Analyzing refractive index profiles of confined fluids by interferometry part II: Multilayer and asymmetric systems

Methods for determining the substrate properties and the optical thickness of thin films or any variation in the refractive index of a fluid or film near a surface for unknown 5-layer symmetric and 3-layer asymmetric interferometers are presented. Both systems can be fully resolved without any known layer properties and without contact or confining the films. The method was tested using realistic simulated interferometer data, and was found to consistently yield accurate values for all desired properties. The method was experimentally validated through analysis of an asymmetric three layer interferometer system of linear polyethyleneimine (LPEI) adsorbed onto mica substrates of differing thickness and identical refractive index. The results were in excellent agreement with the dry polymer film properties measured using conventional SFA contact measurements. More complicated systems were also evaluated for feasibility, and any additional parameter specifications required for analysis were determined. The utility of this method is broad, as a single experiment in a laboratory setting can independently provide non-contact film properties and the effects of confinement on the film structure, which can be correlated to a simultaneously measured interaction force profile.