Double-layer Polarization Research Articles

Swift and precise detection of unlabeled pathogens using a nanogap electrode impedimetric sensor facilitated by electrokinetics

For the protection of human health and environment, there is a growing demand for high-performance, user-friendly biosensors for the prompt detection of pathogenic bacteria in samples containing various substances. We present a nanogap electrode-based purely electrical impedimetric sensor that utilizes the dielectrophoresis (DEP) mechanism. Our nanogap sensor can directly and sensitively detect pathogens present at concentrations as low as 1–10 cells/assay in buffers and drinking milk without the need for separation, purification, or specific ligand binding. This is achieved by minimizing the electrical double-layer effect and electrode polarization in nanogap impedance sensors, reducing signal loss. In addition, even at low DEP voltages, nanogap sensors can quickly establish strong DEP forces between the nanogap electrodes to control the spatial concentration of pathogens around the electrodes. This activates and stabilizes inter-electrode signal transmission along the nanogap-aligned pathogens, increasing sensitivity and reducing errors during repeated measurements. The DEP-enabled nanogap impedance sensor developed in this study is valuable for a variety of pathogen detection and monitoring systems including point-of-care testing (POCT) as it can detect pathogens in diverse samples containing multiple substances quickly and with high sensitivity, is compatible with complex solutions such as food and beverages, and provides highly reproducible results without the need for separate binding and separation processes.

Electrophoresis of a weakly charged dielectric fluid droplet in a cylindrical pore.

Electrophoresis of a weakly charged dielectric droplet with constant surface charge density in a chargeless cylindrical pore is investigated theoretically in this study, focusing on the boundary confinement effect of the double layer, which in turn determines the ultimate motion of the droplet. A patched pseudo-spectral method based on the Chebyshev polynomial is adopted to solve the resulting governing fundamental electrokinetic equations. Mobility reversal, among other interesting phenomena, is observed when the droplet is in a narrow cylindrical pore. No such observation was made in the corresponding motion of a rigid particle. The droplet with a thick double layer may even move against the prediction based on the Coulomb electrostatic law, for instance, a positively charged droplet may move against the electric field. The significant enhancement of the motion-deterring double layer polarization due to the severe steric boundary confinement within a narrow cylindrical pore is found to be responsible for this seemingly peculiar phenomenon. Moreover, smaller droplets may move in the opposite direction of the larger ones. The results are useful in capillary electrophoresis involving droplets in particular and migration of droplets through narrow channels in general.

Frequency-Dependent Microelectrophoresis Study of Colloids with Tunable Surface Charge.

Nonaqueous poly(methyl methacrylate) (PMMA) colloidal suspensions in a solvent that is simultaneously matched in both density and refractive index have been important for real-space studies of colloidal self-assembly, but their complex electrostatic character remains largely unexplored. Electrophoresis is a powerful tool for determining the surface potential and charge of the colloidal suspension; however, because of refractive index matching, standard electrophoresis measurements are not feasible. We carry out microscope-based microelectrophoresis measurements on PMMA colloids in cyclohexyl bromide and cis-trans decalin to measure particle charge as a function of salt concentration in both DC and frequency-variable AC fields. The colloid charge depends on salt concentration and reverses sign near 0.35 μM, providing evidence that solution ions are actively modifying the colloid surface. The frequency dependence of the electrophoretic mobility yields the characteristic time scale for electric double-layer polarization and provides intriguing evidence for Manning condensation and polyion formation.

Analysis of the effect of optical thickness on polarization in a sea fog stratified environment.

Since there are usually multiple layers present in a real-world sea fog environment, and because previous studies have tended to analyze sea fog as a single layer rather than as refined layered sea fog, this paper splits sea fog into two categories: water fog and salt fog double-layer environments. By adjusting the optical thickness of the two layers of media, we may investigate the issue of the law governing the transmission of polarized light. In this paper, the analysis is mainly carried out through a simulation and experimental tests. The simulation portion is based mostly on the improved layered Monte Carlo approach, which builds a simulation model more appropriate for multilayer non-spherical media by using the accumulation principle to determine the scattering and transmission properties between layers. The tests are conducted by altering the double-layer medium's optical thickness, incoming wavelength, and polarization state, and then getting the polarization information of visible light after transmission through the complicated environment. The findings demonstrate that the optical thickness of the sea fog double-layer media affects polarized light transmission in a non-negligible way. Longer wavelength polarized light may keep polarization information better as the optical thickness increases, and circularly polarized light has polarization-preserving properties that are superior to linearly polarized light. By contrasting the simulation findings with the experimental data, the consistency of the two conclusions is confirmed, and the study offers a helpful resource for the transmission of polarized light in the sea fog environment.

Spectral induced polarization responses of carbonate precipitation in gel-saturated media with and without electron conductors

SUMMARY Interest in the use of carbonate precipitation for the purposes of soil stabilization, carbon capture and environmental remediation has created a need for geophysical techniques capable of characterizing the 3-D extent of precipitation in the subsurface. The use of spectral induced polarization, a geophysical technique sensitive to mineral precipitation and dissolution processes, has been investigated in previous studies; nevertheless, debate still exists regarding measured induced polarization (IP) signals relating to the formation of calcite. In this study, calcite was precipitated within gel-saturated media through highly controlled double-diffusion methods. Three experiments were conducted to study both the inherent IP response of calcite in isolation, as well as the IP response when precipitated within a low-polarizability background medium (quartz sand) and a high-polarizability medium (a sand–magnetite mixture). The results support that in isolation, the polarization of the electrical double layer surrounding individual calcite grains is not a significant contributor to IP signatures from calcite precipitation. However, under certain circumstances the precipitation of calcite, in the quartz-sand and sand–magnetite mixtures, can produce a strong IP response when the precipitation blocks a current conduction pathway. If metallic mineral grains are present then they may act as conduits for current if they become embedded in an insulating calcite wall, creating a significant electrode polarization which dominates the IP signal. Equivalent circuit models indicate that the precipitation of calcite as a thin planar front normal to the current path is analogous to a simple lossy capacitor blocking most DC current but being thin enough to measurably polarize. Circuit models also suggest that the total impedance seen from the sand–magnetite sample is simply the sum of the impedance of the disseminated grains and the grains encased in the calcite front. The encased grains also have a higher relaxation time which may indicate a different polarization mechanism than from disseminated magnetite.

Effect of simple cubic, face-centered cubic, and body-centered cubic-electrodes on the electric double layer capacitance of supercapacitors

The continuum theory has been used to analyze the polarization, ion crowding, and electrostatic forces of the electric double layer in the electrode materials having simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC) morphologies. The study manifests the effect of thickness of electrodes, electrode’s particle size, and porosity on electric double-layer specific capacitance (EDLC). Electrochemical interference and the specific capacitance depend on the packing factor. The larger particle size decreases the specific capacitance, but porosity increases due to more surface area. Due to symmetry, SC, BCC, and FCC morphologies have 1, 3, and 5 spheres in a unit cell. The number of unit cells is varied from 1 to 100 in model 1 to analyze the effect of electrode thickness. Model 2 has three unit cells to understand the effect of porosity, and only pore lengths are varied. The critical thickness of the electrodes is the integer multiples of 1.71 μm in all the morphologies. The Stern layer-specific capacitance is 167.6 μF cm−2 in all cases. The EDLC in BCC is around 5.6–7.6 μF cm−2 in the steady state that is intermediate between SC and FCC morphologies. The more dense packing of carbon particles in a unit cell increases the energy storage capabilities of electrodes. The average electrode permittivity slightly decreases due to the combined effect of the high electric field, status of polarization, and electrode particle size. The least optical transmission of electrodes is 98.35%.

Effect of coating nanostructure on the electrokinetics of polyelectrolyte-coated particles. Grafted vs adsorbed polymer

In this work, the electrokinetic response of nanoparticles suspensions under the action of alternating electric fields is analyzed for the case that the particles are coated with a shell of polyelectrolyte. This is an important field of application of colloidal systems, as the coating is necessary or even unavoidable for control of the stability or functionalization of the particles for a specific use. Characterization of the coating in situ is not an easy task, and electrokinetics can help in answering this question, if one goes beyond the simple routine evaluation of the electrophoretic mobility of the particles, which will provide information on the sign (and perhaps a sort of effective amount) of the surface charge and potential, but little more. The richness and rigour of the information is much more significant if AC fields are used. This is the case for the techniques evaluated in the present investigation, namely, AC electrophoresis and dielectric dispersion. They jointly sweep several decades in frequency, and are sensitive in different manners to the size of the particles, the charge of the core and the shell, the thickness and rigidity of the latter, etc. In addition, an important distinction is made between coatings with a soft or adsorbed layer structure and those with a grafted, ideally radially arranged (brush-like) polyelectrolyte. A model is first described for the AC electrokinetics in both cases, and it is demonstrated that the brush structure magnifies the Maxwell-Wagner (or double layer) polarization, leading to an elevation in the AC mobility for frequencies around the MHz, and it very much raises the amplitude of the alpha- or concentration-polarization relaxation detected in the dielectric dispersion. The distribution of the charge in a thicker region for the brush structure explains these results. Experimental investigations are carried out with silica spheres coated by PDADMAC (+) and PSS (-) polyelectrolytes in the soft-layer case, and by (vinylbenzyl)trimethylammonium chloride (+) and sodium 4-vinylbenzenesulfonate (-) monomers that were polymerized (grafted) on the particles. The results show that the brush coating produces the expected MW elevation of the mobility, mostly in the case of the cationic polymer, apparently better attached to the particles by electrostatic attraction to the negative charge of the core silica particles. In contrast, a rather monotonous decrease of the mobility in absolute value is measured for the soft coatings, indicating that the inertia of the particles sets out at lower frequencies because of aggregation. Dielectric spectra confirm the better stability in the presence of the grafted polymer, although a low-frequency elevation in the logarithmic derivative of the permittivity is a proof of the existence of aggregates, less abundant in any case than for soft coatings. Dielectric data also confirm the different amounts of charge, larger for grafted cationic layers than for anionic ones. Finally, the model elaborated can fit the experimental results yielding quantitative values of the main parameters of the coated particles, namely, effective size, overall charge of the coating, and thickness and ionic permeability of the latter.

Frequency-Dependent Dielectric Spectroscopy of Insulating Nanofluids Based on GTL Oil during Accelerated Thermal Aging

Improving the dielectric properties of liquid-insulating materials is a current problem in research into the insulation system of a power transformer. Modern optimization of insulating liquids involves the potential use of unique synthetic esters enriched with nanoparticles. This study presents the results of the dielectric response of liquefied gas-based (GTL) insulating liquids during accelerated thermal aging. The dielectric relaxation spectroscopy method was used in the frequency domain to point out power losses as an imaginary part of a complex electric modulus. The relaxation spectra express the validity of applying this complex dielectric parameter. The polarization processes of the base oil alternately change position in the low-frequency band during thermal aging. Fullerene nanofluid undergoes three phases of dielectric loss changes during thermal aging. In the case of magnetic nanofluid, the effect of electric double-layer polarization disappeared after 500 h of thermal aging. It was found that with the gradual increase in the thermal aging time, there is no gradual increase in the dielectric losses investigated in the measured frequency spectrum. This study shows that the concentration of the two types of nanoparticles independently causes a different dielectric response to an applied AC electric field in the GTL base fluid.

Electrical impedance behaviour of carbon fibre reinforced cement-based sensors at different moisture contents

Carbon fibre reinforced cement-based sensors offer a potential low-cost and high-performance alternative to externally applied sensors for health monitoring of concrete structures. This paper focuses on the moisture dependence of bulk conductivity for carbon fibre reinforced cement-based sensors with fibre contents 0.1–1.5 vol%. Under two-point AC impedance spectroscopy at frequencies 1 Hz–10 MHz, the electrode polarization (EP) process, an electrical double layer polarization process (EDLP), and the bulk Maxwell-Wagner type interfacial polarization process (MWP) were identified. While reducing the saturation degree from 100 % to 0 %, the fractional change of bulk conductivity (FCS) underwent three different stages, which consists of a non-linear stage at 100–80 % saturation, a linear stage at 80–20 % saturation, and a final stage at 20–0 % saturation. The results gave insights into the stability of cement-based sensors under field conditions, and the underlying mechanisms of behaviour. From this, a route towards appropriate use of cement-based sensors in mechanical sensing and potentially moisture sensing without further external measures can be realised.

Diffusiophoresis in suspensions of highly charged soft particles.

Diffusiophoresis phenomenon of aoft particles suspended in binary electrolyte solutions is explored theoretically in this study based on the spherical cell model, focusing on the chemiphoresis component in absence of diffusion potential. Both the electrostatic and hydrodynamic aspects of the boundary confinement, or steric effect, due to the presence of neighboring particles are examined extensively under various electrokinetic conditions. Significant local extrema are found in mobility profiles expressed as functions of the Debye length in general, synchronized with the strength of the motion-inducing double layer polarization. Moreover, a seemingly peculiar phenomenon is observed that the soft particles may move faster in more concentrated suspensions. The competition between the simultaneous enhancement of the motion-inducing electric driving force and the motion-retarding hydrodynamic drag force from the boundary confinement effect of the neighboring particles is found to be responsible for it. The above findings are also demonstrated experimentally in a very recent study on the diffusiophoretic motion of soft particles through porous collagen hydrogels. The results presented here are useful in various practical applications of soft particles like drug delivery.

Fitting the determined impedance in the guinea pig inner ear to Randles circuit using square error minimization in the range of 100 Hz to 50 kHz

Objective. Several lumped and distributed parameter models of the inner ear have been proposed to improve vestibular implant stimulation. The models should account for all significant physical phenomena that influence the current propagation, such as the electrical double layer (EDL) and medium polarization. The electrical properties of the medium are reflected in the electrical impedance; therefore, the study aimed to measure the impedance in the guinea pig inner ear and construct its equivalent circuit. Approach. The electrical impedance was measured from 100 Hz to 50 kHz between a pair of platinum electrodes immersed in 0.9% NaCl saline solution using sinusoidal voltage signals. The Randles circuit was fitted to the measured impedance in the saline solution in order to estimate the EDL parameters () of the electrode interface in saline. Then, the electrical impedance was measured between all combinations of the electrodes located in the semicircular canal ampullae and the vestibular nerve in the guinea pig in vitro. The extended Randles circuit considering the medium polarization () together with EDL parameters () obtained from the saline solution was fitted to the measured impedance of the guinea pig inner ear. The Warburg element was assumed negligible and was not considered in the guinea pig model. Main results. For the set-up used, the obtained EDL parameters were: The average values of intra-, extracellular resistances, and membrane capacitance were respectively. Significance. The obtained values of the model parameters can serve as a good estimation of the EDL for modelling work. The EDL, together with medium polarization, plays a significant role in the electrical impedance of the guinea pig inner ear, therefore, they should be considered in electrical conductivity models to increase the credibility of the simulations.

Diffusiophoresis of a highly charged dielectric fluid droplet

Diffusiophoresis of a dielectric fluid droplet with constant surface charge density in a symmetric binary electrolyte solution is investigated theoretically in this study, focusing on the chemiphoresis component, the very heart of diffusiophoresis. The resultant electrokinetic equations are solved with a pseudo-spectral method based on Chebyshev polynomial in the spirit of a computational fluid dynamic simulation. Reversions of moving directions are found for droplets less viscous than ambient solution when the electrolyte strength is increased due to the buildup of osmosis pressure in front of the moving droplets leading to an osmosis pressure gradient upon the droplet. The upward spouting effect of the spinning droplet surface is also responsible this buildup, which hinders the downward migration of ions and holds them up there. A solid particle may move faster than a gas bubble due to the energy consumption in the formation of an induced exterior vortex flow nearby surrounding the gas bubble. The less viscous the droplet is, the more severe this consumption is. A “solidification” phenomenon is observed where all the droplets move at the same speed with their surfaces and interior fluids motionless like rigid particles. Funnel-shape local extrema of mobility profiles provide solid evidence that the diffusion-induced double layer polarization is the very cause of the droplet motion in chemiphoresis. Excellent agreement with experimental data for a rigid particle is obtained. The study provides insights and guidelines in practical applications like drug delivery and other dead-end-pore types of operations such as EOR.

Relaxation behavior in low-frequency complex conductivity of sands caused by bacterial growth and biofilm formation by Shewanella oneidensis under a high-salinity condition

Complex electrical conductivity is increasingly used to monitor subsurface processes associated with microbial activities because microbial cells mostly have surface charges and thus electrical double layers. Although highly saline environments are frequently encountered in coastal and marine sediments, there are limited data available on the complex conductivity associated with microbial activities under a high-salinity condition. Therefore, we have developed the spectral responses of complex conductivity of sand associated with bacterial growth and biofilm formation under a highly saline condition of approximately 1% salinity and approximately 2 S/m pore water conductivity with an emphasis on relaxation behavior. A column test is performed, in which the model bacteria Shewanella oneidensis MR-1 are stimulated for cell growth and biofilm formation in a sand pack, whereas the complex conductivity is monitored from 0.01 Hz to 10 kHz. The test results indicate that the real conductivity increases in the early stage due to the microbial metabolites and the increased surface conduction with cell growth but soon begin to decrease because of the reduction of charge passages due to bioclogging. However, the imaginary conductivity significantly increases with time, and clear bell-shaped relaxation behaviors are observed with the peak frequency of 0.1–1 Hz, associated with the double-layer polarization of cells and electrically conductive pili and biofilms. The Cole-Cole relaxation model appears to capture such relaxation behaviors well, and the modeling results indicate gradual increases in normalized chargeability and decreases in relaxation time during bacterial growth and biofilm formation in the highly saline condition. Comparison with previous literature confirms that the high-salinity condition further increases the normalized chargeability, whereas it suppresses the phase shift and thus the imaginary conductivity. Our results suggest that the complex conductivity can effectively capture microbial biomass formation in sands under a highly saline condition.

Diffusiophoresis of a highly charged soft particle normal to a conducting plane.

Diffusiophoresis of a soft particle in electrolyte solutions normal to a conducting solid plane is investigated theoretically in this study, focusing on the highly charged particle in particular. A pseudo-spectral method based on Chebyshev polynomial is adopted to solve the resultant governing electrokinetic equations. It was found, among other things, that the closer the soft particle is to the plane, the faster it moves in general, provided only the chemiphoresis component of the diffusiophoresis is involved, i.e., no diffusion potential is present. The presence of the conducting plane is found to have three effects upon the particle motion nearby: the geometric boundary confinement effect, the electrostatic mirror-image force analog effect, and the hydrodynamic retarding effect. The enhancement of the double layer polarization by the first two effects leads to the seeming intriguing observation mentioned above. The particle always moves away from the plane in chemiphoresis. If a diffusion potential is present, however, then it is possible to drive the particle toward the plane. The results have potential applications in drug delivery.

Impact of ion partitioning and double layer polarization on diffusiophoresis of a pH-regulated nanogel

Diffusiophoresis of a charge regulated spherical polyelectrolyte nanogel (PE) due to an externally imposed ionic concentration gradient is considered. The immobile charge density of the nanogel develops through the association/dissociation reactions of their inorganic functional groups. The nanogel is ion and fluid permeable with dielectric permittivity different from that of the surrounding electrolyte medium. This difference in dielectric permittivity creates an ion partitioning due to the difference in self energy of ions. The Nernst–Planck equation for ion transport and the Poisson equation (PNP) for the electric field are modified to take into account the ion partitioning effects. The diffusiophoresis mechanism is governed by the electrophoresis generated by the induced electric field and chemiphoresis develops due to the mitigation of counterions across the double layer of the nanogel. In addition, the convection dominated double layer polarization and the counterion condensation as well as the electroosmotic flow created by the gel immobile charge play a role in the diffusiophoresis. All these effects are incorporated through the modified PNP equations coupled with the Navier–Stokes equations. The governing equations in their full form are solved numerically through a control volume approach. Present computed solutions for the limiting cases are in good agreement with the existing solutions based on the first-order perturbation analysis. In order to illustrate the diffusiophoresis mechanism we have measured the induced electric field and effective charge density of the PE and analyzed its dependence on several electrokinetic parameters. The contribution due to chemiphoresis is low for PE compared to a rigid colloid. For a highly permeable PE the diffusiophoretic velocity increases and approaches a saturation for higher range of the PE fixed charge density. The ion partitioning effect depletes counterions in PE to manifests its diffusiophoretic velocity. The diffusiophoretic velocity of PE for pH > IEP is higher than the case for which pH < IEP.

Mechanistic studies of droplet electrophoresis: A review.

Electrophoresis (EP) of droplets is an intriguing phenomenon that has applications in biological systems, separation strategies, and reactor engineering. Droplet EP is significantly different from the classic particle EP because of droplet characteristics such as a mobile surface charge and the nonrigidity of the interface. Also, the liquid-liquid system, where there is an interplay between the hydrodynamic and electrokinetic forces in both phases, adds to the complexity of electrophoretic motion. Due to the vast amount of potential applications of droplet EP, a mechanistic understanding of the droplet motion in the presence of an external electric field is crucial. This review provides a background on the mechanism of droplet EP and summarizes the intrinsic interplay between the different relevant forces in these systems. The review also describes the key differences between droplet EP and particle EP, and the impact of these differences on droplet mobility. Additionally, we schematically summarize the effects of key parameters on droplet EP mobility, such as electric double layer polarization, the development of internal flow inside a droplet and boundaryeffects.

Diffusiophoresis of a Highly Charged Soft Particle in Electrolyte Solutions.

Diffusiophoresis of a soft particle suspended in an infinite medium of symmetric binary electrolyte solution is investigated theoretically in this study, focusing on the chemiphoresis component when there is no global diffusion potential in the bulk solution. The general governing electrokinetic equations are solved with a pseudo-spectral method based on Chebyshev polynomials, and particle mobility, defined as the particle velocity per unit concentration gradient, is calculated. Parameters of electrokinetic interest are examined, in general, to explore their respective impact upon particle motion, such as the fixed charge density and permeability in the outer porous layer, the surface charge density and size of the inner rigid core, and the electrolyte strength in the solution. Nonlinear phenomena such as the motion-deterring double-layer polarization and the counterion condensation effects are scrutinized, in particular, for highly charged soft particles. Mobility reversal is observed in some range of electrolyte strength for highly charged particles. The generation of an axisymmetric counterclockwise vortex flow across the porous layer is found to be responsible for it. The onset of the mobility reversal is synchronized with the appearance or disappearance of this vortex flow. Mobility reversal may happen more than once, with particle moving toward or away from the region of higher solute concentration. The latter is undesirable in the application of drug delivery and thus should be avoided by delicate control of the electrokinetic environment. A local micro diffusion potential is discovered, which always speeds up the migration of coions and slows down that of counterions to guarantee that there is no net electric current across the double layer. Moreover, multilayer structure of the double-layer polarization is discovered when the electrolyte strength is high. The study presented here provides insight and crucial information for practical applications of soft particles, such as drug delivery.

Diffusiophoresis of a highly charged soft particle in electrolyte solutions induced by diffusion potential

Diffusiophoresis of a single soft particle in an electrolyte solution with induced diffusion potential is investigated theoretically in this study. A pseudo-spectral method based on Chebyshev polynomials is adopted to solve the resultant governing electrokinetic equations. Parameters of electrokinetic interest are examined extensively to explore their respective effect upon the particle motion, such as the fixed charge density and the permeability of the outer porous layer, the surface potential and size of the inner rigid core, and the electrolyte strength and magnitude of the induced diffusion potential in the solution. The nonlinear effects pertinent to highly charged particles, such as the double layer polarization effect and the counterion condensation effect, are scrutinized, in particular. Here, nonlinear effects refer to the effects that can only be properly revealed by accurately solving the complete nonlinear Poisson–Boltzmann equation governing the electric potential instead of the simplified linear Helmholtz equation under the Debye–Hückel approximation, valid for lowly charged particles only. We found, among other things, that characteristic local extrema in mobility profiles are mainly due to these two effects. Moreover, a soft particle moves fastest in dilute electrolyte solutions, in general. The smaller the soft particle is, the faster it moves under otherwise identical structural and electrokinetic conditions, provided that the particle radius is smaller than the Debye length, the characteristic thickness of the double layer. The shape of the double layer polarization takes an undulating multilayer form at large electrolyte strength. The results provided here are useful in practical applications such as drug delivery as well as microfluidic and nanofluidic operations.

Enhanced Oxygen Evolution via Electrochemical Water Oxidation using Conducting Polymer and Nanoparticle Composites.

Nano-Co3 O4 was used for electrocatalytic water oxidation due to its promising features of better performance and low cost. An enhanced electrochemical water oxidation performance of the nanoparticles can be achieved by mixing them with other types of highly conductive nano/micro-structured materials. Conductive polymers would be one of the candidates to achieve this goal. Here, we report our recently developed nano-Co3 O4 and polypyrrole composites for enhanced electrochemical water oxidation. We chose polypyrrole as a support of nano-Co3 O4 to obtain highly active sites of nano-Co3 O4 with high conductivity. Morphological and chemical characterization of the prepared materials were performed using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). After immobilizing them individually on fluorine doped tin oxide (FTO) substrate, their electrocatalytic properties toward water oxidation were investigated. The optimum composite materials showed significantly higher electrocatalytic properties compared to that of pure nano-Co3 O4 and polypyrrole. Electrochemical impedance studies indicated that the composite materials possess significantly less electron transfer resistance toward water oxidation reaction compared to that of only polypyrrole or nano-Co3 O4 , while the higher double-layer capacitance and polarization resistance values obtained from fitting of the impedance data represent the faster electrode kinetics in the composite electrocatalyst. Due to the synergetic effect, the optimum nano-Co3 O4 and polypyrrole composites could be represent a novel and promising material for water oxidation.

Electrokinetic transport of a non-conducting liquid droplet in a polyelectrolyte medium

A numerical study on the electrophoresis of a liquid droplet embedded in a polyelectrolyte hydrogel medium is made by considering the full set of governing equations based on the conservation principle. The surface of the droplet is considered to be charged, and the liquid filling the droplet is non-conducting. The dielectric polarization of the non-conducting droplet is also addressed in the present study. The impact of the surface conduction, double layer polarization, and relaxation effects on the electrophoresis of the non-conducting polarizable uniformly charged droplet is elucidated for a wide range of the gel volume charge density, Debye length, and drop viscosity and size. The presence of the gel immobile charge and slip velocity at the droplet surface leads to a stronger surface conduction, which precludes consideration of a simplified model based on the thin-layer assumption. Our numerical solutions at a low ζ—potential corresponding to a droplet of large viscosity, for which surface conduction is negligible—agree well with the existing analytic solutions for a rigid colloid. The strong electroosmotic flow driven by the immobile charges of the gel medium creates a negatively charged drop in the hydrogel medium to translate along the direction of the applied field. Entrapment of the charged drop can be made by regulating the Debye length and volume charge density of the gel. The charged gel medium is found to be efficient in size-based sorting of the liquid drops.