* Instead of passing constant parameters to our pressure diffusion `Kernel`, we can use the `Material` system to supply the values.
* This allows for properties that vary in space and can be coupled to variables in the simulation.

[MOOSE Material System](/wiki/MooseSystems/Materials)

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# Darcy Pressure Material and Properties

* For the Darcy example, we need two `Material` properties: `permeability` and `viscosity`.
* These can both be computed by the same `Material` object: `PackedColumn`.
* As in the reference article, `permeability` varies with the size of the steel balls. We'll use a `LinearInterpolation` object to implement this.


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# Source Code

[PackedColumn.h](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/include/materials/PackedColumn.h)

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[PackedColumn.C](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/src/materials/PackedColumn.C)

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[DarcyPressure.h](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/include/kernels/DarcyPressure.h)

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[DarcyPressure.C](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/src/kernels/DarcyPressure.C)

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[step3.i](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/problems/step3.i)

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# Create a Test

- Modify the test from Step 2
- DarcyPressure now requires material properties, so create a `GenericConstantMaterial` in the input file and the test passes.

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[/tutorials/darcy_thermo_mech/step03_darcy_material/tests/kernels/darcy_pressure/darcy_pressure.i](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/tests/kernels/darcy_pressure/darcy_pressure.i)

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- Create a simple test that uses the material created.

[tutorials/darcy_thermo_mech/step03_darcy_material/tests/materials/packed_column/tests](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/tests/materials/packed_column/tests)

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[tutorials/darcy_thermo_mech/step03_darcy_material/tests/materials/packed_column/packed_column.i](https://github.com/idaholab/moose/blob/devel/tutorials/darcy_thermo_mech/step03_darcy_material/tests/materials/packed_column/packed_column.i)