly, super hydrophilic substrates should be avoided in practice, because their strong adsorption effects will override the electric field effects and disable the permeation process. On the electric field side, results showed that increased electric field strength enhances the permeation, but varying electric field polarity will result in an asymmetric permeation behavior, which was found to be the result of the different evaporation rate of the ion species that ultimately caused a non-charge-neutral droplet. Our investigation then uncovered the two critical roles of the electric field: elongating the droplet and providing the driving force for the permeation.

Adsorption Isotherms of Low-Pressure H2O on a Low-Temperature Surface Measured by a Quartz Crystal Microbalance.

The low-pressure gas in the vacuum plume produced by the chemical thrusters contaminates the spacecraft when adsorbed on the low-temperature surface. To provide theoretical support for further research on gaseous plume pollutants, the adsorption isotherms of low-pressure H2O were measured by a quartz crystal microbalance (QCM) at temperatures ranging from 233 to 273 K. The measured isotherms are similar to the type-I and type-II isotherms and have been correlated by various models (e.g., the Langmuir, Dubinin-Radushkevich, Brunauer-Emmett-Teller (BET), and universal models). It shows that the universal model has a great advantage in predicting the adsorption at a specific temperature point in our study. To estimate the adsorption at the continuous temperature range, the critical parameters of the multi-Langmuir model were expressed in semiempirical formulas. Since the normalized isotherms of H2O at different temperatures converge well, a simplified multi-Langmuir (SML) model was proposed. The experimental results at the temperature and pressure ranges we explored are consistent with the results predicted by the SML model, suggesting that the SML model is more suitable and convenient to predict the low-pressure adsorption of H2O for a continuous low-temperature range. Moreover, the low-pressure adsorption behaviors of H2O and CO2 on the low-temperature surface are compared and discussed.

Drop Impact on Heated Nanostructures.

Drop impact on a heated surface not only displays intriguing flow motion but also plays a crucial role in various applications and processes. We examine the impact dynamics of a water drop on both heated flat and nanostructured surfaces, with a wide range of impact velocity (V) and surface temperature (Ts) values. Via high-speed imaging and temperature measurements, we construct phase diagrams of different impact outcomes on these heated surfaces. Like those on the heated flat surface, water drops can deposit, spread, rebound, or break-up with atomizing on the heated nanostructures as V and Ts are increased. We find a significant influence of nanostructures on the impact dynamics by generating particular events in specific parameter ranges. For example, events of splashing, gentle central jetting, and violent central jetting are observed on and thus triggered by the heated nanostructures. The heated nanotextures with high roughness can easily trigger the splashing and the central jetting. Our data of the normalized maximum spreading diameter for the heated surfaces display distinct trends at low and high Weber number (We) ranges, where We compares the kinetic to surface energy of the impacting droplet. Finally, compared with the flat surface, the dynamic Leidenfrost temperature (TLD) for We ≈ 10 is decreased (by ≈60 °C) by the high-roughness nanotextures. In addition, our experimental data of TLD is consistent with a model prediction proposed by balancing the droplet dynamic and vapor pressure.

Improving the viability and versatility of the E × B probe with an active cooling system.

A thermostatic E × B probe is designed to protect the probe body from the thermal effect of the plasma plume that has a significant influence on the resolution of the probe for high-power electric thrusters. An active cooling system, which consists of a cooling panel and carbon fiber felts combined with a recycling system of liquid coolants or an open-type system of gas coolants, is employed to realize the protection of the probe. The threshold for the design parameters for the active cooling system is estimated by deriving the energy transfer of the plasma plume-probe body interaction and the energy taken away by the coolants, and the design details are explained. The diagnostics of the LIPS-300 ion thruster with a power of 3 kW and a screen-grid voltage of 1450 V was implemented by the designed thermostatic E × B probe. The measured spectra illustrate that the thermostatic E × B probe can distinguish the fractions of Xe+ ions and Xe2+ ions without areas of overlap. In addition, the temperature of the probe body was less than 306 K in the beam region of the plasma plume during the 200-min-long continuous test. A thermostatic E × B probe is useful for enhancing the viability and versatility of equipment and for reducing uneconomical and complex test procedures.