Electroosmotic Flow in Cylindrical Microchannel Using Electrostatics + TDS + Creeping Flow Gives NaN/Inf Solver Error

Please login with a confirmed email address before reporting spam

Hello everyone,

I am a beginner in COMSOL and trying to simulate electroosmotic flow (EOF) inside a cylindrical microchannel.

Geometry:

  • Cylinder radius = 5 µm
  • Cylinder height = 10 µm

Physics used:

  1. Electrostatics (es)
  2. Transport of Diluted Species (tds)
  3. Creeping Flow (spf)

Goal: I want to model electroosmotic flow caused by:

  • wall zeta potential,
  • electric double layer (EDL),
  • applied electric field.

Current setup:

Electrostatics:

  • Side walls: Electric potential = -10 mV (intended as zeta potential)
  • Inlet: 1 V
  • Outlet: 0 V

Transport of Diluted Species:

  • Two species:

    • c1 = cation (+1)
    • c2 = anion (-1)
  • Migration in electric field enabled
  • Diffusion enabled
  • Convection sometimes enabled/disabled during testing
  • Diffusion coefficients: D1 = D2 = 1e-9 m^2/s

Charge density: rho_v = F_const*(c1-c2)

Creeping Flow:

  • Volume force: F = es.rhoq*es.E

Mesh:

  • Boundary layer mesh added near walls.

Problem:

  • Electrostatics alone converges successfully.
  • But when TDS is coupled with Electrostatics (fully coupled or segregated), solver fails with: "NaN or Inf found" or "Initial guess leads to undefined value"

I already tried:

  • Fully coupled solver
  • Segregated solver
  • Linear discretization
  • Smaller applied voltage
  • Different iterative/direct solvers
  • Lower damping factors
  • Turning convection OFF
  • Refining mesh

Observation:

  • Smaller nanoscale geometry (100 nm radius/height) converged.
  • Larger micrometer geometry diverges.
  • TDS concentration sometimes becomes extremely large near walls.

My confusion:

  1. Is applying wall zeta potential as electric potential physically correct in this formulation?
  2. Should EOF be modeled instead using Helmholtz–Smoluchowski slip velocity?
  3. Is full Poisson–Nernst–Planck coupling required here?
  4. Is my charge density formulation correct?
  5. What is the best solver strategy for this multiphysics problem?

I would greatly appreciate guidance on:

  • proper EOF implementation,
  • correct boundary conditions,
  • stable solver settings,
  • whether to use slip velocity or explicit EDL resolution.

Thank you.



Reply

Please read the discussion forum rules before posting.

Please log in to post a reply.

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.