The accuracy of measuring confined fluid properties using white light interferometry

Area of Science:

• Physics
• Materials Science
• Nanotechnology

Background:

• Fabry-Pérot thin-film interferometry with white light offers sub-nanometer resolution for studying nano-confined fluids.
• This technique is central to the surface forces apparatus (SFA) for measuring forces between surfaces at nanometer separations.
• Historically, the absolute accuracy of white light interferometry, particularly for refractive index (RI) measurements of thin films, has been overlooked.

Purpose of the Study:

• To systematically investigate accuracy-limiting factors in white light interferometry for nano-confined fluids.
• To address the critical need for improved accuracy in RI measurements of nanometer-thin fluid films.
• To identify and analyze sources of systematic error in the interferometric definition and experimental setup.

Main Methods:

• Utilizing Fabry-Pérot interferometry with visible white-light illumination.
• Applying the surface forces apparatus (SFA) to confine fluids between mica sheets.
• Developing a theoretical framework for spectral evaluation requiring accurate thickness and dispersive RI values for all optical layers.

Main Results:

• The accuracy of RI measurements is critically dependent on precise detection of secondary spectral modulations.
• Systematic errors arise from the choice of mathematical models for dispersive RI and experimental factors like mechanical deformations.
• An accurate optical description of the mica surfaces is identified as the primary source of systematic error.

Conclusions:

• The current thin-film interferometry methodology lacks sufficient accuracy for detecting density changes below 10% in nano-confined fluids.
• Accurate optical characterization of confining surfaces is paramount for improving interferometric measurements.
• Further advancements are needed to enhance the precision of white light interferometry for nanoscale fluid analysis.