Discussion Forum

Hello all,

I'm fairly new to comsol, so I was wondering if you guys could help me with something I'm working on. I'm trying to model electroosmotic flow through a 2d rectangular channel. I understand how to do the actual fluid flow part, but I'm having a problem modeling the electrical potential using the coefficient pde. Here is what I'm doing:

coefficient PDE:

f= -2*z*e*n*sinh(z*e*potential/(kb*T))/(er*eo)

c=1

all other coefficients are zero

boundaries:

inlet & outlet: neumann type with zero coefficients (so dpotential/dx = 0)

upper & lower wall: dirichlet type with h=1 & r=potential at walls

When I run it like this, comsol just spits out that the potential everywhere is equal to the potential at the walls. So I tried putting in another boundary right in the middle of the channel and setting both the neumann and dirichlet coefficients to zero, but then comsol just solves the potential as a linear function increasing to zero from my potential at the walls (which is negative). The potential should look more like a rapidly decaying exponential but for some reason comsol is not solving it that way. I tried playing around with the solver by checking the "highly nonlinear" box in the solver's menu but it just gave the same solution.

Is there something I'm doing wrong or something I need to tell comsol to do in order to solve it as an exponential?

For reference, here's the wikipedia page on electroosmotic flow: en.wikipedia.org/wiki/Electroosmotic_flow

If you look at that, consider my rhoe (electric charge density) to be the hyperbolic sine function i put in the f coefficient


Thanks for your time,

-Jack