Space Electroosmotic Thrusters in Ion Partitioning Soft Nanochannels

Space electroosmotic thrusters (EOTs) are theoretically investigated in a soft charged nanochannel with a dense polyelectrolyte layer (PEL), which is considered to be more realistic than a low-density PEL. When the PEL is dense, its permittivity is smaller than the one of the electrolyte solution layer, leading to rearrangement of ions in the channel, which is denoted as the ion partitioning effect. It is noted that fluid viscosity becomes high within the PEL owing to the hydration effect. An analytical solution for electroosmotic velocity through the channel is obtained by utilizing the Debye–Hückel linearization assumption. Based on the fluid motion, thruster performances, including thrust, specific impulse, thrust-to-power ratio, and efficiency, are calculated. The ion partitioning effect leads to enhancement of the thruster velocity, while increase of the dynamic viscosity inside the PEL reduces the flow rate of the fluid. Therefore, these performances are further impacted by the dense soft material, which are discussed in detail. Moreover, changes or improvements of the thruster performances from the dense PEL to the weak PEL are presented and compared, and distributions of various energy items are also provided in this study. There is a good result whereby the increase in electric double layer thickness promotes the development of thruster performances. Ultimately, the simulated EOTs produce thrust of about 0 to 20 μN and achieve thruster efficiency of 90.40%, while maintaining an appropriate thrust–power ratio of about 1.53 mN/W by optimizing all design parameters.

Electroosmotic Mixing of Non-Newtonian Fluid in a Microchannel with Obstacles and Zeta Potential Heterogeneity

This paper investigates the electroosmotic micromixing of non-Newtonian fluid in a microchannel with wall-mounted obstacles and surface potential heterogeneity on the obstacle surface. In the numerical simulation, the full model consisting of the Navier–Stokes equations and the Poisson–Nernst–Plank equations are solved for the electroosmotic fluid field, ion transport, and electric field, and the power law model is used to characterize the rheological behavior of the aqueous solution. The mixing performance is investigated under different parameters, such as electric double layer thickness, flow behavior index, obstacle surface zeta potential, obstacle dimension. Due to the zeta potential heterogeneity at the obstacle surface, vortical flow is formed near the obstacle surface, which can significantly improve the mixing efficiency. The results show that, the mixing efficiency can be improved by increasing the obstacle surface zeta potential, the flow behavior index, the obstacle height, the EDL thickness.

Assessment and Prediction of EOF Mixing in Binary Electrolytes

A two dimensional simulation is made to analyse the mixing enhancement due to surface roughness and geometric modulation in a sufficiently long rectangular nano-channel filled with electrolyte solutions of different concentrations. Geometric modulation is made by mounting non-conducting rectangular blocks on the bottom wall of the channel. An overpotential patch is placed on the upper wall of each block to create surface heterogeneity. Based on a finite volume staggered grid approach, the flow characteristics and mixing efficiency are discussed by a complete numerical solution of coupled nonlinear set of PDEs involving Nernst-Planck equation for ion distribution, Navier-Stokes equation for velocity components and Maxwells equation for potential distribution. A linear pressure drop is observed above the overpotential region which creates a recirculating zone. Mixing efficiency is improved with increasing vortex strength which is enhanced by decreasing EDL (electric double layer) thickness and increasing overpotential patch strength.

Coagulation kinetics and the stability of surfactant-free emulsion-polymerised latexes

The stability factor of styrene/sulfopropyl methacrylate (styrene/SPM) and styrene/3-[N,N-dimethyl-N-(methacryloxyethyl)ammonium]propane sulfate (styrene/ SPE) latexes was studied using the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and the critical coagulation concentration, CCC, was deduced for the latexes. Photon correlation spectroscopy was used to study ionic stabilities. The stability factor was determined as the ratio of the rate constant for rapid coagulation to that of slow coagulation, obtained from the coagulation kinetics data. The log-log plot of the stability factor, W, as a function of NaCl electrolyte concentration shows an asymptotic decrease in W of both latexes. DLVO theory was successfully employed, whereby the characteristic properties of the diffuse electric double layers or the Stern layers around latex particles in terms of their Hamaker constants and diffuse potentials were determined. The electric double layer thickness decreases with increasing NaCl concentration. Stability curves revealed a higher CCC value for the styrene/SPM latex than for the styrene/SPE latex, which is in accordance with the higher surface charge density and an indication of a better stability.

Taylor-Like Dispersion of Charged Species in Electrokinetically-Driven Nanoflows

In this paper, electrokinetic dispersion of charged and uncharged species in nanochannels with finite electric double layers is modelled numerically. The relatively thick electrical double layers in these flows influence dispersion through the coupled effects of both cross-stream electromigration and advection in the presence of cross-stream velocity gradients. It is found that valence charge has a significant effect on axial dispersion in these flows, in addition to other established dependencies. Effective diffusion coefficients were found to vary over 30% from the case of neutral species for single charged ions. An effective diffusion coefficient similar to Taylor dispersion is calculated and a relationship between effective diffusion coefficient, Peclet number, relative electric double layer thickness, and valence charge is plotted.