An Ohmic model for electrokinetic flows of binary asymmetric electrolytes

Isotachophoresis: Theory and Microfluidic Applications

Isotachophoresis (ITP) is a versatile electrophoretic technique that can be used for sample preconcentration, separation, purification, and mixing, and to control and accelerate chemical reactions. Although the basic technique is nearly a century old and widely used, there is a persistent need for an easily approachable, succinct, and rigorous review of ITP theory and analysis. This is important because the interest and adoption of the technique has grown over the last two decades, especially with its implementation in microfluidics and integration with on-chip chemical and biochemical assays. We here provide a review of ITP theory starting from physicochemical first-principles, including conservation of species, conservation of current, approximation of charge neutrality, pH equilibrium of weak electrolytes, and so-called regulating functions that govern transport dynamics, with a strong emphasis on steady and unsteady transport. We combine these generally applicable (to all types of ITP) theoretical discussions with applications of ITP in the field of microfluidic systems, particularly on-chip biochemical analyses. Our discussion includes principles that govern the ITP focusing of weak and strong electrolytes; ITP dynamics in peak and plateau modes; a review of simulation tools, experimental tools, and detection methods; applications of ITP for on-chip separations and trace analyte manipulation; and design considerations and challenges for microfluidic ITP systems. We conclude with remarks on possible future research directions. The intent of this review is to help make ITP analysis and design principles more accessible to the scientific and engineering communities and to provide a rigorous basis for the increased adoption of ITP in microfluidics.

PEO of AZ31 Mg Alloy: Effect of Electrolyte Phosphate Content and Current Density

In this work, the quality of coatings prepared by plasma electrolytic oxidation (PEO) on an AZ31 magnesium alloy were evaluated. This was done by studying the effects of the chemical composition of phosphate-based process electrolytes in combination with different applied current densities on coating thickness, porosity, micro-cracking and corrosion resistance in 0.1 M NaCl. Both processing parameters were studied in four different levels. Mid-term corrosion resistance in 0.1 M NaCl was examined by electrochemical impedance spectroscopy and based on this, corrosion mechanisms were hypothesized. Results of performed experiments showed that the chosen processing parameters and electrolyte composition significantly influenced the morphology and corrosion performance of the prepared PEO coatings. The PEO coating prepared in an electrolyte with 12 g/L Na3PO4·12H2O and using an applied current density 0.05 A/cm2 reached the highest value of polarization resistance. This was more than 11 times higher when compared to the uncoated counterpart.

Electrorotation of leaky-dielectric and conducting microspheres in asymmetric electrolytes and angular velocity reversal

We consider the central problem of polarizable and leaky-dielectric uncharged spherical particle freely suspended in an unbounded nonsymmetric binary electrolyte, which is forced by an ambient time-harmonic uniform electric field. Under the assumption of a "weak field," we employ the linearized standard electrokinetic model of binary electrolytes to account for such anion/cation asymmetry. A simplified generalized asymmetric dipole-term approximation, valid for a dielectric/conducting microsphere, is analytically derived for an arbitrary Debye scale and for any mismatch between ion diffusivities and valances. A two-peak unified dispersion spectrum covering all range of practical frequencies (KHz to MHz), is found for the case of a rotating electric field (ROT). The angular velocity of a free polarized particle is composed of dielectrophoretic contribution, resulting from the electrical torque (dipole term) as well as from the induced electroosmotic (ICEO) flow field. The two effects usually act in opposite directions. Under ROT excitation, we obtain a cofield rotation at high frequencies (MHz) and a counter-field behavior at low frequencies (KHz). The low-frequency dispersion is generally governed by electric double-layer charging and the high frequency by a Maxwell-Wagner relaxation process. ICEO generally dominates the low-frequency cofield response; however, it can be shown that depending on the electrolyte asymmetry, yet another dielectrophoretic related switching (reversal) point might exist. Furthermore, for large frequencies and depending on the complex permittivity ratio between the particle and electrolyte, we find a second switching point. Explicit expressions for the above two frequency reversal values are obtained in terms of the problem physical parameters and are compared against experimental results. Finally, we provide an analytical solution for the ROT ICEO velocity field of a microsphere as a function of electrolyte asymmetry and Debye length and compare it with numerical simulations.

Diffusion of multiple electrolytes cannot be treated independently: model predictions with experimental validation

We study the diffusion of multiple electrolytes in a one-dimensional pore. We model the scenario where an electrolyte is in contact with a reservoir of another electrolyte, such that the cation of the two electrolytes is common. The model reveals that several factors influence the ion concentration profiles: (i) relative diffusivities of the ions, (ii) ratio of the electrolyte concentrations in the pore and the reservoir, and (iii) the valence of the ions. We demonstrate that it is crucial to consider the interaction between ion fluxes as treating the electrolytes independently, as is sometimes proposed, does not completely capture the dynamics of ion transport. We validate our numerical predictions by conducting experiments with sodium fluorescein salt in the pore and sodium chloride/sodium sulphate/sodium hydroxide in the reservoir. Our visualization and results demonstrate that ion diffusivities and concentrations in the reservoir can influence the diffusion rates of fluorescein, which underscores that ion fluxes are coupled and that multiple electrolytes cannot be treated independently. These results should be useful to the wide range of situations where concentration variations are imposed on systems with an existing background electrolyte.

Mathematical Model for the Electrokinetic Instability of Electrolyte Flow

We study a one-dimensional model of the dilute aqueous solution of KCl in the electric field. Our model is based on a set of Nernst-Planck-Poisson equations and includes the incompressible fluid velocity as a parameter. We demonstrate instability of the linear electric potential variation for the uniform ion distribution and compare analytical results with numerical solutions. The developed model successfully describes the stability loss of the steady state solution and demonstrates the emerging of spatially non-uniform distribution of the electric potential. However, this model should be generalized by accounting for the convective movement via the addition of the Navier-Stokes equations in order to substantially extend its application field.