Select the Hertz–MD with Adhesion from the Contact Force Model list in the physics interface Force section to compute the contact forces using the Hertz–MD with adhesion model. A widely used adhesive contact force model used in DEM is the Johnson–Kendall–Roberts (JKR) contact model, which is an extension of the Hertz–MD contact force model and allows for tensile force to develop due to surface adhesion, γ, which is calculated here as (Ref. 2)

where i and j are indices for the grains in contact and γij is the interface energy density.

The normal contact force is given by (Ref. 3)

where a is the radius of the circular contact patch area. For the definition of other parameters used, see Contact Force: Hertz–MD (Mindlin and Deresiewicz) Model and Contact Force: Linear Elastic Model.

An analytical solution to calculate a for given δn is given in Ref. 4. The contact force between grains (or between grain and wall) starts when there is a physical contact and the contact force remains active as the gap between the neighboring grains is less than or equal to the cutoff distance, δc, which is given by (Ref. 3),

where a0 is the equilibrium contact patch radius when |Fn|=0 and no other external forces are acting and is given by

The tangential force is exactly the same as that of Contact Force: Hertz–MD (Mindlin and Deresiewicz) Model except the definition of kt, which is given by