Electrical Double Layer Theory Research Articles

Ion detection in a DNA nanopore FET device.

An ion detection device that combines a DNA-origami nanopore and a field-effect transistor (FET) was designed and modeled to determine sensitivity of the nanodevice to the local cellular environment. Such devices could be integrated into a live cell, creating an abiotic-biotic interface integrated with semiconductor electronics. A continuum model is used to describe the behavior of ions in an electrolyte solution. The drift-diffusion equations are employed to model the ion distribution, taking into account the electric fields and concentration gradients. This was matched to the results from electric double layer theory to verify applicability of the model to a bio-sensing environment. The FET device combined with the nanopore is shown to have high sensitivity to ion concentration and nanopore geometry, with the electrical double layer behavior governing the device characteristics. A logarithmic relationship was found between ion concentration and a single FET current, generating up to 200nA of current difference with a small applied bias.&#xD.

Spontaneous Imbibition and an Interface-Electrostatics-Based Model of the Transition Zone Thickness of Hydrocarbon Reservoirs and Their Theoretical Interpretations

The transition zone (TZ) of hydrocarbon reservoirs is an integral part of the hydrocarbon pool which contains a substantial fraction of the deposit, particularly in carbonate petroleum systems. Consequently, knowledge of its thickness and petrophysical properties, viz. its pore size distribution and wettability characteristic, is critical to optimizing hydrocarbon production in this zone. Using classical formation evaluation techniques, the thickness of the transition zone has been estimated, using well logging methods including resistivity and Nuclear Magnetic Resonance, among others. While hydrocarbon fluids’ accumulation in petroleum reservoirs occurs due to the migration and displacement of originally water-filled potential structural and stratigraphic traps, the development of their TZ integrates petrophysical processes that combine spontaneous capillary imbibition and wettability phenomena. In the literature, wettability phenomena have been shown to also be governed by electrostatic phenomena. Therefore, given that reservoir rocks are aggregates of minerals with ionizable surface groups that facilitate the development of an electric double layer, a definite theoretical relationship between the TZ and electrostatic theory must be feasible. Accordingly, a theoretical approach to estimating the TZ thickness, using the electrostatic theory and based on the electric double layer theory, is attractive, but this is lacking in the literature. Herein, we fill the knowledge gap by using the interfacial electrostatic theory based on the fundamental tenets of the solution to the Poisson–Boltzmann mean field theory. Accordingly, we have used an existing model of capillary rise based on free energy concepts to derive a capillary rise equation that can be used to theoretically predict observations based on the TZ thickness of different reservoir rocks, using well-established formation evaluation methods. The novelty of our work stems from the ability of the model to theoretically and accurately predict the TZ thickness of the different lithostratigraphic units of hydrocarbon reservoirs, because of the experimental accessibility of its model parameters.

A Mini Review on Electroosmotic Phenomena in Porous Media

Electroosmosis phenomenon in porous media finds widespread applications in various fields such as microfluidic systems, polymer electrolyte membrane fuel cells, oil and gas engineering, wastewater sludge dewatering and groundwater dynamics etc. Therefore, the electroosmotic flow mechanism in porous media have attracted broad interests from multiple disciplines. This paper provides an overview on the physical mechanisms and mathematical models for electroosmosis in porous media. The background of electric double layer theory and state-of-art research progress on pore-scale models for electroosmotic flow through porous media are reviewed. Two typical and significant research topics, electroosmosis under pressure coupling effect and nanoscale electroosmotic phenomena, are then focused on. The advances in theoretical analysis, numerical simulation and experimental measurements are summarized. Finally, the potential research directions for the electroosmotic flow in porous media are addressed out.

Ion-Solvent Interactions under Confinement Hold the Key to Tuning the DNA Translocation Speeds in Polyelectrolyte-Functionalized Nanopores.

DNA sequencing and sensing using nanopore technology delves critically into the alterations in the measurable electrical signal as single-stranded DNA is drawn through a tiny passage. To make such precise measurements, however, slowing down the DNA in the tightly confined passage is a key requirement, which may be achieved by grafting the nanopore walls with a polyelectrolyte layer (PEL). This soft functional layer at the wall, under an off-design condition, however, may block the DNA passage completely, leading to the complete loss of output signal from the nanobio sensor. Whereas theoretical postulates have previously been put forward to explain the essential physics of DNA translocation in nanopores, these have turned out to be somewhat inadequate when confronted with the experimental findings on functionalized nanopores, including the prediction of the events of complete signal losses. Circumventing these constraints, herein we bring out a possible decisive role of the interplay between the inevitable variabilities in the ionic distribution along the nanopore axis due to its finite length as opposed to its idealized "infinite" limit as well as the differential permittivity of PEL and bulk solution that cannot be captured by the commonly used one-dimensional variant of the electrical double layer theory. Our analysis, for the first time, captures variations in the ionic concentration distribution across multidimensional physical space and delineates its impact on the DNA translocation characteristics that have hitherto remained unaddressed. Our results reveal possible complete blockages of DNA translocation as influenced by less-than-threshold permittivity values or greater-than-threshold grafting densities of the PEL. In addition, electrohydrodynamic blocking is witnessed due to the ion-selective nature of the nanopore at low ionic concentrations. Hence, our study establishes a functionally active regime over which the PEL layer in a finite-length nanopore facilitates controllable DNA translocation, enabling successful sequencing and sensing through the explicit modulation of translocation speed.

Interrelationship of Electric Double Layer Theory and Microfluidic Microbial Fuel Cells: A Review of Theoretical Foundations and Implications for Performance

Microbial fuel cells and their related microfluidic systems have emerged as promising greener energy alternatives for the exploitation of avenues related to combined power and wastewater treatment operations. Moreover, the potential for their application in biosensing technology is large. However, while the fundamental principles of science that govern the design and operation of microbial fuel cells (MFCs) and microfluidic microbial fuel cells (MMFCs) are similar to those found in colloid science, the literature shows that current research lacks sufficient reference to the electrostatic and electrokinetic aspects, focusing mostly on aspects related to the architecture, design, anodes, microbial growth and metabolism, and electron transfer mechanisms. In this regard, research is yet to consider MFCs and MMFCs in the context of electrostatic and electrokinetic aspects. In this extensive review, we show, for the first time, the interrelationship of MFCs and MMFCs with electric double layer theory. Consequently, we show how the analytical solution to the mean field Poisson–Boltzmann theory relates to these systems. Moreover, we show the interrelationship between MFC and MMFCs’ performance and the electric double layer and the associated electrostatic and electrokinetic phenomena. This extensive review will likely motivate research in this direction.

Effect of surface water on wollastonite carbonation: Activated dissolution and mass transfer

Carbonation of non-hydraulic calcium silicate to produce cement represents a crucial strategy for mitigating carbon emissions in the cement industry. However, the dissolution rate of Ca2+ is the main constraint on the reaction kinetics. In this study, the carbonation of wollastonite (CS) was investigated, with an emphasis on the role of water in the Ca2+ dissolution process. Firstly, we investigated carbonation performance of CS and its influencing factors. Results revealed a significant impact of the water-to-solid (w-s) ratio on carbonation. In the carbonate products, we observed that the dissolution of Ca2+ adsorbed on the particle surface, referred to as “pseudocrystalline replacement”, which can serve as supplementary cementitious material. Additionally, this observation signifies the accordance of Ca2+ dissolution with the electric double layer (EDL) theory. Subsequently, we quantitatively assessed the influence of w-s ratio on carbonation using kinetic models. Based on EDL theory and the kinetic model, we categorized water film effect into three stages and determined the influence of water film thickness on the rate-determining factors of CS carbonation. This provides theoretical support for preparation of CS-based cement. Interestingly, we found that reaction rate constant varies with changes in the w-s ratio, indicating a catalytic role of water. Further analysis using density functional theory demonstrated that the free water combined with the O/Ca sites on CS to form hydroxylated surfaces, thereby decreasing and increasing the abilities of the local and neighboring sites, respectively, to absorb H2CO3. These findings provide a theoretical foundation for the curing method of low-carbon cements and offer a methodology for the preparation of supplementary cementitious materials.

Lead (II) ions enable the ion-specific effects of monovalent anions on the molecular structure and interactions at silica/aqueous interfaces

HypothesisThe adsorption of heavy metal ions such as Pb(II) onto negatively charged minerals such as silica is expected to alter the structure and the interactions at the silica/aqueous interfaces. Besides the solution pH, the inner-sphere sorption of Pb(II) is expected to regulate the surface charge/potential, hypothesized to control the actions of monovalent anions in the aqueous environment. These complex pictures can be probed directly using surface-sensitive sum-frequency generation (SFG) spectroscopy. ExperimentsThe pH-dependent water structure within the double layer at silica/aqueous interfaces under the influence of different ions was examined using SFG. The recorded SFG spectra were deconvoluted into the Stern layer (SL) and diffuse layer (DL) using the maximum entropy method in conjunction with the electrical double-layer theory. FindingsStandalone monovalent sodium salts do not exhibit ion-specific effects on the silica/aqueous interfaces. However, the mixture of Pb(II) species and each of these salts display profound ion-specific effects on the structure of silica/aqueous interfaces, indicating the role of Pb(II) as an enabler of the ion-specificity of the investigated monovalent anions. The interesting effect arises from a complex interplay between the physical processes (i.e., electrostatic interactions, screening effects, etc.) and chemical processes such as the hydrolysis of Pb(II) ions, ion complexation, protonation and deprotonation of the surface silanol group.

Impact of Pt(hkl) Electrode Surface Structure on the Electrical Double Layer Capacitance.

The classical theory of the electrical double layer (EDL) does not consider the effects of the electrode surface structure on the EDL properties. Moreover, the best agreement between the traditional EDL theory and experiments has been achieved so far only for a very limited number of ideal systems, such as liquid metal mercury electrodes, for which it is challenging to operate with specific surface structures. In the case of solid electrodes, the predictive power of classical theory is often not acceptable for electrochemical energy applications, e.g., in supercapacitors, due to the effects of surface structure, electrode composition, and complex electrolyte contributions. In this work, we combine ab initio molecular dynamics (AIMD) simulations and electrochemical experiments to elucidate the relationship between the structure of Pt(hkl) surfaces and the double-layer capacitance as a key property of the EDL. Flat, stepped, and kinked Pt single crystal facets in contact with acidic HClO4 media are selected as our model systems. We demonstrate that introducing specific defects, such as steps, can substantially reduce the EDL capacitances close to the potential of zero charge (PZC). Our AIMD simulations reveal that different Pt facets are characterized by different net orientations of the water dipole moment at the interface. That allows us to rationalize the experimentally measured (inverse) volcano-shaped capacitance as a function of the surface step density.

A dynamic soil freezing characteristic curve model for frozen soil

The soil freezing characteristic curve (SFCC) plays a fundamental role in comprehending thermo-hydraulic behavior and numerical simulation of frozen soil. This study proposes a dynamic model to uniformly express SFCCs amidst varying total water contents throughout the freezing-thawing process. Firstly, a general model is proposed, wherein the unfrozen water content at arbitrary temperature is determined as the lesser of the current total water content and the reference value derived from saturated SFCC. The dynamic performance of this model is verified through test data. Subsequently, in accordance with electric double layer (EDL) theory, the theoretical residual and minimum temperatures in SFCC are calculated to be −14.5 °C to −20 °C for clay particles and −260 °C, respectively. To ensure that the SFCC curve ends at minimum temperature, a correction function is introduced into the general model. Furthermore, a simplified dynamic model is proposed and investigated, necessitating only three parameters inherited from the general model. Additionally, both general and simplified models are evaluated based on a test database and proven to fit the test data exactly across the entire temperature range. Typical recommended parameter values for various types of soils are summarized. Overall, this study provides not only a theoretical basis for most empirical equations but also proposes a new and more general equation to describe the SFCC.© 2023 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Physical analysis of self-discharge mechanism for supercapacitor electrode for hybrid electric energy storage system

Self-discharge is a spontaneous process that has considerable adverse effects on the performance of supercapacitors. In order to quantitatively investigate the contribution of self-discharge mechanism, this paper proposed a theoretical self-discharge model for carbon electrode of supercapacitors based on electric double layer theory. Three physical contributions, i.e., side reactions, ion diffusion, and ohmic leakage, were investigated. In addition, self-discharge measurement of carbon electrode was performed to validate such theoretical model. The results indicated that the potential drop due to side reactions was negligible throughout the self-discharge process in all cases. In addition, ion diffusion dominated for low initial potential and short holding time, accounting for 50%–80% of self-discharge. On the other hand, ohmic leakage dominated for high initial potential and long holding time. Furthermore, the potential drop due to ion diffusion increased monotonously with time while the potential drop due to ohmic leakage remained constant throughout the self-discharge. Moreover, the potential drop due to ohmic leakage increased with the increase in holding time, which compensated for the decreasing potential drop due to ion diffusion. This could explain the fact that the total electrode potential decay during the self-discharge was independent of holding time. Finally, dimensional analysis was performed to predict self-discharge. Time constants of side reactions, ion diffusion, and ohmic leakage were derived. Overlapping dimensionless electrode potential versus dimensionless time indicated that such dimensional analysis was generally applicable to predict self-discharge of carbon-based supercapacitors at a given initial potential and time.

Theoretical Interpretation of pH and Salinity Effect on Oil-in-Water Emulsion Stability Based on Interfacial Chemistry and Implications for Produced Water Demulsification

The petroleum industry produces thousands of barrels of oilfield waters from the initial stage driven by primary production mechanisms to the tertiary stage. These produced waters contain measurable amounts of oil-in-water emulsions, the exact amounts being determined by the chemistry of the crude oil. To meet strict environmental regulations governing the disposal of such produced waters, demulsification to regulatory permissible levels is required. Within the electric double layer theory, coupled with the analytical solutions to the Poisson–Boltzmann Equation, continuum electrostatics approaches can be used to describe the stability and electrokinetic properties of emulsions. In the literature, much of the surface charge density and zeta potential relationship to emulsion stability has been confined to systems with less salinity. In this paper, we have exploited the theoretical foundations of the electric double layer theory to carry out theoretical evaluations of emulsion salinity based on zeta potential and surface charge density calculations. Most importantly, our approaches have enabled us to extend such theoretical calculations to systems of the higher salinity characteristic of oil-in-water emulsions found in oilfield-produced waters, based on crude oil samples from the literature with varying surface chemistry. Moreover, based on the definition of acid crude oils, our choice of samples represents two distinct classes of crude oils. This approach enabled us to evaluate the stability of emulsions associated with these produced oilfield waters in addition to predicting the potential of demulsification using demulsifiers. Given that the salinity range of this study is that encountered with the vast majority of produced oilfield waters, the findings from our theoretical predictions are perfect guides as far as emulsion stability is concerned.

Ion adsorption on nanofiltration membrane surface and its effect on rejection of charged solutes: A zeta potential approach

The net charge on nanofiltration (NF) membrane surface, which differs from the inherent fixed charges due to the adsorption of ions from the feed water, has a great effect on the rejection selectivity of the membrane. In this study, by taking advantage of the electric double layer theory, streaming zeta potentials of the membrane varying with solution composition and concentration were utilized to investigate the characteristics and mechanisms of ion adsorption on the typical negatively charged NF membrane surface, which were then utilized to interpret the (selective) rejection behaviors for different solutes. Results showed that the cation adsorption all followed the Langmuir isotherm, indicative of monolayer adsorption mechanism. Both Langmuir adsorption fitting results and Gibbs free energy calculation showed that the cation adsorption is primarily of Coulomb force origin, rather than the chemical complexation. In addition, Ca2+ and Mg2+ could occupy the membrane surface much more facilely than Na+, with the adsorption of divalent cations reaching near saturation at relatively low concentrations in the magnitude of 1 mmol L−1. Contrary to the original presumption, no remarkable difference was observed between the adsorptions of Ca2+ and Mg2+, as was also the case between -COOH and -SO3H, the two common types of acidic functional groups on membrane surface for cation adsorption. These mechanisms of ion adsorption, when incorporated with the existing NF theory, could well interpret the seemingly “strange” rejection behaviors including the counter-ion competition effect. This study evidenced that zeta potential measurements could act as a useful approach for the study of ion adsorption on NF membrane surface and selective rejection mechanisms of NF membranes as well.

Ag/Reduced Graphene Oxide/Carbon Fiber Composites as Electrodes for Highly Stable and Responsive Marine Electric Field Sensors

The increasing demand for maintenance-free, cost-effective, and high-stability flexible electrodes is highly anticipated for the research of marine electric-field sensors, and various carbon fibers (CFs) are considered the most suitable carbon-based electrode for the purpose. Nevertheless, the conventional fabrication of acidified or nitrogenized CF electrodes did not attract great attention for commercial application, which has limitations in the case of simultaneous cost-effectiveness and long-term stability. In this study, with homemade reduced graphene oxide (rGO) load, the prepared double carbon substrate Ag-rGO-CF composite electrodes have a honeycomb rGO conductive skeleton and the dispersed Ag nanoparticles via a facile hermetic oxidation process and hydrothermal method. The prepared Ag-rGO-CF composite electrode demonstrated an outstanding electrode performance, including low resistance, high electrochemical reaction rate, and stable potential with long-term stability and low cost. The electric double-layer (EDL) structure theory from the Gouy–Chapman–Stern model and double carbon substrate mechanism between CFs and rGO with the help of Ag were clarified, whose principle is to strengthen the adsorption force and electrostatic attraction of the electrode and to offset the residual charge while reducing the thickness of the electric double layer to achieve the purpose of potential balance. The Ag-rGO-CF composite electrode has presented long-term stability and high sensitivity, especially a wide application bandwidth. The Ag-rGO-CF composite electrode made in this study is a competitive candidate material as a flexible marine electrode field sensor.

Probing interfacial water structure induced by charge reversal and hydrophobicity of silica surface in the presence of divalent heavy metal ions using sum frequency generation spectroscopy

HypothesisAdsorption of divalent heavy metal ions (DHMIs) at the mineral–water interfaces changes interfacial chemical species and charges, interfacial water structure, Stern (SL), and diffuse (DL) layers. These molecular changes can be detected by probing changing orientation and hydrogen-bond network of interfacial water molecules in response to changing local charges and hydrophobicity. ExperimentsSum-frequency generation (SFG) spectroscopy was used to probe changes in vibrational resonances of interfacial OH vs. DHMI concentration and pH. SFG spectra were deconvoluted using the measured surface potential and maximum entropy method in conjunction with the electrical double-layer theory for the SL and DL structures and correlated by hydrophobicity. FindingsThree surface charge reversals (CRs) were detected at low (CR1), medium (CR2), and high (CR3) pHs. Unlike CR1, SFG signals were minimized at CR2 and CR3 for DHMIs-silica systems highlighting considerable alterations in the structure of interfacial waters due to the inner-sphere sorption of metal hydroxo complexes. SFG results showed “hydrophobic-like” stretching modes at > 3600 cm−1 for Pb-, Cu-, and Zn-treated silica. However, contact angle measurements revealed the hydrophobization of silica only in the presence of Pb(II), as confirmed by an in-depth SFG analysis of the hydrogen-bond network of the interfacial water molecules in the SL.

Effect of original crystal size of kaolinite on the formation of intercalation compounds of coal-measure kaolinite

Coarse crystalline (14.8–19.8 µm) and crypto crystalline (0.51–0.92 µm) coal-measure kaolinite were selected to explore the effect of original crystal sizes on intercalation. Characterization methods of SEM, XRF, XRD, FT-IR, and TG-DTG were utilized to study the original crystal size distribution, chemical composition, crystal structure changes, vibrational characteristics of functional groups, and thermal stability before and after intercalation. The test results reflected that the original crystal size significantly affects the formation of intercalated compounds. Coal-measure kaolinite intercalation compounds have been successfully prepared. Nonlinear fitting curves between the different characterization methods and the original average crystal size showed a significant positive correlation. The results showed that the intercalation effect of coarse crystalline was more significant than those of crypto crystalline, that is to say, the larger the original crystal sizes of coal-measure kaolinite, the higher the intercalation rate, the higher the grafting rate and the more pronounced the mass loss rate. Based on the theory of surface energy and surface charge of mineral particles, the electrical double layer theory which was different from the previous researchers was proposed to explain the intrinsic mechanism of the effect of original crystal size on intercalation.

Research on optimization test of electrode material in desulfurization wastewater purification system of coal-fired power plant

The traditional coal-fired power plant wastewater treatment technology is faced with the problems of complex system, high operating cost, easy scaling and corrosion, etc. Therefore, coal-fired power plants need to adopt an integrated multi-functional coupling system, taking into account the desalination, anti-scaling, etc. function to remove pollutants from wastewater. Electro-adsorption technology is a technology that can realize water purification and water desalination. It is a new type of water treatment technology. It can effectively remove impurity ions in water without scaling under the premise of low energy consumption. In this paper, the development history of electrosorption theory, the principle of electrosorption, the main points of electric double layer theory, the structure of electrosorption and its working process are reviewed. Several main electrode materials and their advantages and disadvantages are introduced in this paper. Based on the increasingly stringent environmental protection requirements, electro-adsorption technology is expected to be widely used in the power industry due to its advantages of low energy consumption, low cost, and no secondary pollution.

Electrokinetic energy conversion in nanochannels with surface charge-dependent slip

Electrokinetic energy conversion in nanochannels has attracted increasing attention in recent years. However, few scholars have studied the impact of the intrinsic distinction between surface charge-dependent and independent slip on power generation performance. In this work, we consider three kinds of boundary conditions, including no slip, independent slip and surface charge-dependent slip, to investigate the ion concentration distribution, velocity and energy conversion performance in nanochannels combined with the electric double layer theory. The results show that the boundary slip can improve the output performance significantly. Besides, due to the restriction of surface charge on boundary slip, the power generation performance under independent slip and dependent slip exhibits a significant deviation. Thus, the independent slip model is not suitable for predicting ion transport and fluidic flow in practical applications. Moreover, the ion concentration and channel height also play an optimizing role in the performance of electrokinetic energy conversion. We find the boundary slip is a benefit to promote the ion flux due to the velocity mechanism but has little effect on ion concentration distribution. The findings of this study can help for better understanding the ion transfer behavior in nanochannels, which provides useful information for the design and optimization of nanochannel power generation.

Mg2+ and Cu2+ Charging Agents Improving Electrophoretic Deposition Efficiency and Coating Adhesion of Nano-TbF3 on Sintered Nd-Fe-B Magnets

In order to prepare nano-TbF3 coating with high quality on the surface of Nd-Fe-B magnets by electrophoretic deposition (EPD) more efficiently, Mg2+ and Cu2+ charging agents are introduced into the electrophoretic suspension and the influence on the electrophoretic deposition is systematically investigated. The results show that the addition of Mg2+ and Cu2+ charging agents can improve the electrophoretic deposition efficiency and coating adhesion of nano-TbF3 powders on sintered Nd-Fe-B magnets. The EPD efficiency increases by 116% with a relative content of Mg2+ as 3%, while it increases by 109% with a relative content of Cu2+ as 5%. Combining the Hamaker equation and diffusion electric double layer theory, the addition of Mg2+ and Cu2+ can change the zeta potential of charged particles, resulting in the improvement of EPD efficiency. The relative content of Mg2+ below 3% and Cu2+ below 5% can increase the thickness of the diffusion electric double layer, the excessive addition of a charging agent will compress the diffusion electric double layer, and thicker diffusion layer represents higher zeta potential. Furthermore, the addition of Mg2+ and Cu2+ charging agents greatly improves the coating adhesion, and the critical load for the cracking of the coating increases to 146.4 mN and 40.2 mN from 17.9 mN, respectively.

Concentration dependence of yield stress of bentonite suspension and corresponding particle interactions

The prediction of the concentration dependence of the yield stress of bentonite suspension is important for both practical use and fundamental study. Whilst a great amount of research work has been undertaken concerning the theoretical prediction based on the Face-Face repulsive interactions alone, the effect of inclination angle α and separation distance D between clay particles has seemingly received little attention in the research literature. This article presents the prediction model of yield stress developed using the strength of interparticle interactions between two non-parallel platy particles of finite dimensions, aimed at providing insights into the evolution of interparticle interaction with increasing the mass fraction. The repulsive force between the particles for a range of values of geometrical parameters computed using the Gouy-Chapman theory of electrical double layer, along with the calculation presented on van der Waals force, provide a more quantitative means of evaluating the interparticle force in bentonite suspension. It was found that as α and D between particles increase, the magnitude of interparticle force including repulsive and attractive force decreases quickly. The ultimate comprehensive investigation indicated that as the mass fraction of bentonite suspension increases, interparticle interaction propagates from attractive interaction to repulsive attraction, and finally shifts to attractive interaction. In this process, inter-particle separation distance decreases, accompanied by a transition process from a larger inclination angle to a nearly parallel orientation between the particles. The prediction model of yield stress considering the influence of concentration variation on α and D yields a good prediction of experimental data. The results of the present study should help interpret test results in cases where it may not be entirely possible to maintain a parallel particle orientation.

A Pore-Scale Physical Model for Electric Dewatering of Municipal Sludge Based on Fractal Geometry

Water in municipal sludge has an electric double layer (EDL), and can be rapidly removed via electroosmosis. However, the understanding of the physical mechanisms of electroosmosis is limited owing to the complex pore structure of sludge porous media, which may restrict the development of electric dewatering (EDW) technology. In this study, the sludge microstructure was measured using the computerized tomography technique and quantitatively characterized using fractal geometry. A new pore-scale physical model for the electroosmotic flow through sludge porous media was developed based on the EDL theory, and the analytical expressions for the electroosmotic flow rate and permeability coefficient were determined. The proposed fractal pore-scale model for electroosmotic flow was validated by comparing it with the measured electroosmotic flow rate using a self-designed EDW device for municipal sludge composed of a hydraulic system and applied electric field. The results show that the electroosmotic flow rate of sludge porous media depends not only on the applied electric field, but also on the sludge microstructure including porosity, maximum pore size, and pore and tortuosity fractal dimensions. However, the correlation between the electroosmotic flow rate and the voltage gradient was usually nonlinear, and became linear only if the tortuosity fractal dimension equaled to one (straight flow path). The electroosmotic permeability coefficient increases with an increase in porosity; however, it is lowered by the increased pore and tortuosity fractal dimensions under a certain porosity. The electroosmosis phenomenon can effectively remove part of the interstitial water and surface water in sludge porous media, and the proposed electroosmotic physical model provides a useful theoretical basis for the development of EDW technology for municipal sludge.