Hygroscopic Swelling
Hygroscopic swelling is an internal strain caused by changes in moisture content. This strain can be written as
where βh is the coefficient of hygroscopic swelling, Mm is the molar mass, cmo is the moisture concentration, and cmo,ref is the strain-free reference concentration.
The Hygroscopic Swelling subnode is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).
Shell Properties

This section is only present when Hygroscopic Swelling is used as a subnode to:
Linear Elastic Material in the Layered Shell interface. See the documentation for the Hygroscopic Swelling node in the Layered Shell chapter.
Linear Elastic Material, Layered in the Membrane interface. See the documentation for the Hygroscopic Swelling node in the Membrane chapter.
Model Inputs
From the Concentration c list, select an existing concentration variable from another physics interface, if any concentration variables exist. For User defined enter a value or expression for the concentration. The unit for the input depends on the setting of Concentration type in the Hygroscopic Swelling Properties section. Only concentration variables having the chosen physical dimension are available in the Concentration list.
When adding a Hygroscopic swelling subnode in a Layered Shell or Membrane interface, and the concentration field is computed by another physics interface; use a discretization one order lower for the concentration field than what is used for the displacement field.
Hygroscopic Swelling Properties
In the Concentration type list, select Molar concentration or Mass concentration, depending on the units used for the concentration.
Enter a Strain reference concentration cref . This is the concentration at which there are no strains due to hygroscopic swelling.
If Molar concentration is selected as the Concentration type, enter also the Molar mass of the fluid, Mm. The default value is 0.018 kg/mol, which is the molar mass of water.
The default Coefficient of hygroscopic swelling βh uses values From material. For User defined, select Isotropic, Diagonal, or Symmetric from the list to enter one or more components for a general coefficient of hygroscopic swelling tensor βh. The default value for the User defined case is 1.5e-4 m3/kg. When a nonisotropic coefficient of hygroscopic swelling is used, the axis orientations are given by the coordinate system selection in the parent node.
The Include moisture as added mass check box is selected by default. When selected, the mass of the fluid is included in a dynamic analysis, and when using mass proportional loads. It will also contribute when computing mass properties.
Location in User Interface
Context Menus
Ribbon
Physics tab with Linear Elastic Material, Linear Elastic Material, Layered, Nonlinear Elastic Material, or Hyperelastic Material node selected in the model tree: