Using COMSOL within CatiNT

There is a range of predefined COMSOL variables which will be written by default to the COMSOL input file. These can be used when defining new COMSOL variables or requesting specific output.

General COMSOL Functions

There are some generalized COMSOL functions available for use, some of them listed here ([var] stands for the variable name):

Functional Call	Equation	Description
[var]x, d([var],x])	\(\frac{\mathrm{d}[\mathrm{var}]}{\mathrm{d}x}\)	derivative with respect to x-direction
[var].at0(0,[var])	\([\mathrm{var}](x=0)\)	value of the variable at \(x=0\)
intop2([var]*(x<=dest(x)))	\(\displaystyle\int\limits_0^x [\mathrm{var}] \mathrm{d}x\)	integral of variable from 0 to x

intop2 is defined as a domain integral as default integral.

Species-Dependent Variables:

Species dependent variables can be requested by putting the species name sp into double brackets behind the physical property symbol:

Variable Name CatINT	Variable Name COMSOL	Equation	Description
j[[sp]]	j1,j2,...	\(j_i(x)\)	Flux of species sp
cp[[sp]]	cp1,cp2,...	\(c_i(x)\)	Concentration of species sp
ci[[sp]]	ci1,ci2,...	\(c_i(t=0)\)	Initial Concentration

Global Variables

Variable Name CatINT	Mathmatical Expression	Equation	Description
phi	–	Electrostatic Potential in Solution
rho_s	\(\left(\Phi^\mathrm{M}-\Phi^\mathrm{PZC}\right)\cdot C_\mathrm{S}\)	Surface Charge Density
rho_c	\(\rho_\mathrm{c}=F^2 \sum_i z_i^2 u_i c_i(x)\)	Electrolyte Conductivity
i_el	\(i_\mathrm{el}=F \sum\limits_i z_i j_i\)	Electrolyte Current Density
delta_phi_iR	\(\Delta \Phi_\mathrm{iR}(x)=\displaystyle\int\limits_0^x \frac{i(x)}{\rho_\mathrm{c}} \mathrm{d}x\)	iR Drop in the Electrolyte as a function of distance to the electrode
delta_phi_diff	\(\Delta\Phi_\mathrm{diff}(x)=\displaystyle\int\limits_0^x \frac{\sum\limits_iz_i D_i \nabla c_i}{\rho_\mathrm{c}} \mathrm{d}x\)	Diffusional Potential Drop