Ionic Drag Research Articles

Ionic coulomb drag in nanofluidic semiconductor channels for energy harvest

The onset of electronic current in a doped silicon membrane induced by the long-range Coulomb interaction of ions flowing through a nanofluidic channel is established by a combined computational and analytical approach based on Green’s function technique and Boltzmann transport formalism. Characterized by an open circuit voltage and short circuit current, the electronic Coulomb drag provides a new paradigm for power harvesting. In addition, our model predicts a current amplification of the ionic drag current because of the large momentum transfer from heavy ions to charge carriers in silicon, which is achieved for both anions and cations flowing in the nanochannel irrespective of the dopant type in the semiconductor. The analysis indicates the versatility of this effect with respect to the nature of the electrolyte and the semiconducting materials, providing proper tuning of their structures and design configurations.

Diffusiophoresis of a moderately charged spherical colloidal particle.

An analytic expression is obtained for the diffusiophoretic mobility of a charged spherical colloidal particle in a symmetrical electrolyte solution. The obtained expression, which is expressed in terms of exponential integrals, is correct to the third order of the particle zeta potential so that it is applicable for colloidal particles with low and moderate zeta potentials at arbitrary values of the electrical double-layer thickness. This is an improvement of the mobility formula derived by Keh and Wei, which is correct to the second order of the particle zeta potential. This correction, which is related to the electrophoresis component of diffusiophoresis, becomes more significant as the difference between the ionic drag coefficients of electrolyte cations and anions becomes larger and vanishes in the limit of thin or thick double layer. A simpler approximate mobility expression is further obtained that does not involve exponential integrals.

Electrokinetics of non-Newtonian fluids in poly-electrolyte grafted nanochannels: Effects of ion-partitioning and confinement

Fluid flow inside nano-channels coated with a polyelectrolyte brush layer (PEL) is dictated by the brush structure and its interaction with the counter-ion dominated electrical double layer (EDL) near the walls of the PEL channel. In this work, we bring out the confluence of spatial permittivity variation, the influence of proximity between ions in charged PEL channels and non-Newtonian constitutive behavior of the fluid towards altering the electrokinetics of power-law fluids as well as their mechanical to electrical energy conversion capabilities. Our estimates are based on considering the fluidic drag experienced by the ions mediated by effective confinement induced by other ions and fluid rheology. For small electrical double layer (EDL) thicknesses (corresponding to high ionic concentrations), the energy conversion efficiency is shown to be higher for shear-thinning fluids, with a crossover in these characteristics for higher EDL thicknesses. Results show that lower bulk to PEL charge density ratio corresponds to higher conversion efficiencies. PEL also dictates ionic drag in that higher PEL thickness corresponds to higher ionic drag. It has been observed that although the electrokinetic parameters such as streaming potential (electrical potential generated across a channel due to a pressure-driven flow) and energy conversion efficiency (based on the ratio of electrical energy harvested to mechanical energy input) are significantly influenced by the fluid and the nanochannel properties, there exists a cross-over EDL thickness for the variation in the conversion efficiency, which is solely dependent on the properties of the nanochannel. The consideration of the ionic confinement effect allows us to have a more realistic estimate of energy conversion efficiency and yields better estimates of critical regimes of operation depending on EDL thickness for optimal efficiency. These results may yield insight towards designing functional nanofluidic devices with biological interfaces.

Characterization of gold nanoparticles with different hydrophilic coatings via capillary electrophoresis and Taylor dispersion analysis. Part I: Determination of the zeta potential employing a modified analytic approximation

Taking gold nanoparticles with different hydrophilic coatings as an example, it is investigated whether capillary electrophoresis in combination with Taylor dispersion analysis allows for the precise determination of mean electrophoretic mobilities, electrophoretic mobility distributions, and zeta potentials in a matrix of exactly known composition and the calibration-free determination of number-weighted mean hydrodynamic radii. Our experimental data confirm that the calculation of the zeta potential for colloidal nanoparticles with ζ>25mV requires to take the relaxation effect into account. Because of the requirement to avoid particle–wall interactions, a solution of disodiumtetraborate decahydrate (borax) in deionized water had been selected as suitable electrolyte. Measurements of the electrophoretic mobility at different ionic strength and application of the analytic approximation developed by Ohshima show that in the present case of a buffered solution with a weak electrolyte co-ion and a strong electrolyte counterion, the effective ionic drag coefficient should be approximated with the ionic drag coefficient of the counterion. The obtained results are in good agreement with theoretical expectations regarding the dependence of the zeta potential and the electrokinetic surface charge density on the ionic strength. We also show that Taylor dispersion analysis (besides estimation of the number-weighted mean hydrodynamic radius) provides additional information on the type and width of the number-weighted particle distribution.

Electrophoresis of bubbles

AbstractSmoluchowski’s celebrated electrophoresis formula is inapplicable to field-driven motion of drops and bubbles with mobile interfaces. We here analyse bubble electrophoresis in the thin-double-layer limit. To this end, we employ a systematic asymptotic procedure starting from the standard electrokinetic equations and a simple physicochemical interface model. This furnishes a coarse-grained macroscale description where the Debye-layer physics is embodied in effective boundary conditions. These conditions, in turn, represent a non-conventional driving mechanism for electrokinetic flows, where bulk concentration polarization, engendered by the interaction of the electric field and the Debye layer, results in a Marangoni-like shear stress. Remarkably, the electro-osmotic velocity jump at the macroscale level does not affect the electrophoretic velocity. Regular approximations are obtained in the respective cases of small zeta potentials, small ions, and weak applied fields. The nonlinear small-zeta-potential approximation rationalizes the paradoxical zero mobility predicted by the linearized scheme of Booth (J. Chem. Phys., vol. 19, 1951, pp. 1331–1336). For large (millimetre-size) bubbles the pertinent limit is actually that of strong fields. We have carried out a matched-asymptotic-expansion analysis of this singular limit, where salt polarization is confined to a narrow diffusive layer. This analysis establishes that the bubble velocity scales as the $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}2/3$-power of the applied-field magnitude and yields its explicit functional dependence upon a specific combination of the zeta potential and the ionic drag coefficient. The latter is provided to within an $O(1)$ numerical pre-factor which, in turn, is calculated via the solution of a universal (parameter-free) nonlinear flow problem. It is demonstrated that, with increasing field magnitude, all numerical solutions of the macroscale model indeed collapse on the analytic approximation thus obtained. Existing measurements of clean-bubble electrophoresis agree neither with present theory nor with previous models; we discuss this ongoing discrepancy.

Dynamic Electrophoretic Mobility of Spherical Colloidal Particles in Salt-Free Concentrated Suspensions

In this contribution, the dynamic electrophoretic mobility of spherical colloidal particles in a salt-free concentrated suspension subjected to an oscillating electric field is studied theoretically using a cell model approach. Previous calculations focusing the analysis on cases of very low or very high particle surface charge are analyzed and extended to arbitrary conditions regarding particle surface charge, particle radius, volume fraction, counterion properties, and frequency of the applied electric field (sub-GHz range). Because no limit is imposed on the volume fractions of solids considered, the overlap of double layers of adjacent particles is accounted for. Our results display not only the so-called counterion condensation effect for high particle charge, previously described in the literature, but also its relative influence on the dynamic electrophoretic mobility throughout the whole frequency spectrum. Furthermore, we observe a competition between different relaxation processes related to the complex electric dipole moment induced on the particles by the field, as well as the influence of particle inertia at the high-frequency range. In addition, the influences of volume fraction, particle charge, particle radius, and ionic drag coefficient on the dynamic electrophoretic mobility as a function of frequency are extensively analyzed.

Temporal evolution of the electron diffusion coefficient in electrolyte-filled mesoporous nanocrystalline TiO 2 films

Electron transport in electrolyte-filled mesoporous TiO 2-based solar cells is described quantitatively from the perspective of the continuous-time random walk model. An analytical expression is derived for the time-dependent diffusion coefficient of electrons, which transforms at a characteristic (Fermi) time from strongly time-dependent values (dispersive transport) at short times to relatively time-independent values (nondispersive transport) at long times. At short times, the diffusion coefficient displays a power-law behavior with time. The timescale for the diffusion coefficient to reach its steady-state value is substantially longer than the Fermi time. The Fermi time and the steepness of the distribution of waiting times associated with trap sites have a strong influence on both the steady-state diffusion coefficient of electrons and on the dispersiveness of electron transport. At short timescales, ionic drag, associated with the ambipolar effect, slows electron transport through the TiO 2 matrix, whereas at steady state, transport is trap limited. Decreasing the electron density lowers the steady-state limit of the diffusion coefficient and increases the timescale over which transport is dispersive.

Dielectric relaxation pattern of dilute colloidal suspensions

An immediate method of analysis of the relaxation characteristics of a colloidal suspension, like of any dielectric, is based on the so-called Cole-Cole representation (imaginary part versus real part) of its complex dielectric constant in a wide frequency range. In this work, we show theoretical plots calculated according to the models developed by DeLacey and White (J Chem Soc Faraday Trans 2 77:2007–2039), and by Rosen et al. (J Chem Phys 98: 4183–4194; this model uses the dynamic Stern layer theory). Both theoretical approaches to the dielectric relaxation pattern of a colloidal suspension are compared to each other, and to experimental data obtained on polystyrene suspensions. Although no significant differences are found between the theoretical predictions of the relaxation patterns (except for the values of the dielectric constant; the DSL model yields higher polarizabilities of the suspensions), none of the models can exactly reproduce the frequency dependence of the dielectric constant of a colloidal system. We propose a modification of DeLacey and White's model to include the possibility that the ionic drag coefficients depend on the ion position in the double layer. The final results show that the general trends of the frequency dependence of the quantities involved are not modified, irregardless of the changes in ionic drag coefficients.

Potato slab dehydration by air ions from corona discharge

Space charge (air ions) produced by single corona electrodes was used to enhance drying rates from fresh slabs of potato. The drying path was traced by a beta-ray gauge which provided both sensitivity and reproducibility to the measurements of drying time. The rate of evaporation was increased 2.2 to 3.0 times when subjected to fluxes of 3.02×1012 positive ions alone or in combination with 7.31×1012 negative air ions/cm2 per s compared to that from an air-drying control slab. Electric wind caused by an ionic drag force seems to be the principal driving force for the observed enhancement in drying rates.

AN ELECTROSTATIC METHOD OF DRYING SALINE WATER

ABSTRACT Times for drying and nucleation of saline solutions exposed to relatively high fluxes of unipolar air ions, produced by corona electrodes, were studied with a beta-ray gauge. Ion-treated solutions of NaCl of various concentrations dried about 3.2 times faster when exposed to either 2.54 x 1012;positive or 4.42 x 1012 negative air ions cm-2 s-1 than the control samples. The nucleation site in a solution was observed to be directly below the emitting electrode; whereas nucleation appeared to be spatially random for untreated solutions. The solutions nucleated earlier when treated with the ion fluxes than the control. Electric wind caused by the ionic drag is proposed as the principal driving force for the enhancement of the drying process. The phenomenon of elctrostriction might have also aided in nucleating the solutions.

A comparative study between an electrostatic and conventional methods of drying soil samples

Abstract Drying of soil samples exposed to high fluxes of air ions of both polarity, produced by corona electrodes, were studied by a beta‐ray gauge. The conventional methods of air‐drying and oven‐drying of soil samples were compared with the new technique. Fluxes of 0.94 x 1012 positive and 1.83 x 1012 negative air ions cm‐2 s‐1 reduced drying time of samples by at least three times over the air‐drying controls. Although oven‐drying at a high temperature was the fastest drying method, air ions maintained steady‐state soil temperature during the drying process at 4.5° and 7.3°C below that of the control and ambient temperatures, respectively. There was no significant change in pH, electrical conductivity, and organic matter content values of the dried soil samples of Dalhousie clay, Macdonald sandy loam and St. Sophie sand. Electric wind produced by the ionic drag is proposed to be the principal driving force for the enhancement of evaporation rates from the soil samples. Electroconvection and electrostr...

Impact of air ions of both polarity on evaporation of certain organic and inorganic liquids

Air ions of both polarity, produced by corona electrodes, were used to evaporate to dryness liquid samples of ethyl alcohol (EA), water (W), and carbon tetrachloride (CTC). Drying times were determined with a beta-ray gauge. Ion exposed samples of EA, W, and CTC dried, respectively, 2.3, 3.2, and 5.4 times faster than the corresponding control samples when exposed simultaneously to 0.94×1012 positive and 1.83×1012 negative air ions cm−2s−1 under the same laboratory conditions. Drying by corona discharge could be explained by three different mechanisms. Electric wind caused by the ionic drag is proposed as the principal driving force for the observed enhancement of evaporation. The decrease in free energy of a dielectric in the presence of an electric field compared to its absence may have increased the escaping tendency of the molecules of the treated liquids. The turbulence in the liquids created by the rotational effect on the dielectric molecules by the electric field may also be a factor in further enhancing the mass transfer rates from the samples.

Electrostatically Enhanced Evaporation Rates of Saline Water

Corona-produced air ions of each polarity were used to evaporate to dryness samples of KCl solutions of 10, 15, and 20 % (w/V) concentrations. Surface drying times were determined with a beta-ray gauge. Samples exposed to fluxes of either 1.81 x 10 12 positive or 4.20 x 10 12 negative air ions cm −2 s −1 dried 2.5 to 3.9 time faster than control samples under different relative humidity conditions in the laboratory. Evaporation rate enhancement of solutions, exposed to bipolar air ions, occurred at an equilibrium temperature that was about 6 oC lower than the ambient temperature. Electric wind caused by the ionic drag is proposed as the principal driving force for the observed enhancement of evaporation. Thermodynamic considerations showed that a concentraion gradient developed in the solution with the highest concentration at directly below a single point electrode where nucleation and crystallization of the solution first occurred. An entropic effect also led to an exothermic process which partly explained the low temperature obtained by the exposed solution compared to the temperature of a freely evaporating sample.

A novel technique for drying soil samples

Abstract Evaporation rates and drying times of soil samples exposed to high fluxes of unipolar air ions of either polarity, produced by corona electrodes, were studied with a beta‐ray gauge. Ion‐treated soils dried more than three times faster than the control samples under the same laboratory conditions. An analysis of the experimental curves on Rideau clay showed evaporation rates of 2.96 x 10‐4and 8.84 x 10‐4g cm‐2min‐1for control and treated samples, respectively, when exposed to 4.39 x 1012negative air ions cm‐2s‐1. An objective criterion was defined for a state of soil dryness when the sample acquired a steady‐state value of transmission of beta particles with time. Electric wind caused by the ionic drag is proposed as the principal driving force for the enhancement of evaporation rates from soil samples used.

Diffusiophoresis of a rigid sphere through a viscous electrolyte solution

Numerical predictions of the migration velocity are presented for slightly non-uniform aqueous solutions of KCl or NaCl over a wide range of zeta potential (ζ) and particle radius divided by Debye length (κa). The characteristic speed of migration is comparable in magnitude to the ensemble mean diffusion velocity of ions forming the concentration gradient. In the absence of a macroscopic electric field induced by the gradient, the particle usually migrates toward higher salt concentration (the chemiphoretic contribution). However, for moderate values of κa, the particle reverses direction when |ζ| becomes sufficiently large. With an induced electric field, competition between electrophoresis and chemiphoresis can result in as many as four reversals in direction over the range of ζ. In some cases, the direction of migration was found to depend on the magnitude of the ionic drag coefficients, which seems to preclude predicting the direction on the basis of equilibrium thermodynamics.

Mechanism of Ion Transport in Cooled and Supercooled Water at High Pressure

ABSTRACTAs predicted by the Hubbard-Onsager continuum dielectric friction theory for ionic mobility in solution, the observed pressure coefficient of the difference ζ between the observed ionic drag coefficient and the Stokes law friction (4πηR), is negative for the small ion Li+ even at low temperatures where the water structure is highly developed. For the large ion Cs+, however, it is positive at any temperature in contrast to the theoretical prediction, its magnitude being larger at lower temperatures and pressures. The anomaly is shown also by the medium-sized ion K+ below room temperature. The passing-through-cavities (PTC) mechanism has been proposed to explain the limitations of the continuum theory.

Electrocooling in gases

Significant surface-temperature decreases may be achieved by generating a corona discharge near a heated object. We demonstrate that this electrocooling phenomenon may be ascribed to an electric wind which results from ionic drag and which produces a convective heat-transfer coefficient that is proportional to the fourth root of corona current.