The Number Density node adds a boundary condition for the number density at the boundary.

The Number Density node is not available in 0D.

Select User defined from the list to enter a value or expression that is applied to all size intervals. When this option is selected, the text field also accepts an expression written as a function of the particle size, pop.L. Select Define individually instead to define number densities individually for each size interval. For Define individually, select the checkbox for the number density of a specific size interval and enter a value or an expression in the corresponding field to specify the number density.

To display this section, click the Show More Option button () and select Advanced Physics Options. You can find details about the different constraint settings in the section Constraint Reaction Terms in the COMSOL Multiphysics Reference Manual.