The void ratio ( e ) of a mixture of solids and fluids (gases and liquids), or of a porous composite material such as concrete, is the ratio of the volume of the voids ( V V ) filled by the fluids to the volume of all the solids ( V S ). It is a dimensionless quantity in materials science and in soil science, and is closely related to the porosity (often noted as ϕ ϕ , or η η , depending on the convention), the ratio of the volume of voids ( V V ) to the total (or bulk) volume ( V T ), as follows: