Contact with Walls
Use the Contact with Walls node to define the contact pair properties between grains and walls.
Grain–Wall Pair Selection
Select All pairs (default), Pair between all walls and selected grain types, Pair between all grains and selected wall types, or Manual from the Selection list.
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When All pairs is selected, the contact properties defined applies to all possible pairs between grains and walls.
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When Pair between all walls and selected grain types is selected, a table is available with one column titled Specify properties of grain where you can add any species of grains. You can specify contact pair properties between selected grain species and all types of walls defined by different Wall nodes.
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When Pair between all walls and selected wall types is selected, a table is available with one column titled Specify properties of wall where you can add any types of walls. You can specify contact pair properties between selected type of wall and all species of grains defined by different Grain Properties nodes.
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When Manual is selected, a table is available with two columns: Specify properties for wall and Specify properties for grain. You can specify any pairs of grain species and wall types to have pair properties defined by this node.
All the tables mentioned above have options to move rows up or down, add a new row, delete selected rows, and clear the entire table.
The Selection list is disabled in the default Contact with Walls node. The default node always specifies contact pair properties for all possible pairs between grains and walls. If you want to specify different contact pair properties for different grain–wall pairs, you can add Contact with Walls nodes. Then select an appropriate option from the Selection list and edit the table with desired pairs between walls and grains.
For example, if you set up a model with two Grain Properties nodes to define two species of grains with labels Grain Properties 1 and Grain Properties 2, and one Wall node labeled Wall 1 to define a type of wall, then there are two possible pairs between grain and wall:
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 Wall 1 and Grain Properties 1
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 Wall 1 and Grain Properties 2
Assume that each pair has a different set of contact pair properties. The default Contact with Walls node specifies properties for both pairs. Now, if you want to specify different pair properties for the second pair in the above list, you can add a new Contact with Walls node and then specify the pair using one of the following two ways:
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Select the Pair between all walls and selected grain type option from the Selection list in the Grain–Wall Pair Selection section. Choose Grain Properties 2 for the Specify properties for grain column in the table.
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Select the Manual option from the Selection list in the Grain–Wall Pair Selection section. Choose Wall 1 for the Specify properties for wall column and Grain Properties 2 for the Specify properties for grain column in the table.
Now all the values entered in this node will only apply to this selected pair, and the previous values assigned for this specific pair by the default node get overridden.
Contact Properties
Enter a value or expressions for the following:
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 Normal coefficient of restitution en. The default value is 1. This is the value of the coefficient of restitution in the normal direction or direction along the line connecting center of the grain and the point of contact at wall. It determines the amount of energy loss in the normal direction when the grain and wall come in contact.
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 Tangential coefficient of restitution et. The default value is 1. This is the value of the coefficient of restitution in the tangential direction or direction perpendicular to the line connecting center of the grain and the point of contact at wall. It determines the amount of energy loss in the tangential direction when the grains come in contact.
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 Normal spring constant kn (SI unit: N/m). The default value is 10 MN/m. This setting is only available when Linear elastic is selected from the Contact force model list in the physics interface node’s Force section. The normal spring constant for other contact force models is automatically calculated based on the material properties and overlapping distance between grain and wall in contact.
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 Tangential spring constant kt (SI unit: N/m). The default value is 10 MN/m. This setting is only available when Linear elastic is selected from the Contact force model list in the physics interface node’s Force section. The tangential spring constant for other contact force models is automatically calculated based on the material properties and overlapping distance between grain and wall in contact.
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 Static friction coefficient μn. The default value is 0.09.
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 Rolling friction coefficient μr. The default value is 0.1. This setting is only available when the option selected in Rotational resistance model is either Constant torque model or Varying torque model in the physics interface node’s Rotational Resistance section
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 Twisting friction coefficient μtw. The default value is 0.1. This setting is only available in 3D when the option selected in Rotational resistance model is either Constant torque model or Varying torque model in the physics interface node’s Rotational Resistance section.
For theory see:
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Contact Force: Linear Elastic Model
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Contact Force: Hertz–MD (Mindlin and Deresiewicz) Model
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Contact Force: Coulomb’s Criterion
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Rotational Resistance Theory
Adhesion Properties
This section is only available when
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 the Compute van der Waals force checkbox is selected and/or
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 Hertz–MD with adhesion is selected from the Contact force model list
in the physics interface node’s Force section. Enter the value or expression for the following:
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 Interface energy density γij. The default value is 0.
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 Cutoff distance Dmax (SI unit: m), only available when the Compute van der Waals force checkbox is selected in the physics interface node’s Force section. The default value is 1 μm.
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 Minimal distance Dmin (SI unit: m), only available when the Compute van der Waals force checkbox is selected in the physics interface node’s Force section. The default value is 1.65 angstrom.
For theory see:
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Contact Force: Hertz–MD with Adhesion Model
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Contact Force: van der Waals Force
Temperature Correction Factor
This section is only available when the  Compute grain temperature and Compute conductive heat transfer checkboxes are selected in the physics interface node’s Additional Variables section.
Enter the value or expression for the Temperature correction factor for contact radius Cr. The default is 1. This correction factor can be used to account for the large contact radius that often results from utilizing artificially low values of Young’s modulus.
Contact Heat Transfer