97 research outputs found
Direct numerical simulation of electrokinetic transport phenomena: variational multi-scale stabilization and octree-based mesh refinement
Finite element modeling of charged species transport has enabled the
analysis, design, and optimization of a diverse array of electrochemical and
electrokinetic devices. These systems are represented by the
Poisson-Nernst-Planck (PNP) equations coupled with the Navier-Stokes (NS)
equation. Direct numerical simulation (DNS) to accurately capture the
spatio-temporal variation of ion concentration and current flux remains
challenging due to the (a) small critical dimension of the electric double
layer (EDL), (b) stiff coupling, large advective effects, and steep gradients
close to boundaries, and (c) complex geometries exhibited by electrochemical
devices.
In the current study, we address these challenges by presenting a direct
numerical simulation framework that incorporates: (a) a variational multiscale
(VMS) treatment, (b) a block-iterative strategy in conjunction with
semi-implicit (for NS) and implicit (for PNP) time integrators, and (c) octree
based adaptive mesh refinement. The VMS formulation provides numerical
stabilization critical for capturing the electro-convective instabilities often
observed in engineered devices. The block-iterative strategy decouples the
difficulty of non-linear coupling between the NS and PNP equations and allows
using tailored numerical schemes separately for NS and PNP equations. The
carefully designed second-order, hybrid implicit methods circumvent the harsh
timestep requirements of explicit time steppers, thus enabling simulations over
longer time horizons. Finally, the octree-based meshing allows efficient and
targeted spatial resolution of the EDL. These features are incorporated into a
massively parallel computational framework, enabling the simulation of
realistic engineering electrochemical devices. The numerical framework is
illustrated using several challenging canonical examples
New porous medium Poisson-Nernst-Planck equations for strongly oscillating electric potentials
We consider the Poisson-Nernst-Planck system which is well-accepted for
describing dilute electrolytes as well as transport of charged species in
homogeneous environments. Here, we study these equations in porous media whose
electric permittivities show a contrast compared to the electric permittivity
of the electrolyte phase. Our main result is the derivation of convenient
low-dimensional equations, that is, of effective macroscopic porous media
Poisson-Nernst-Planck equations, which reliably describe ionic transport. The
contrast in the electric permittivities between liquid and solid phase and the
heterogeneity of the porous medium induce strongly oscillating electric
potentials (fields). In order to account for this special physical scenario, we
introduce a modified asymptotic multiple-scale expansion which takes advantage
of the nonlinearly coupled structure of the ionic transport equations. This
allows for a systematic upscaling resulting in a new effective porous medium
formulation which shows a new transport term on the macroscale. Solvability of
all arising equations is rigorously verified. This emergence of a new transport
term indicates promising physical insights into the influence of the microscale
material properties on the macroscale. Hence, systematic upscaling strategies
provide a source and a prospective tool to capitalize intrinsic scale effects
for scientific, engineering, and industrial applications
Electrokinetically forced turbulence in microfluidic flow.
While laminar flow heat transfer and mixing in microfluidic geometries has been investigated experimentally, as has the effect of geometry-induced turbulence in microfluidic flow (it is well documented that turbulence increases convective heat transfer in macrofluidic flow), little literature exists investigating the effect of electrokinetically-induced turbulence on heat transfer at the micro scale. Successful research in this area could be invaluable in creating more efficient heat exchangers for emerging microscale electronics as well as to fields requiring greater control of mixing in microfluidic devices
Charge relaxation dynamics of an electrolytic nanocapacitor
Understanding ion relaxation dynamics in overlapping electric double layers
(EDLs) is critical for the development of efficient nanotechnology based
electrochemical energy storage, electrochemomechanical energy conversion and
bioelectrochemical sensing devices as well as controlled synthesis of
nanostructured materials. Here, a Lattice Boltzmann (LB) method is employed to
simulate an electrolytic nanocapacitor subjected to a step potential at t = 0
for various degrees of EDL overlap, solvent viscosities, ratios of cation to
anion diffusivity and electrode separations. The use of a novel, continuously
varying and Galilean invariant, molecular speed dependent relaxation time
(MSDRT) with the LB equation recovers a correct microscopic description of the
molecular collision phenomena and enhances the stability of the LB algorithm.
Results for large EDL overlaps indicated oscillatory behavior for the ionic
current density in contrast to monotonic relaxation to equilibrium for low EDL
overlaps. Further, at low solvent viscosities and large EDL overlaps, anomalous
plasma-like spatial oscillations of the electric field were observed that
appeared to be purely an effect of nanoscale confinement. Employing MSDRT in
our simulations enabled a modeling of the fundamental physics of the transient
charge relaxation dynamics in electrochemical systems operating away from
equilibrium wherein Nernst-Einstein relation is known to be violated.Comment: Accepted for publication in the Journal of Physical Chemistry C on
October 30 2014. Supplementary info available free of charge via the Internet
at http://pubs.acs.org. Revised version includes more details on the
computation of the molecular speed dependent relaxation time (MSDRT) and
emphasizes the Galilean invariance of the computed MSDR
Augmented saddle point formulation of the steady-state Stefan--Maxwell diffusion problem
We investigate structure-preserving finite element discretizations of the
steady-state Stefan--Maxwell diffusion problem which governs diffusion within a
phase consisting of multiple species. An approach inspired by augmented
Lagrangian methods allows us to construct a symmetric positive definite
augmented Onsager transport matrix, which in turn leads to an effective
numerical algorithm. We prove inf-sup conditions for the continuous and
discrete linearized systems and obtain error estimates for a phase consisting
of an arbitrary number of species. The discretization preserves the
thermodynamically fundamental Gibbs--Duhem equation to machine precision
independent of mesh size. The results are illustrated with numerical examples,
including an application to modelling the diffusion of oxygen, carbon dioxide,
water vapour and nitrogen in the lungs.Comment: 27 pages, 5 figure
Nonlinear Dynamic Modeling, Simulation And Characterization Of The Mesoscale Neuron-electrode Interface
Extracellular neuroelectronic interfacing has important applications in the fields of neural prosthetics, biological computation and whole-cell biosensing for drug screening and toxin detection. While the field of neuroelectronic interfacing holds great promise, the recording of high-fidelity signals from extracellular devices has long suffered from the problem of low signal-to-noise ratios and changes in signal shapes due to the presence of highly dispersive dielectric medium in the neuron-microelectrode cleft. This has made it difficult to correlate the extracellularly recorded signals with the intracellular signals recorded using conventional patch-clamp electrophysiology. For bringing about an improvement in the signalto-noise ratio of the signals recorded on the extracellular microelectrodes and to explore strategies for engineering the neuron-electrode interface there exists a need to model, simulate and characterize the cell-sensor interface to better understand the mechanism of signal transduction across the interface. Efforts to date for modeling the neuron-electrode interface have primarily focused on the use of point or area contact linear equivalent circuit models for a description of the interface with an assumption of passive linearity for the dynamics of the interfacial medium in the cell-electrode cleft. In this dissertation, results are presented from a nonlinear dynamic characterization of the neuroelectronic junction based on Volterra-Wiener modeling which showed that the process of signal transduction at the interface may have nonlinear contributions from the interfacial medium. An optimization based study of linear equivalent circuit models for representing signals recorded at the neuron-electrode interface subsequently iv proved conclusively that the process of signal transduction across the interface is indeed nonlinear. Following this a theoretical framework for the extraction of the complex nonlinear material parameters of the interfacial medium like the dielectric permittivity, conductivity and diffusivity tensors based on dynamic nonlinear Volterra-Wiener modeling was developed. Within this framework, the use of Gaussian bandlimited white noise for nonlinear impedance spectroscopy was shown to offer considerable advantages over the use of sinusoidal inputs for nonlinear harmonic analysis currently employed in impedance characterization of nonlinear electrochemical systems. Signal transduction at the neuron-microelectrode interface is mediated by the interfacial medium confined to a thin cleft with thickness on the scale of 20-110 nm giving rise to Knudsen numbers (ratio of mean free path to characteristic system length) in the range of 0.015 and 0.003 for ionic electrodiffusion. At these Knudsen numbers, the continuum assumptions made in the use of Poisson-Nernst-Planck system of equations for modeling ionic electrodiffusion are not valid. Therefore, a lattice Boltzmann method (LBM) based multiphysics solver suitable for modeling ionic electrodiffusion at the mesoscale neuron-microelectrode interface was developed. Additionally, a molecular speed dependent relaxation time was proposed for use in the lattice Boltzmann equation. Such a relaxation time holds promise for enhancing the numerical stability of lattice Boltzmann algorithms as it helped recover a physically correct description of microscopic phenomena related to particle collisions governed by their local density on the lattice. Next, using this multiphysics solver simulations were carried out for the charge relaxation dynamics of an electrolytic nanocapacitor with the intention of ultimately employing it for a simulation of the capacitive coupling between the neuron and the v planar microelectrode on a microelectrode array (MEA). Simulations of the charge relaxation dynamics for a step potential applied at t = 0 to the capacitor electrodes were carried out for varying conditions of electric double layer (EDL) overlap, solvent viscosity, electrode spacing and ratio of cation to anion diffusivity. For a large EDL overlap, an anomalous plasma-like collective behavior of oscillating ions at a frequency much lower than the plasma frequency of the electrolyte was observed and as such it appears to be purely an effect of nanoscale confinement. Results from these simulations are then discussed in the context of the dynamics of the interfacial medium in the neuron-microelectrode cleft. In conclusion, a synergistic approach to engineering the neuron-microelectrode interface is outlined through a use of the nonlinear dynamic modeling, simulation and characterization tools developed as part of this dissertation research
- …