Use the Thin Low Permittivity Gap node

to model a thin gap of a material with a small permittivity compared to the adjacent domains. The layer has the thickness ds and the relative permittivity εr. The indices 1 and 2 refer to the two sides of the boundary.

The default is to take the Relative permittivity εr (dimensionless) values From material.  For User defined, enter a different value or expression. Enter a Surface thickness ds (SI unit: m). The default is 5 mm.