Small Debye Length Research Articles

Peculiarities of charged particle kinetics in spherical plasma

We describe transient kinetic simulations of partially ionized weakly-collisional plasma around spherical bodies absorbing or emitting charged particles. Numerical solutions of kinetic equations for electrons and ions in 1D2V phase space are coupled to an electrostatic solver using the Poisson equation or quasineutrality condition for small Debye lengths. The formation of particle groups and their contributions to electric current flow and screening of charged bodies by plasma are discussed for applications to Langmuir probes and solar wind.

Understanding the dynamic rheological property of cement paste blended with steel slag powder: From interparticle force and physico-chemical parameters of view

The blending of steel slag powder with cement can mitigate the carbon footprint of cementitious composites. The rheological properties of cement pastes are well known to be related to interparticle forces. However, the magnitude of the interparticle force depends on many physico-chemical factors. The influence of various factors on interparticle forces is not yet clear, making it difficult to control the rheological properties. In this paper, the interparticle force of steel slag powder blended cement paste was quantified and its relationships with dynamic rheological properties and basic physico-chemical parameters were explored. By means of this, the interparticle force was confirmed to could explain the effect of steel slag powder on the dynamic rheological properties of cement paste within 1 h. Moreover, how physico-chemical parameters affect the interparticle forces was clarified. A cement paste with large particle size and apolar surface energy, as well as small potential square, Debye length and small polar surface energy, can exhibit large interparticle force and high dynamic yield stress and plastic viscosity. A simple formula calculating interparticle force from basic parameters was proposed, which is believed to effectively and practically regulate the interparticle force. The relationships between interparticle forces, physico-chemical parameters, and rheological properties provide an effective reference for regulating the dynamic rheological properties of cement paste through fundamental parameters.

Quantum well formation in AgBr for holes in the region of volumetric charge of intrinsic defects

The paper shows that in the area of the volumetric charge of the Frenkel defects, due to the large bending of the zones and the small Debye length, a quantum well for holes is formed. It is shown that the potential near the surface behaves linearly. The energies of the quantum energy levels and the hole localization regions in the triangular potential well approximation for the {200} and {111} surfaces are calculated. The energies of the levels increase with increasing temperature, and the localization regions of holes decrease due to decreasing Debye length.

The combined quasineutral and incompressible limit for the Navier–Stokes–Poisson system in critical spaces

In this paper, we study the quasineutral and incompressible limit for the Navier–Stokes–Poisson system with ill-prepared initial data in whole space. Based on Strichartz estimates for the Klein–Gordon equation and the pseudo-scaling property of modified Besov norms, we prove that the density tends to a constant, the compressible part of velocity vanishes, and the incompressible part of velocity converges to the solution of the incompressible Navier–Stokes equation. We get a relation of the lifespan of the solution between the Navier–Stokes–Poisson and the incompressible Navier–Stokes equations for small Debye length and low Mach number.

Incoherent and Collective Thomson Scattering for the Determination of Electron and Ion Properties in Low-Temperature Plasma

In this lecture an overview of applications of incoherent Thomson scattering (TS) as well as collective Thomson scattering (CTS) will be given. These are the most accurate methods for measuring the electron and ion properties, because the method is direct and non-intrusive. A CTS system based on the fundamental wavelength of a seeded Nd:YAG laser, being developed for the high density, low-temperature plasma of the linear plasma generator Magnum-PSI will be described also. The small Debye length of dense low temperature plasma enables application of CTS at relatively short laser wavelength. The combination of this CTS system and existing incoherent TS system enables determination of electron density and temperature as well as ion temperature and plasma velocity of the near surface plasma. In this lecture, the theoretical background and experimental challenges of the work will be given along with some examples that demonstrate the capabilities of such systems.

A new generalized membrane polarization frequency-domain impedance formula

A frequency-domain membrane polarization impedance formula was suggested previously by Marshall and Madden for a sequence of ion selective and non-selective regions of equal cross-sections. We generalized the formula for the case of non-equal cross-sections and small Debye lengths (common case). This formula is a simplification of a general analysis of the impedance, given by Buchheim and Irmer, which is tedious for computations and does not give physical insight into the process. The obtained results are physically consistent for the effect of the different cross-sections on the limiting resistance at high frequencies. It is, also, consistent in behaviour with the chargeability formula given by Titov. The impedance model considering regions of non-equal cross-sections may be useful for modeling complex impedance of partially and fully water-saturated rocks. Our study also gives, in detail, the essential equations not given in Buchheim and Irmer derivation, while the lengthy intermediation steps are omitted. Some typing errors in the equations of the previous derivation are rectified. Our approach and formula for the impedance is more rigourous than the formula given by Bücker and Hördt.

A numerical study on ion concentration polarization and electric circuit performance of an electrokinetic battery.

Ion concentration polarization (ICP) imposes remarkable adverse effects on the energy conversion performance of the pressure-driven electrokinetic (EK) flows through a capillary system that can be equivalently treated as a battery. An optimized dimensionless numerical method is proposed in this study to investigate the causes and the effects of the ICP. Results show that remarkable ICP phenomena are induced under certain conditions such as high applied pressure, high surface charge density, and small inversed Debye length at dimensionless values of 6000, -10, and 0.5. Meanwhile, different factors influence the ICP and the corresponding electric properties in different ways. Particularly for the overall electric resistance, the applied pressure and the surface charge density mainly affect the variation amplitude and the level of the overall electric resistance when varying the output electric potential, respectively. Differently, the Debye length affects the overall electric resistance in both aspects. Ultimately, the induced ICP leads to significant nonlinear current-potential curves.

Derivation of an effective dispersion model for electro-osmotic flow involving free boundaries in a thin strip

Since dispersion is one of the key parameters in solute transport, its accurate modeling is essential to avoid wrong predictions of flow and transport behavior. In this research, we derive new effective dispersion models which are valid also in evolving geometries. To this end, we consider reactive ion transport under dominate flow conditions (i.e. for high Peclet number) in a thin, potentially evolving strip. Electric charges and the induced electric potential (the zeta potential) give rise to electro-osmotic flow in addition to pressure-driven flow. At the pore-scale a mathematical model in terms of coupled partial differential equations is introduced. If applicable, the free boundary, i.e. the interface between an attached layer of immobile chemical species and the fluid is taken into account via the thickness of the layer. To this model, a formal limiting procedure is applied and the resulting upscaled models are investigated for dispersive effects. In doing so, we emphasize the cross-coupling effects of hydrodynamic dispersion (Taylor–Aris dispersion) and dispersion created by electro-osmotic flow. Moreover, we study the limit of small and large Debye length. Our results improve the understanding of fundamentals of flow and transport processes, since we can now explicitly calculate the dispersion coefficient even in evolving geometries. Further research may certainly address the situation of clogging by means of numerical studies. Finally, improved predictions of breakthrough curves as well as facilitated modeling of mixing and separation processes are possible.

Discrete-element model of electrophoretic deposition in systems with small Debye length: effective charge, lubrication force, characteristic scales, and early-stage transport

Electrophoresis was recently proposed as a new method for controlling properties and structure of the interface between cement and steel elements in petroleum and construction industries (Lavrov A et al., 2018). Cement slurries typically contain a wide range of particle sizes (micron to hundred micron), and the particles have small Debye lengths (nanometer). The relative magnitude of different forces acting on particles in such systems was examined. A formulation based on the DLVO theory was used to calculate van der Waals forces and electric repulsion forces between particles. It was shown that the following forces need to be included in a discrete-element model of electrophoretic deposition in this case: viscous drag force, force due to the external electric field, gravity + buoyancy force, lubrication force, van der Waals force, and direct mechanical contact force. The recommended cut-off gaps for van der Waals and lubrication forces are equal to the radius of the larger particle participating in the interaction. An example DEM simulation has revealed that deposition starts with deposing finer particles. Shortly after, larger particles are deposited on the finer substrate. This is due to the larger speed-up time of larger particles. The difference in the speed-up time leads to some size segregation at the early stage of deposition, even though the particle mobility is independent of the particle size. The model enables some insight into the processes that take place during the earlier stages of electrophoretic deposition that are difficult to capture and analyze in laboratory experiments.

Multicomponent fluid model for two-temperature plasmas derived from kinetic theory : application to magnetic reconnection

This contribution deals with the fluid modeling of multicomponent magnetized plasmas in thermo-chemical non-equilibrium from the partially- to fully-ionized collisional regimes, aiming at the predictive simulation of magnetic reconnection in Sun chromosphere conditions. Such fluid models are required for large-scale simulations by relying on high performance computing. The fluid model is derived from a kinetic theory approach, yielding a rigorous description of the dissipative and non-equilibrium effects and a well-identified mathematical structure. We start from a general system of equations that is obtained by means of a multiscale Chapman-Enskog method, based on a non-dimensional analysis accounting for the mass disparity between the electrons and heavy particles, including the influence of the electromagnetic field and transport properties. The latter are computed by using a spectral Galerkin method based on a converged Laguerre-Sonine polynomial approximation. Then, in the limit of small Debye length with respect to the characteristic scale in the Sun chromosphere, we derive a two-temperature single-momentum multicomponent diffusion model coupled to Maxwell’s equations, which is able to describe fully- and partially-ionized plasmas, beyond the multi-fluid model of Braginskii, valid for the whole range of the Sun chromosphere conditions. The second contribution is the development and verification of an accurate and robust numerical strategy that is based on CanoP, a massively parallel code with adaptive mesh refinement capability, which is able to cope with the full spectrum of scales of the magnetic reconnection process, without additional constraint on the time steps compared to single-fluid Magnetohydrodynamics (MHD) models. The final contribution is a study of the physics of magnetic reconnection in collaboration with the heliophysics team of NASA Ames Research Center. We show that the model and methods allow us to retrieve the results of usual single-fluid MHD models in the highly collisional case at equilibrium, while achieving a more detailed physics description relevant to such applications in the weakly collisional case, where non-equilibrium effects become important.

Boundary asymptotics for a non-neutral electrochemistry model with small Debye length

This article addresses the boundary asymptotics of the electrostatic potential in non-neutral electrochemistry models with small Debye length in bounded domains. Under standard physical assumptions motivated by non-electroneutral phenomena in oxidation–reduction reactions, we show that the electrostatic potential asymptotically blows up at boundary points with respect to the bulk reference potential as the scaled Debye length tends to zero. The analysis gives a lower bound for the blow-up rate with respect to the model parameters. Moreover, the maximum potential difference over any compact subset of the physical domain vanishes exponentially in the zero-Debye-length limit. The results mathematically confirm the physical description that electrolyte solutions are electrically neutral in the bulk and are strongly electrically non-neutral near charged surfaces.

Biomolecule Detection in High Ionic Strength With Enhanced Sensitivity Using Graphene Nanogrid FET Immunosensor

Detecting charges in high ionic strength solutions is a challenge due to the inherent problem of ionic screening in small Debye length. There have been some efforts to improve the sensing in high ionic strength buffer but the sensitivity and detection limit is poor. To address this challenge, we report high frequency sensing using smooth graphene nanogrid FET immunosensor. These sensors have certain intrinsic advantages: firstly, in the nanopores, the biomolecules usually reside within a distance shorter than the original pore radius which statistically raises the charge transfer probability between the biomolecules and the surface even at high ionic strength of the buffer and secondly, such a graphene nanostructure has a size dependent effective energy gap which increases the transconductance of the device. These features coupled with high frequency sensing have enabled detection down to 0.1 fMHep-B in a high ionic strength buffer (150mM) at 8 MHz with a sensitivity as high as 445.45%. This is an enhancement by more than 18 times compared to the existing reports.

Fast Nanoscale Surface Charge Mapping with Pulsed-Potential Scanning Ion Conductance Microscopy.

A vast range of interfacial systems exhibit charge heterogeneities on the nanoscale. These differences in local surface charge density are challenging to visualize, but recent work has shown the scanning ion conductance microscope (SICM) to be a very promising tool to spatially resolve and map surface charge and topography via a hopping potential sweep technique with a single nanopipette probe, with harmonic modulation of a bias applied between quasi-reference counter electrodes in the nanopipette and bulk solution, coupled with lock-in detection. Although powerful, this is a relatively slow process, with limitations on resolution and the size of the images that can be collected. Herein, we demonstrate a new scanning routine for mapping surface charge and topography with SICM, which increases the data acquisition rate by an order of magnitude and with the potential for further gains. Furthermore, the method is simplified, eliminating the need for bias modulation lock-in detection, by utilizing a potential-pulse, chronoamperometric approach, with self-referencing calibration of the response at each pixel in the image. We demonstrate the application of this new method to both a model substrate and living PC-12 cells under physiological (high ionic strength) conditions, where charge mapping is most challenging (small Debye length). This work contributes significantly to the emergence of SICM as a multifunctional technique for simultaneously probing interfacial structure and function with nanometer resolution.

Self-electrophoresis of spheroidal electrocatalytic swimmers

Using the method of matched asymptotic expansions, we derive a general expression for the speed of a prolate spheroidal electrocatalytic nanomotor in terms of interfacial potential and physical properties of the motor environment in the limit of small Debye length and Péclet number. This greatly increases the range of geometries that can be handled without resorting to numerical simulations, since a wide range of shapes from spherical to needle-like, and in particular the common cylindrical shape, can be well-approximated by prolate spheroids. For piecewise-uniform distribution of surface cation flux with fixed average absolute value, the mobility of a prolate spheroidal motor with a symmetric cation source/sink configuration is a monotonically decreasing function of eccentricity. A prolate spheroidal motor with an asymmetric sink/source configuration moves faster than its symmetric counterpart and can exhibit a non-monotonic dependence of motor speed on eccentricity for a highly asymmetric design.

Electrophoresis of Heteropolymers. Effect of Stiffness

In the limit of a small Debye length, the electrophoretic mobility of a charged heteropolymer is a weighted average of the mobilities of its parts. The weights are a function of the position along the chain that depends on the properties of the polymer, such as its stiffness. It is known that for a flexible polymer the weights diverge as a power law at the ends, while for a rodlike polymer the divergence is only logarithmic. Using the Kirkwood–Riseman approximation, we study in detail the intermediate case of a semiflexible polymer. We find that for sufficiently small persistence lengths, the position dependence of the weights matches that for flexible polymers away from the ends, but crosses over to the logarithmic dependence characteristic of rods near the ends. While this crossover is broad and the exact points at which it occurs can be defined in different ways, we give an operational definition of the distance of the crossover points from the respective end that turns out to be nearly independent of ...

Non-linear plasma wake growth of electron holes

An object's wake in a plasma with small Debye length that drifts across the magnetic field is subject to electrostatic electron instabilities. Such situations include, for example, the moon in the solar wind and probes in magnetized laboratory plasmas. The instability drive mechanism can equivalently be considered drift down the potential-energy gradient or drift up the density-gradient. The gradients arise because the plasma wake has a region of depressed density and electrostatic potential into which ions are attracted along the field. The non-linear consequences of the instability are analysed in this paper. At physical ratios of electron to ion mass, neither linear nor quasilinear treatment can explain the observation of large-amplitude perturbations that disrupt the ion streams well before they become ion-ion unstable. We show here, however, that electron holes, once formed, continue to grow, driven by the drift mechanism, and if they remain in the wake may reach a maximum non-linearly stable size, beyond which their uncontrolled growth disrupts the ions. The hole growth calculations provide a quantitative prediction of hole profile and size evolution. Hole growth appears to explain the observations of recent particle-in-cell simulations.

Scale analysis of a hydrodynamic model of plasma

We examine a hydrodynamic model of the motion of ions in plasma in the regime of small Debye length, a small ratio of the ion/electron temperature, and high Reynolds number. We analyze the associated singular limit and identify the limit problem — the incompressible Euler system. The result leans on careful analysis of the oscillatory component of the solutions by means of Fourier analysis.

Ionic liquid confined in silica nanopores: molecular dynamics in the isobaric–isothermal ensemble

Molecular dynamics simulations in the isobaric–isothermal ensemble are used to investigate the structure and dynamics of an ionic liquid confined at ambient temperature and pressure in hydroxylated amorphous silica nanopores. The use of the isobaric–isothermal ensemble allows estimating the effect of confinement and surface chemistry on the density of the confined ionic liquid. The structure of the confined ionic liquid is investigated using density profiles and structural order parameters while its dynamics is assessed by determining the mobility and ionic conductivity of the confined phase. Despite the important screening of the electrostatic interactions (owing to the small Debye length in ionic liquids), the local structure of the confined ionic liquid is found to be mostly driven by electrostatic interactions. We show that both the structure and dynamics of the confined ionic liquid can be described as the sum of a surface contribution arising from the ions in contact with the surface and a bulk-like contribution arising from the ions located in the pore centre; as a result, most properties of the confined ionic liquid are a simple function of the surface-to-volume ratio of the host porous material. In contrast, the ionic conductivity of the confined ionic liquid, which is a collective dynamical property, is found to be similar to the bulk. This study sheds light on the complex behaviour of hybrid materials made up of ionic liquid confined in inorganic porous materials.

Asymptotic theory of double layer and shielding of electric field at the edge of illuminated plasma

The method of matched asymptotic expansions is applied to the problem of a collisionless plasma generated by UV illumination localized in a central part of the plasma in the limiting case of small Debye length λD. A second-approximation asymptotic solution is found for the double layer positioned at the boundary of the illuminated region and for the un-illuminated plasma for the plane geometry. Numerical calculations for different values of λD are reported and found to confirm the asymptotic results. The net integral space charge of the double layer is asymptotically small, although in the plane geometry it is just sufficient to shield the ambipolar electric field existing in the illuminated region and thus to prevent it from penetrating into the un-illuminated region. The double layer has the same mathematical nature as the intermediate transition layer separating an active plasma and a collisionless sheath, and the underlying physics is also the same. In essence, the two layers represent the same physical object: a transonic layer.

(Invited) Planar Field Effect Transistor Biosensors: Toward Single Molecular Detection and Clinical Applications

Field effect transistor (FET) based biosensors that can translate directly specific bindings between receptors and target analytes to electrical signals have attracted considerable interests for label free, rapid, low-cost detection of biomolecules. However, it is challenging to detect biomarkers at low concentrations in physiological buffers with high ionic strengths due to the small Debye length. FET biosensors on a planar configuration are highly preferable because of the process compatibility with microelectronic manufacturing. III-nitride semiconductor materials have been considered as a promising candidate for biosensing applications due to their unique properties, such as chemical inertness, non-toxicity, and excellent thermal stability. In particular, III-nitrides FETs are ion-impermeable and highly stable in electrolytes, making them ideal for detection at ultralow concentrations in fluids with high ionic strengths. Here we present a recessed gate planar AlGaN/GaN heterojunction FET (HFET) for biodetection. The gate area of the AlGaN/GaN HFET was recessed by inductively coupled plasma (ICP) etching to shift the threshold voltage to near zero volt regime for sensitivity enhancement. As such, the device works at the subthreshold regime. The biosensing measurement was performed on a non-bias gate setting to eliminate the noise and electrochemical activities from the gate electrode. The signal-to-noise ratio of these recessed AlGaN/GaN FET biosensors is two orders of magnitude higher than that of the Si nanowire FET biosensors previously reported. Detection of streptavidin at concentration of 16 aM in PBS physiological buffer has been demonstrated. To the best of our knowledge, this is the highest sensitivity ever reported on a field effect-based biosensor in physiological buffer. These devices have demonstrated their potential in biochemical and clinical applications. We have successfully detected MIG (monokine induced by interferon gamma, an indicator of transplant rejection in patients when detected at a concentration level above nano Molar) in clinical samples. Utilizing of ion impermeable semiconductors like GaN opens up opportunities of FET-based biosensors for direct detection of biomarkers in physiological fluids at low concentrations for early disease diagnosis with excellent manufacturing compatibility.