Contact Between Grains
Use the Contact Between Grains node to define the contact pair properties between different species of grains.
Grains Pair Selection
Select All pairs (default) or Manual option from the Selection list.
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When All pairs is selected, the contact properties defined applies to all possible pairs between different species of grains.
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When Manual is selected, a table is available with two columns Specify properties for first grain type and Specify properties for second grain type. You can specify any pairs of grain species to have pair properties defined by this node. This table also has 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 Between Grains node. The default node always specifies contact pair properties for all possible pairs between grains. If you want to specify different contact pair properties for different grains pairs, you can add Contact Between Grains nodes. Then select Manual from the Selection list and edit the table with desired pairs between different species of 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, there are three possible pairs between different grain species:
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 Grain Properties 1 and Grain Properties 1
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 Grain Properties 1 and Grain Properties 2
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 Grain Properties 2 and Grain Properties 2
Assume that the first two pairs in the above list have the same contact pair properties but the third pair has a different set of contact pair properties. The default Contact Between Grains node specifies properties for all three pairs. Now, if you add one more Contact Between Grains node and select Manual from the Selection list in the Grains Pair Selection section, then you can specify Grain Properties 2 in Specify properties for first grain type and again Grain Properties 2 in Specify properties for second grain type. 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 values or expressions for the following quantities:
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 Normal coefficient of restitution en. The default value is 1. This is the value of coefficient of restitution in normal direction or direction along the line connecting centers of two grains in contact. It determines the amount of energy loss in the normal direction when the grains 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 centers of two grains in contact. 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 two grains 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 Force section. The tangential spring constant for other contact force models is automatically calculated based on the material properties and overlapping distance between two grains 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 selected 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 selected 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 values or expressions for the following quantities:
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 Interface energy density γij. The default value is 0.
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 Cutoff distance for van der Waals force 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 for van der Waals force 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 conductive heat transfer checkbox is selected in the physics interface node’s Additional Variables section.
Enter a value or expression for 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