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https://github.com/evstigneevnm/overscreening_breakdown

This is a repository for the numerical method for the overscreening breakdown problem.
https://github.com/evstigneevnm/overscreening_breakdown

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This is a repository for the numerical method for the overscreening breakdown problem.

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# Overview of overscreening_breakdown



#### This is a repository for the pseudo-spectral collocation numerical method for the overscreening breakdown problem. For the discription, see bellow.



## Requirements and Installation

* #### python 3.8 or newer with modules:

- numpy

- matplotlib

- scipy

- pickle

* #### clone repository locally into your local folder



## Testing:

#### For manufactured glabal tests run the following from your local folder:

```

python3 test_manufactures_solutions.py

```

resutls will be stored in "./figures" folder.



Other tests are:

- test_basis.py

- test_matrices.py

- test_solve_nonlinear_problem.py

- test_different_mu.ipynb



## Description

#### Problem formulation

Overscreening breakdown problem describes the electric double layer (EDL) near electrodes with heterogeneous surface structure. We investigate the interplay between short-range ion correlations, which provides charge density oscillations and overscreening regime, with the impact of electrode surface morphology.



We solve the Modified Poisson-Fermi equation in dimensionless form:

$$(1-\delta_c^2 \nabla^2) \nabla^2 u = \frac{\sinh(u)}{1 +2\gamma \sinh^2\left(u/2\right)} - \frac{0.5 \mu g(x)e^{u}}{1 +2\gamma \sinh^2\left(u/2\right)}$$

with boundary condition: $$u(x = 0) = V, \quad u'''(x = 0) = 0$$ where $u$ is electrostatic potential, $\delta_c$ is correlation length describing electrostatic correlations, $\gamma$ is the compacity parameter, $\mu$ is the magnitude of ion separation caused by electrode surface geometry and ion size asymmetry, $g(x) = \frac{1}{\sqrt{2\pi}} \exp\{-\frac {x^2}{2\sigma^2}\}$ has the form of the Gauss distribution function describing the area of electrode surface structure impact with roughness deviation $\sigma$, and $V$ is the potential on the electrode. 



The main parameters, treated as the input of physical problem, are:

- $\delta_c$ to control the scale of electrostatic correlations

- $\sigma$ to control the scale, where surface roughness acts on the EDL

- $\mu$ to control the magnitude of electrode surface roughness contribution

- $V$ to control the initial structure of EDL

- $\gamma$ to control the EDL density



As the output you obtain the solution on electrostatic potential $u$ and then using classical electrostatic definition you can also calculate: charge density of EDL, ion concentrations, cumulative charge or differential capacitance.



For more details we refer to our article: https://arxiv.org/abs/2401.13458



#### Numerical method

The problem is solved on the $R^+$ domain by using an analytical mapping to the domain [0, pi].

Newton's method with spectral discretization over the collocation method is used in order to find the solution from some initial guess. The method includes homotopy for difficult cases that allow one to converge to the solution.

For the example-driven manual, see:

```

execution_example.ipynb

```

For the tests on the stability of parameters, see:

```

test_different_mu.ipynb

```



## Contacts

The numerical method is implemented by Evstigneev Nikolay and Ryabkov Oleg.

Physical formulation by: Alexey Khlyupin, Irina Nesterova, Kirill Gerke.

If you have any questions, please contact [email protected] or [email protected]



## Funding

Partially, Evstigneev Nikolay and Ryabkov Oleg appreciate the support of the work by the Russian Science Foundation (RSF), project number 23-21-00095.