442 research outputs found
Modeling transport of charged species in pore networks: solution of the Nernst-Planck equations coupled with fluid flow and charge conservation equations
A pore network modeling (PNM) framework for the simulation of transport of
charged species, such as ions, in porous media is presented. It includes the
Nernst-Planck (NP) equations for each charged species in the electrolytic
solution in addition to a charge conservation equation which relates the
species concentration to each other. Moreover, momentum and mass conservation
equations are adopted and there solution allows for the calculation of the
advective contribution to the transport in the NP equations.
The proposed framework is developed by first deriving the numerical model
equations (NMEs) corresponding to the partial differential equations (PDEs)
based on several different time and space discretization schemes, which are
compared to assess solutions accuracy. The derivation also considers various
charge conservation scenarios, which also have pros and cons in terms of speed
and accuracy. Ion transport problems in arbitrary pore networks were considered
and solved using both PNM and finite element method (FEM) solvers. Comparisons
showed an average deviation, in terms of ions concentration, between PNM and
FEM below with the PNM simulations being over times faster
than the FEM ones for a medium including about pores. The improved
accuracy is achieved by utilizing more accurate discretization schemes for both
the advective and migrative terms, adopted from the CFD literature. The NMEs
were implemented within the open-source package OpenPNM based on the iterative
Gummel algorithm with relaxation.
This work presents a comprehensive approach to modeling charged species
transport suitable for a wide range of applications from electrochemical
devices to nanoparticle movement in the subsurface
Fluctuation-enhanced electric conductivity in electrolyte solutions
In this letter we analyze the effects of an externally applied electric field
on thermal fluctuations for a fluid containing charged species. We show in
particular that the fluctuating Poisson-Nernst-Planck equations for charged
multispecies diffusion coupled with the fluctuating fluid momentum equation,
result in enhanced charge transport. Although this transport is advective in
nature, it can macroscopically be represented as electrodiffusion with
renormalized electric conductivity. We calculate the renormalized electric
conductivity by deriving and integrating the structure factor coefficients of
the fluctuating quantities and show that the renormalized electric conductivity
and diffusion coefficients are consistent although they originate from
different noise terms. In addition, the fluctuating hydrodynamics approach
recovers the electrophoretic and relaxation corrections obtained by
Debye-Huckel-Onsager theory, and provides a quantitative theory that predicts a
non-zero cross-diffusion Maxwell-Stefan coefficient that agrees well with
experimental measurements. Finally, we show that strong applied electric fields
result in anisotropically enhanced velocity fluctuations and reduced
fluctuations of salt concentrations.Comment: 12 pages, 1 figur
Controlling turbulent drag across electrolytes using electric fields
Reversible in operando control of friction is an unsolved challenge crucial
to industrial tribology. Recent studies show that at low sliding velocities,
this control can be achieved by applying an electric field across electrolyte
lubricants. However, the phenomenology at high sliding velocities is yet
unknown. In this paper, we investigate the hydrodynamic friction across
electrolytes under shear beyond the transition to turbulence. We develop a
novel, highly parallelised, numerical method for solving the coupled
Navier-Stokes Poisson-Nernest-Planck equation. Our results show that turbulent
drag cannot be controlled across dilute electrolyte using static electric
fields alone. The limitations of the Poisson-Nernst-Planck formalism hints at
ways in which turbulent drag could be controlled using electric fields.Comment: Accepted by the Faraday Discussions on Chemical Physics of
Electroactive Material
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