Under the Interaction Model subtab, properties of the particles as well as their interaction with the solid and porous materials, available in the structure, are defined. The choice of parameters under this subtab affects the entries and columns shown in the table under the Size Distribution subtab.

For all materials in the model, a Pass Through Model and a Collision Model must be set. It depends on the choice of the Pass Through Model whether a Bounce Model needs to be selected additionally, or not.

Solid materials are always Impassable for particles, and for them, only the collision model among Caught on first touch, Hamaker, Sieving and Random must be chosen. The Hamaker model can only be selected for particles with finite size diameter, while Random can only be chosen for molecules. Additional columns appear in the table under the Size Distribution subtab when Hamaker (columns Restitution and Adhesion), Sieving (column Restitution), or Random (column Deposition Probability) are chosen as Collision Model.

Additional collision models can be defined by the user in user defined functions (UDF) and accessed by adding an UDF Search Folder by clicking the file button on the top right of this tab. UDFs are more commonly used for filter simulations. Thus, more details can be found in the FilterDict user guide.

For porous materials, one of three Pass Through Models can be selected:

• When set to All particles pass, particles can move through the material. In that case, no Collision Model can be selected.
• Choosing Impassable sets the material to be treated as a solid material and particles cannot enter.
• When Bounce Probability is selected, part of the particles can enter the porous material, while others collide with the material. Here the expected behavior of a fully resolved porous materials is simulated. The part of the particles that enter the material are the particles that hit the porous material at a pore. The part of the particles that collide with the material are the particles that hit the solid. This option should only be chosen if the particles are smaller than the pore sizes of the porous material, i.e., they can enter the material. The percentage of the particles that enter the material, depends on the choice of the Bounce Model.

For the Bounce Model three different options are available:

• Selecting Manual allows the user to define the probability of a particle to be reflected at the interface between pore space and porous material by clicking the Edit button. In the example shown, the probability of a particle to be reflected, if it reaches the interface between the pore space (ID 00) and the porous material (ID 02), is 20%. Vice versa the probability that the particle enters the porous material is 80%.
  
• For Connected Porosity, the bounce probability depends on the porosity of the material the particle is entering () and the material the particle is leaving (). The reflection probability is . If the particle is moving from a region with low porosity to one with higher porosity, it will never be reflected, i.e., the reflection probability is always kept between 0% and 100%. This Bounce Model should be selected for the case of materials with different porosities, i.e., pores that are connected and change only their width at the interface between materials. The porosity value(s) of the porous material(s) need to be set correctly on the Constituent Materials tab.
• If Independent Porosity is selected, the reflection probability is , i.e., if the porous material has a porosity of 30%, 30% of the particles reaching the interface enter the porous material and 70% are reflected. The porosity value(s) of the porous material(s) need to be set correctly on the tab Constituent Materials.
  In the example shown here, with only an interface between pore and porous material, Connected and Independent Porosity both give the same value. Clicking the Edit button shows the computed value.
  

For the Particle Diameter two options are available. If Finite Size is set for the Particle Diameter, the momentum equation is solved to model the particle movement. For Molecules (Limit d=0) the simplified movement equation is solved to model the particle movement. In this case, no Particle Density value has to be entered, Particle Charges are set to zero (i.e., the subtab Electrostatic Effects becomes inactive), and Diffusivities have to be defined individually per particle type in the Size Distribution subtab.

For finite sized particles, a Particle Density has to be given. If Constant is chosen, one density is set for all particle sizes. If Individual per particle type densities are chosen, the field to set the Density in (kg/m³) disappears and individual density values have to be entered in the Size Distribution subtab.

For Particle Diffusivity, the user may choose between Brownian Motion and Individual per particle type. For Brownian motion, the diffusivity coefficient  is computed following the equation described in the theoretical basis. For Individual per particle type, the value of must be entered under the Size Distribution subtab for each particle type. The values entered in the Diffusivity in Pore column are used to model the diffusion of the particles in the pores and the values entered in a Diffusivity in Media column are used to model the diffusion inside of this porous material. In the table, one column appears for each porous material which allows particles to pass, i.e., for which the Pass Through Model is set to All particles pass or to Bounce Probability.

For Particle Sliding, None, Sieving or Sieving and Hamaker can be selected. The selection defines for which material the sliding will be considered. For none of the materials, for those with Collision Model Sieving or for those with Collision Models Sieving and Hamaker.