A system for investigating changes in optical properties of a porous effective substrate that comprises a surface, related to at least one selection from the group consisting of: effective surface depth and refractive index; pore size; pore volume; and pore size distribution at atmospheric pressure; said system enabling flowing of a gas that contains at least one solvent therein which is flowed over said substrate surface at a monitored temperature, over a sequence of different known solvent partial pressures, said system comprising: a) at least one source of a flow of at least one solvent and a controller configured to control the mass flow of a solvent; b) a source of a flow of gas and a controller configured to control the mass flow of the gas; c) an atomizing nozzle configured to receive both the flow of at least one solvent and of the flow of gas, and to expell a mixture of said at least one solvent in said gas over said substrate at a sequence of solvent partial pressures; d) an open chamber comprising said substrate that comprises a surface, and a temperature monitoring sensor; e) an optical device arranged to direct a polarised beam of electromagnetic radiation at said surface of the substrate and to investigate the change of optical properties of said substrate based on a detected polarisation state of the electromagnetic radiation, said system not comprising a partial pressure sensor feedback loop to control partial pressure values in said open chamber, but rather, said system being configured so that partial pressure values in said open chamber are set entirely by direct control of solvent flow rate of said at least one solvent in view of a measured temperature; such that in use said temperature monitoring sensor provides a measurement of temperature, and said atomizing nozzle receives both a flow of gas and at least one solvent from said gas and solvent mass flow causing controllers associated with said source of a flow of gas, and of said at least one source of a flow of at least one solvent, respectively, and in response expels a mixture of said at least one solvent in said gas, which mixture is caused to pass over said substrate surface in said open chamber in a manner such that said solvent transiently permeates said substrate surface long enough for data to be acquired therefrom, for each of a sequence of solvent partial pressures; said partial pressure of said solvent being determined from said measured temperature and solvent flow rate. effective surface depth and refractive index; pore size; pore volume; and pore size distribution at atmospheric pressure; said system enabling flowing of a gas that contains at least one solvent therein which is flowed over said substrate surface at a monitored temperature, over a sequence of different known solvent partial pressures, said system comprising: a) at least one source of a flow of at least one solvent and a controller configured to control the mass flow of a solvent; b) a source of a flow of gas and a controller configured to control the mass flow of the gas; c) an atomizing nozzle configured to receive both the flow of at least one solvent and of the flow of gas, and to expell a mixture of said at least one solvent in said gas over said substrate at a sequence of solvent partial pressures; d) an open chamber comprising said substrate that comprises a surface, and a temperature monitoring sensor; e) an optical device arranged to direct a polarised beam of electromagnetic radiation at said surface of the substrate and to investigate the change of optical properties of said substrate based on a detected polarisation state of the electromagnetic radiation, a) at least one source of a flow of at least one solvent and a controller configured to control the mass flow of a solvent; b) a source of a flow of gas and a controller configured to control the mass flow of the gas; c) an atomizing nozzle configured to receive both the flow of at least one solvent and of the flow of gas, and to expell a mixture of said at least one solvent in said gas over said substrate at a sequence of solvent partial pressures; d) an open chamber comprising said substrate that comprises a surface, and a temperature monitoring sensor; e) an optical device arranged to direct a polarised beam of electromagnetic radiation at said surface of the substrate and to investigate the change of optical properties of said substrate based on a detected polarisation state of the electromagnetic radiation, said system not comprising a partial pressure sensor feedback loop to control partial pressure values in said open chamber, but rather, said system being configured so that partial pressure values in said open chamber are set entirely by direct control of solvent flow rate of said at least one solvent in view of a measured temperature; such that in use said temperature monitoring sensor provides a measurement of temperature, and said atomizing nozzle receives both a flow of gas and at least one solvent from said gas and solvent mass flow causing controllers associated with said source of a flow of gas, and of said at least one source of a flow of at least one solvent, respectively, and in response expels a mixture of said at least one solvent in said gas, which mixture is caused to pass over said substrate surface in said open chamber in a manner such that said solvent transiently permeates said substrate surface long enough for data to be acquired therefrom, for each of a sequence of solvent partial pressures; said partial pressure of said solvent being determined from said measured temperature and solvent flow rate.