Chemical Potential
The Chemical Potential node adds the equations to model diffusive flux in terms of the gradient of the chemical potential. This node is active only when Concentration is selected as Transported Quantity.
Coordinate System Selection
The Global coordinate system is selected by default. The Coordinate system list contains all applicable coordinate systems in the component. The coordinate system is used for interpreting directions of orthotropic and anisotropic material data presented in a local system. The coordinate system must have orthonormal coordinate axes, and be defined in the material frame.
Mobility
Select a Mobility model — Linear, Quadratic, or User defined.
For Linear, enter expressions for the Mobility, ηc, (s mol/kg) for each of the species.
If Quadratic is selected as the Mobility model, enter also the Maximum concentration cmax for each of the species. The default value is Inf mol/m3.
For User defined, enter expressions for the Mobility coefficient, Mc, for each of the species. Select Isotropic, Diagonal, or Symmetric to enter one or more components. When an anisotropic input is used, the axis orientations are given by the Coordinate System Selection.
Chemically stored free energy
Under Chemically stored free energy model, select User defined or Flory–Huggins when there is only one species. For User defined, enter expressions for the Molar volume, Vm,c, one expression per species. For Flory–Huggins, enter expressions for the Interaction parameter, χc, the Molar volume, Vm,c, and the Stretch, λ0.
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The Interaction parameter, χc, is a dimensionless measure of the enthalpy of mixing between the solvent and polymer network. An expression can be inserted to simulate dependence from other parameters such as temperature.
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The Stretch, λ0, or free swelling stretch, is used to take into account that the meshed geometry is a result of an unconstrained expansion (free swelling) from a dry configuration (zero species concentration). This is necessary because the Flory–Huggins model is singular at zero concentration, so an initial condition different from zero should be used in the Initial Values node. The stretch value should be compatible with the selected initial conditions.
For details, see the Chemical Potential section in the Structural Mechanics Theory chapter.
If multiple species are present, only the User defined option is available as the Chemically stored free energy model.
Advanced
This setting is only available when the model includes deforming or rotating domains, for example when using a Moving Mesh node or a Structural Mechanics interface. From the Frame list, select Reference configuration or Deformed configuration to specify the mobility tensor in the material frame or in the spatial frame. They are related as follows,
Constraint Settings
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog.
In the COMSOL Multiphysics Reference Manual:
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Constraint Reaction Terms
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Weak Constraints
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Constraint Settings
Location in User Interface
Context Menus
Transport in Solids > Chemical Potential
Ribbon
Physics tab with Transport in Solids selected:
Domain > Chemical Potential