The Partition Condition node can be used to prescribe the relation between the concentration of a solute species in two adjoining immiscible phases. It can for example be used on interior boundaries separating two liquid phases, a gas-liquid interface, or on a boundary separating a liquid phase and a solid or porous media.

Select Partition coefficient from the Relation list to prescribe a linear relationship between the concentration on either side of a boundary. For a species concentration ci, the ratio between the concentration on the upside and on the downside of the boundary (ci,u and ci,d respectively) is then defined in terms of a partition coefficient Ki in the manner of

Select User defined from the Relation list to set up a nonlinear relation between the up and downside concentrations. For such relations f(ci)u=f(ci)d, where f(ci)u is the expression enforced on the upside. Use the associated input fields to enter the upside and downside expressions for each species.

The up and downside of the selected boundary is indicated with a red arrow in the Graphics window. The arrow points from the downside into the upside. Select the Reverse direction check box to reverse the direction of the arrow on the selected boundary, and update the definition of the up and downside concentrations accordingly.