Debye Screening Length Research Articles

Starting Electroosmosis in a Fibrous Porous Medium with Arbitrary Electric Double-Layer Thickness

The transient electroosmotic response in a charged porous medium consisting of a uniform array of parallel circular cylindrical fibers with arbitrary electric double layers filled with an electrolyte solution, for the stepwise application of a transverse electric field, is analyzed. The fluid momentum conservation equation is solved for each cell by using a unit cell model, where a single cylinder is surrounded by a coaxial shell of the electrolyte solution. A closed-form expression for the transient electroosmotic velocity of the bulk fluid in the Laplace transform is obtained as a function of the ratio of the cylinder radius to the Debye screening length and the porosity of the fiber matrix. The effect of the fiber matrix porosity on the continuous growth of the electroosmotic velocity over time is substantial and complicated. For a fiber matrix with larger porosity, the bulk fluid velocity takes longer to reach a certain percentage of its final value. Although the final value of the bulk fluid velocity generally increases with increasing porosity, early velocities may decrease with increasing porosity. For a given fiber matrix porosity, the transient electroosmotic velocity is a monotonically increasing function of the ratio of the cylinder radius to the Debye length.

Tunable Casimir Equilibria in a Dual-Liquid System

The Casimir effect, a macroscopic manifestation of quantum phenomena, arises from zero-point energy and thermal fluctuations. When two objects are brought into close proximity, the Casimir effect manifests as a repulsive force, while at greater separations, it transitions to an attractive force. There exists a specific distance at which the Casimir force vanishes, referred to as the stable Casimir equilibrium. Stable Casimir equilibria arise from the curve minima of the Casimir energy, which can create spatial trapping. The manipulation of stable Casimir equilibria offers promising applications in areas such as tunable optical resonators and self-assembly. This paper presents a scheme for achieving tunable Casimir equilibria in a dual-liquid system. The system comprises a multilayered stratified structure with a gold substrate. Above the gold substrate, a stratified liquid system is formed due to the immiscibility between organic solutions and water. The lower-density solution is on top, while the higher-density one lies beneath. Our results suggest that a stable Casimir equilibrium for a suspended gold nanoplate can be realized, when the suspended gold nanoplate is immersed in organic solutions of toluene or benzene. Moreover, the height of the suspended gold nanoplate, determined by the stable Casimir equilibrium, can be precisely tuned by varying the thickness of the water layer. The effects of finite temperature and ionic concentration on the Casimir equilibria are also analyzed in this work. The results suggest that the separation heights of Casimir equilibria decrease with increasing the temperature. Interestingly, the ionic concentration in water significantly affects the Casimir pressure when the Debye screening length is comparable or smaller than the separation, allowing for a wide range of modulations on Casimir equilibria. This work opens a new avenue for tuning Casimir equilibria, with significant applications for "quantum trapping" of micro-nano particles.

Biosensing beyond Debye screening length using epitaxial graphene field-effect transistors on SiC substrate

We demonstrated the antigen detection beyond Debye screening length using antibody-modified epitaxial graphene field-effect transistors (FETs), which have been considered to be impossible because of the Debye screening length. The transfer characteristics of the graphene FETs with a single crystal epitaxial graphene film showed no concentration dependence of buffer solutions. The capacitance measurements in an epitaxial graphene FET also showed no concentration dependence, indicating that the solution-gate capacitance of the epitaxial graphene FET was independent on the Debye screening length. In addition, the minimum capacitance values were also almost independent for the concentration change. Since the Debye screening length depends on the ionic strength of the solution, which is proportional to the buffer solution concentration, the electrical characteristics of solution-gated epitaxial graphene FETs are almost independent on the Debye screening length owing to their small quantum capacitance. The antibody-modified epitaxial graphene FETs indeed detected the antigen, indicating that the epitaxial graphene FETs can detect the target beyond the Debye screening length without any complicated device fabrication processes and other desalted apparatuses.

Strong Impact of Particle Size Polydispersity on the Thermal Conductivity of Yukawa Crystals

Control of thermal transport in colloidal crystals plays an important role in modern technologies. A deeper understanding of the governing heat transport processes in various systems, such as polydisperse colloidal crystals, is required. This study shows how strongly the particle size polydispersity of a model colloidal crystal influences the thermal conductivity. The thermal conductivity of model colloidal crystals has been calculated using molecular dynamics simulations. The model crystals created by particles interacting through Yukawa (screened-Coulomb) interaction are assumed to have a face-centered cubic structure. The influence of the Debye screening length, contact potential, and particle size polydispersity on the thermal conductivity of Yukawa crystals was investigated. It was found that an increase in particle size polydispersity causes a strong—almost fivefold—decrease in the thermal conductivity of Yukawa crystals. In addition, the obtained results showed that the effect of the particle size polydispersity on reducing the thermal conductivity of Yukawa crystals is stronger than changes in values of the Debye screening length or the contact potential.

Frequency Response of the Diffuse-Charge Dynamics in Electrochemical Systems with a Graphene Electrode

We investigate the frequency response of diffuse-charge dynamics related to a 1:1 symmetric electrolyte containing a graphene electrode by solving the governing Poisson-Nernst-Planck equations subjected to appropriate boundary conditions in the asymptotic limit ε = λ D /H → 0, where λ D is the Debye screening length and H is the half-thickness of the electrolyte. Using the method of matched asymptotic expansion, we first solve the leading order non-linear problem for equilibrium state at a nonzero applied DC voltage in the presence of Stern layer(s). Then, we extend the leading order asymptotic analysis to derive an analytic expression for the impedance of the graphene-based electrochemical cell when a small AC voltage perturbation is added to the applied DC voltage. Finally, we use a suitable scaling of the impedance parameters to expose the impacts of the ion concentration and the DC bias voltage on the frequency response for possible applications involving the graphene-electrolyte interface.

Theoretical Investigation of the Potential-Dependent CO Adsorption on Copper Electrodes.

Despite the importance of CO adsorption in many electrocatalytic reaction mechanisms, there has been little investigation of the dependence of the free energy of CO adsorption on the applied potential. Herein, we report on the potential-dependent adsorption of CO on Cu electrodes using a grand-canonical density functional theory approach. We demonstrate that, within the working potential range of electrocatalytic CO2 reduction on Cu(111) and Cu(100), the CO adsorption strength can change by over 0.1 eV. Our analyses explain the potential dependence through an interfacial capacitance loss upon CO adsorption as well as orbital relaxation induced by the electrode potential. Via sensitivity analysis with respect to two electrolyte model parameters (solvent dielectric constant and Debye screening length), we find that the surface excess charge density is a useful descriptor of the CO adsorption free energy.

Diffusiophoresis of a Charged Soft Sphere in a Charged Spherical Cavity

The quasi-steady diffusiophoresis of a soft particle composed of an uncharged hard sphere core and a uniformly charged porous surface layer in a concentric charged spherical cavity full of a symmetric electrolyte solution with a concentration gradient is analyzed. By using a regular perturbation method with small fixed charge densities of the soft particle and cavity wall, the linearized electrokinetic equations relevant to the fluid velocity field, electric potential profile, and ionic concentration distributions are solved. A closed-form formula for the diffusiophoretic (electrophoretic and chemiphoretic) velocity of the soft particle is obtained as a function of the ratios of the core-to-particle radii, particle-to-cavity radii, particle radius to the Debye screening length, and particle radius to the permeation length in the porous layer. In typical cases, the confining charged cavity wall significantly influences the diffusiophoresis of the soft particle. The fluid flow caused by the diffusioosmosis (electroosmosis and chemiosmosis) along the cavity wall can considerably change the diffusiophoretic velocity of the particle and even reverse its direction. In general, the diffusiophoretic velocity decreases with increasing core-to-particle radius ratios, particle-to-cavity radius ratios, and the ratio of the particle radius to the permeation length in the porous layer, but increases with increasing ratios of the particle radius to the Debye length.

NAGase sensing in 3% milk: FET-based specific and label-free sensing in ultra-small samples of high ionic strength and high concentration of non-specific proteins

Biosensing with biological field-effect transistors (bioFETs) is a promising technology toward specific, label-free, and multiplexed sensing in ultra-small samples. The current study employs the field-effect meta-nano-channel biosensor (MNC biosensor) for the detection of the enzyme N-acetyl-beta-D-glucosaminidase (NAGase), a biomarker for milk cow infections. The measurements are performed in a 0.5 μL drops of 3% commercial milk spiked with NAGase concentrations in the range of 30.3 aM–3.03 μM (Note that there is no background NAGase concentration in commercial milk). Specific and label-free sensing of NAGase is demonstrated with a limit-of-detection of 30.3 aM, a dynamic range of 11 orders of magnitude and with excellent linearity and sensitivity. Additional two important research outcomes are reported. First, the ionic strength of the examined milk is ∼120 mM which implies a bulk Debye screening length <1 nm. Conventionally, a 1 nm Debye length excludes the possibility of sensing with a recognition layer composed of surface bound anti-NAGase antibodies with a size of ∼10 nm. This apparent contradiction is removed considering the ample literature reporting antibody adsorption in a predominantly surface tilted configuration (side-on, flat-on, etc.). Secondly, milk contains a non-specific background protein concentration of 33 mg/ml, in addition to considerable amounts of micron-size heterogeneous fat structures. The reported sensing was performed without the customarily exercised surface blocking and without washing of the non-specific signal. This suggests that the role of non-specific adsorption to the BioFET sensing signal needs to be further evaluated. Control measurements are reported.

Impedance of nanocapacitors from molecular simulations to understand the dynamics of confined electrolytes

Nanoelectrochemical devices have become a promising candidate technology across various applications, including sensing and energy storage, and provide new platforms for studying fundamental properties of electrode/electrolyte interfaces. In this work, we employ constant-potential molecular dynamics simulations to investigate the impedance of gold-aqueous electrolyte nanocapacitors, exploiting a recently introduced fluctuation-dissipation relation. In particular, we relate the frequency-dependent impedance of these nanocapacitors to the complex conductivity of the bulk electrolyte in different regimes, and use this connection to design simple but accurate equivalent circuit models. We show that the electrode/electrolyte interfacial contribution is essentially capacitive and that the electrolyte response is bulk-like even when the interelectrode distance is only a few nanometers, provided that the latter is sufficiently large compared to the Debye screening length. We extensively compare our simulation results with spectroscopy experiments and predictions from analytical theories. In contrast to experiments, direct access in simulations to the ionic and solvent contributions to the polarization allows us to highlight their significant and persistent anticorrelation and to investigate the microscopic origin of the timescales observed in the impedance spectrum. This work opens avenues for the molecular interpretation of impedance measurements, and offers valuable contributions for future developments of accurate coarse-grained representations of confined electrolytes.

Theoretical analysis of the electronic structure and electron collision ionization process in a strongly coupled semi-classical plasma environment

In this work, we propose a distorted wave method in the relativistic framework to analyze the electron collision ionization process under the influence of strongly coupled semi-classical plasmas. A pseudopotential is used to represent the screened interactions between the core and outer electron, together with solving the Dirac equation to take care of the relativistic effects. This potential characterizes the plasma in terms of the Debye screening length and the de Broglie wavelength, which takes into account both the quantum mechanical effects of diffraction (short distance) and the collective effects (long distance). With this method, the traditional central-field potential is modified, the bound- and continuum-electron wavefunctions and the collision matrix elements are estimated under a relativistic Dirac–Coulomb framework. The strongly coupled semi-classical plasma effects on the energies, transition rates, and the electron impact ionization cross sections are investigated in detail, using the hydrogen atom as an example. Our results agree well with other available theoretical data. The reported results and the computational scheme are not only of interest to the communities such as atomic and plasma physics, but also have implications for many other areas, including astrophysical objects, inertial confinement fusion plasmas, etc.

Transient electrophoresis of a conducting cylindrical colloidal particle suspended in a Brinkman medium

The time-dependent electrophoresis of an infinitely cylindrical particle in an electrolyte solution, saturated in a charged porous medium after the sudden application of a transverse or tangential step electric field, is investigated semi-theoretically with an arbitrary double-layer thickness in an arbitrary direction relative to the cylinder. The time-dependent modified Brinkman equation with an electric force term, which governs the fluid flow field, is used to model the porous medium and is solved by using the Laplace transform technique. Explicit formulas, for the time-dependent electrophoretic velocity of the cylindrical particle in Laplace’s transform domain, have been derived for both axially and transversely when the uniform electric fields are imposed. They can also be linearly superimposed for an arbitrarily oriented relative to the electric field. Semi-analytical results for the electrophoretic velocities are presented as functions of the dimensionless elapsed time, the ratio of the particle radius to the Debye length, the particle-to-medium density ratio, and the permeability parameter of the porous medium. The results demonstrate, in general, that the growth of the electrophoretic velocities with the time scale are more slower for high permeability, and the effect of the relaxation time for unsteady electrophoresis is found to be negligible, regardless of the thickness of the double layer, the relative mass density or the permeability of the medium. The normalized transient electrophoretic velocities exhibit a consistent upward trend as the ratio of the particle radius to the Debye screening length increases. Conversely, they display a consistent downward trend as the particle-to-fluid density ratio increases, while all other parameters remain constant. The effect of the relaxation time for the transient electrophoresis is much more important for a cylindrical particle than for a spherical particle due to its smaller specific surface area.

Mechanistic elucidation of the catalytic activity of silver nanoclusters: exploring the predominant role of electrostatic surface.

The core and the ligand shell of metal nanoclusters (MNCs) have an influential role in modulating their spectroscopic signatures and catalytic properties. The aspect of electrostatic interactions to regulate the catalytic properties of MNCs has not been comprehensively addressed to date. Our present work conclusively delineates the role of the metal core and the electrostatic surface of MNCs involved in the reduction of nitroarenes. A facile surface modification of mercaptosuccinic acid (MSA)-templated AgNCs has been selectively achieved through Mg2+ ions (Mg-AgNCs). Microscopic studies suggest that the size of Mg-AgNCs is ∼3.3 nm, which is considerably higher than that of MSA-templated AgNCs (∼1.75 nm), confirming the formation of a nano-assembled structure. Our spectroscopic and microscopic experiments revealed that the negatively charged AgNCs efficiently catalyze the reduction of 4-nitrophenol (4-NP) with a rate constant of 0.23 ± 0.01 min-1. However, upon surface modification, the catalytic efficiency almost doubles due to the formation of Mg-AgNCs. Catalysis through AgNCs and Mg-AgNCs collectively portrays the role of the core and electrostatic surfaces. Furthermore, the role of electrostatic interaction has been substantiated by varying the ionic strength of the medium, as well as employing different molecular systems. A quantitative assessment of the Debye screening length asserts the correlation between the ionic strength of the medium and the role of electrostatic interactions involved herein. This highly enhanced catalytic aspect has been utilized for the real sample analysis, wherein AgNCs unexpectedly outperform Mg-AgNCs. This approach of real sample analysis also emanates the role of electrostatics involved. This comprehensive investigation represents the influential role of the core and ligand shell of MNCs as well as the role of electrostatics on its catalytic activities, which is relevant for the rational design of highly efficient catalysts.

Power and Sensitivity Management of Carbon Nanotube Transistor Glucose Biosensors.

Continuous glucose monitoring (CGM), which is significant for the daily management of diabetes, requires a low-power-consumption sensor system that can track low nanomolar levels of glucose in physiological fluids, such as sweat and tears. However, traditional electrochemical methods are limited to analytes in micromolar to millimolar ranges and entail high power consumption. Carbon nanotube (CNT) film field-effect transistors (FETs) are promising for constructing extremely sensitive biosensors, but their wide applications in CGM are limited by the strong screening effect of physiological fluids and the zero charge of glucose molecules. In this study, we demonstrate a glucose aptamer-modified CNT FET biosensor to realize a highly sensitive CGM system with sub-nW power consumption by applying a suitable gate voltage. A positive gate voltage can enlarge the effective Debye screening length at the double layer to reduce the local ion population nearby and then improve the sensitivity of the FET-based biosensors by 5 times. We construct CNT FET sensors for CGM with a limit of detection of 0.5 fM, a record dynamic range up to 109, and a power consumption down to ∼100 pW. The proposed field-modulated sensing performance scheme is applicable to other aptamer-based FET biosensors for detecting neutral or less charged molecules and opens opportunities to develop facilely modulated, highly sensitive, low-power, and noninvasive CGM systems.

Label-Free Neuropeptide Detection beyond the Debye Length Limit.

Biosensors with high selectivity, high sensitivity, and real-time detection capabilities are of significant interest for diagnostic applications as well as human health and performance monitoring. Graphene field-effect transistor (GFET) based biosensors are suitable for integration into wearable sensor technology and can potentially demonstrate the sensitivity and selectivity necessary for real-time detection and monitoring of biomarkers. Previously reported DC-mode GFET biosensors showed a high sensitivity for sensing biomarkers in solutions with a low salt concentration. However, due to Debye length screening, the sensitivity of the DC-mode GFET biosensors decreases significantly during operation in a physiological fluid such as sweat or interstitial fluid. To overcome the Debye screening length limitation, we report here alternating current (AC) mode heterodyne-based GFET biosensors for sensing neuropeptide-Y (NPY), a key stress biomarker, in artificial sweat at physiologically relevant ionic concentrations. Our AC-mode GFET biosensors show a record ultralow detection limit of 2 × 10-18 M with an extensive dynamic range of 10 orders of magnitude in sensor response to target NPY concentration. The sensors were characterized for various carrier frequencies (ranging from 30 kHz to 2 MHz) of the applied AC voltages and various salt concentrations (10, 50, and 100 mM). Contrary to DC-mode sensing, the AC-mode sensor response increases with an increase in salt concentration in the electrolyte. The sensor response can be further enhanced by tuning the carrier frequency of the applied AC voltage. The optimum response frequency of our sensor is approximately 400-600 kHz for salt concentrations of 50 and 100 mM, respectively. The salt-concentration- and frequency-dependent sensor response can be explained by an electrolyte-gated capacitance model.

Structure of ionic liquids and concentrated electrolytes from a mesoscopic theory

Recently, underscreening in concentrated electrolytes was discovered in experiments and confirmed in simulations and theory. It was found that the correlation length of the charge-charge correlations, λs, satisfies the scaling relation λs/λD∼(a/λD)n, where λD is the Debye screening length and a is the ionic diameter. However, different values of n were found in different studies. In this work we solve this puzzle within the mesocopic theory that yielded n = 3 in agreement with experiments, but only very high densities of ions were considered [A. Ciach and O. Patsahan, J.Phys.: Condens. Matter33, 37LT01 (2021)]. Here we apply the theory to a broader range of density of ions and find that different values of n in the above scaling can yield a fair approximation for λs/λD for different ranges of a/λD. The experimentally found scaling holds for 2<a/λD<4, and we find n = 3 for the same range of the reduced Debye length. For smaller a/λD, we find n = 2 obtained earlier in several simulation and theoretical studies, and still closer to the Kirkwood line we obtain n = 1.5 that was also predicted in different works. It follows from our theory that n = 3 (i.e. λs is proportional to the density of ions) when the variance of the local charge density is large, and λs is proportional to this variance times the Bjerrum length. Detailed derivation of the theory is presented.

Crystal nucleation of highly screened charged colloids.

We study the nucleation of nearly hard charged colloidal particles. We use Monte Carlo simulations in combination with free-energy calculations to accurately predict the phase diagrams of these particles and map them via the freezing density to hard spheres, then we use umbrella sampling to explore the nucleation process. Surprisingly, we find that even very small amounts of charge repulsion can have a significant effect on the phase behavior. Specifically, we find that phase boundaries and nucleation barriers are mostly dependent on the Debye screening length and that even screening lengths as small as 2% of the particle diameter are sufficient to show marked differences in both. This work demonstrates clearly that even mildly charged colloids are not effectively hard spheres.

Dispersion of modified fumed silica in elastomeric nanocomposites

In polymer nanocomposites, surface modification of silica aggregates can shield Coulombic interactions that inhibit agglomeration and formation of a network of agglomerates. Surface modification is usually achieved with silane coupling agents although carbon-coating during pyrolytic silica production is also possible. Pyrogenic silica with varying surface carbon contents were dispersed in styrene-butadiene (SBR) rubber to explore the impact on hierarchical dispersion, the emergence of meso-scale structures, and the rheological response. Pristine pyrogenic silica aggregates at concentrations above a critical value (related to the Debye screening length) display correlated meso-scale structures and poor filler network formation in rubber nanocomposites due to the presence of silanol groups on the surface. In the present study, flame synthesized silica with sufficient surface carbon monolayers can mitigate the charge repulsion thereby impacting network structural emergence. The impact of the surface carbon on the van der Waals enthalpic attraction, a∗, is determined. The van der Waals model for polymer nanocomposites is drawn through an analogy between thermal energy, kBT, and the accumulated strain, γ. The rheological response of the emergent meso-scale structures depends on the surface density of both carbon and silanol groups.

Structure of molten NaCl and the decay of the pair-correlations.

The structure of molten NaCl is investigated by combining neutron and x-ray diffraction with molecular dynamics simulations that employed interaction potentials with either rigid or polarizable ions. Special attention is paid to the asymptotic decay of the pair-correlation functions, which is related to the small-k behavior of the partial structure factors, where k denotes the magnitude of the scattering vector. The rigid-ion approach gives access to an effective restricted primitive model in which the anion and cation have equal but opposite charges and are otherwise identical. For this model, the decay of the pair-correlation functions is in qualitative agreement with simple theory. The polarizable ion approach gives a good account of the diffraction results and yields thermodynamic parameters (density, isothermal compressibility, Debye screening length, and heat capacity) in accord with experiment. The longest decay length for the partial pair-distribution functions is a factor of ≃2.5 times greater than the nearest-neighbor distance. The results are commensurate with the decay lengths found for the effective restricted primitive model, which are much shorter than those found in experiments on concentrated electrolytes or ionic liquids using surface force apparatus.

A New Approach toward the Realization of Specific and Label‐Free Biological Sensing Based on Field‐Effect Devices

AbstractSpecific and label‐free detection of biological interactions is paramount to a plethora of technological applications ranging from home‐care diagnostics to smart agriculture and home security. The biologically‐modified field‐effect transistor (BioFET) is a promising sensing platform due to its inherent signal amplification, low power, and miniaturization. In the following a low‐cost meta‐nano‐channel BioFET (MNC BioFET) is reported that provides means to electrostatically control the size, shape, and location of the conducting channel such as to enhance the coupling between the locally‐occurring electrostatics of the biological interactions and the electrodynamics of the underlying conducting channel. Moreover, it provides means to electrostatically control the Debye screening length at the sensing area to increase the readout signal. The MNC BioFET is fabricated in a large‐scale silicon chip foundry that ensures robustness and stability, optimal noise levels and signal amplification, repeatability, and ultimate miniaturization with the potential for high‐end multiplexing in ultra‐small samples. In the current study, a specific and label‐free sensing of prostate specific antigen with the MNC BioFET is demonstrated, and show the dependency of the sensor signal on the channel configuration. Finally, an order of magnitude enhancement in readout signal is demonstrated by the electrostatic control of the screening length at the sensing area.

Streaming Potential with Ideally Polarizable Electron-Conducting Substrates.

With nonconducting substrates, streaming potential in sufficiently broad (vs Debye screening length) capillaries is well known to be a linear function of applied pressure (and coordinate along the capillary). This study for the first time explores streaming potential with ideally polarizable electron-conducting substrates and shows it to be a nonlinear function of both coordinate and applied pressure. Experimental manifestations can be primarily expected for streaming potentials arising along thin porous electron-conducting films experiencing solvent evaporation from the film side surface. Model predictions are in good qualitative agreement with literature experimental data.