Discussion Forum

Thank you for your time and help in advance!

I'm trying to model the pH-induced swelling of hydrogels in COMSOL (in 2D and 3D). The rectangular hydrogel is confined by walls, except in the top direction, where it's free to move. When I tried to define the confinement by using boundary conditions to limit swelling direction, there's a discrepancy between the simulation and experimental results: experimentally, we noticed that the hydrogel arched to form a domed surface with a hollow centre underneath, but the COMSOL model just showed swelling.

I thought that this might be due to an interplay between the friction forces at confining walls and swelling, so instead of a boundary condition, we tried to model the walls by coupling a contact pair in the structural mechanics module, but we're having trouble with this. I'm wondering if we can create a user-defined material to model the swelling hydrogel, so we can somehow bypass the weak form PDE that's involved?

Another problem is that during parametric sweep, when we're trying to vary the material flexibility for different pH, it seems that COMSOL is simultaneously solving for both the weak form PDE and the roots of a cubic polynomial equation to determine the concentration of H+ in the hydrogel. COMSOL easily errors out depending on the initial concentration guess, and doesn't give us all the 3 roots from the cubic polynomial.

Any thoughts or advice on how to approach this problem is greatly appreciated, thank you again for your time reading this!