#Surface Contacts
The interaction properties of two ContactGeometries, belonging to two different Body’s, is defined with a SurfaceContact.Model. For example, the collision between two wooden logs, and one log and one rock should be defined by two different SurfaceContact models. It is then fundamental that the geometries of the objects have the intended material assigned. If there is no SurfaceContact model declared for the material pair of two geometries colliding, the behavior should be reduced to a default definition defined by the runtime environment.
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SurfaceContact.Model defines the pairwise contact properties between geometries assigned materials. The SurfaceContact model will describe the behavior in a contact normal direction, the friction definition between the materials, and if there is any stickiness (adhesive forces) involved in the interaction. It is undefined to have more than one SurfaceContact model declared for one pair of materials, resulting in the runtime showing an error.
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The normal_flexibility is modeled as a DefaultFlexibility. There are alternative elasticity types for the surface contact. The [PointwiseElasticity][PointwiseElasticity.openplx] model the surface normal interaction as individual elastic springs. The [PatchElasticity][PatchElasticity.openplx] does scale the contact stiffness to behave consistent independent of the contact area and the number of contact points.
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The friction is modeled as a DefaultFriction attribute define the friction forces between the two colliding surfaces. The friction acts in an orthogonal plane to the contact normal. DefaultDryFriction is a velocity independent friction which results in a force that is maximum of the friction coefficient multiplied with the normal force, while not doing any work. There are a few variants of the dry friction, describing various fidelity for the equations of motion. The DryBoxFriction is independent of the normal force, while the DryScaleBoxFriction depends on the normal force. The DryConeFriction is the most sophisticated model, where the friction bounds is symmetric in all directions. ConstantViscousSurfaceFriction is a fully velocity dependent friction type.
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The adhesion is modeled as a DefaultAdhesion template describes how two materials stick together, generating forces opposite to the contact normal. The ConstantForceAdhesion defines a constant adhesive force active when the volumes of two contact geometries overlap.
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