Accurate refractive index measurements in nanoconfined spaces Peng Zhang 1 , Michal Góra 2 , Yinfeng Xu 1 , Himanshu Mishra 1 , Manfred Heuberger 2 1 Interfacial Lab, King Abdullah University of Science and Technology, Saudi Arabia, 2 Laboratory for Advanced Fibers, Swiss Federal Laboratories for Materials Science and Technology, Switzerland When a liquid is confined at the nanoscale within an immiscible phase–solid, liquid, or gas–its non-continuum or molecular nature can become apparent. These "nanoconfined" liquids are relevant in numerous natural and applied contexts, such as molecular biology, lubrication, soil physics, aquaporins, evapotranspiration, nanofluidic devices, and catalysis. Several studies on nanoconfined liquids have reported properties deviations from bulk, including fluctuations in refractive indices, reduction in dielectric constants, and ultrafast transport. However, the molecular basis for these findings remains unclear. Designing accurate and precise experiments to probe nanoconfined liquids is a daunting task that is often prone to artifacts. In this contribution, we investigate the refractive indices of confined liquids using the Surface Force Apparatus (SFA), a technique that has been evolving since the 1960s to measure surface forces at an angstrom-scale resolution. We will report on our efforts to improve the precision and accuracy of refractive index measurements based on SFA and identify the key sources of errors. References 1. Heuberger, M., Zach, M. & Spencer, N. D. Density fluctuations under confinement: when is a fluid not a fluid? Science 292 , 905-908, doi:10.1126/science.1058573 (2001). 2. Heuberger, M. & Zach, M. Nanofluidics: Structural forces, density anomalies, and the pivotal role of nanoparticles. Langmuir 19 , 1943-1947, doi:10.1021/la026645p (2003). 3. Zhu, Y. & Granick, S. Reassessment of solidification in fluids confined between mica sheets. Langmuir 19 , 8148-8151, doi:10.1021/la035155 (2003). 4. Israelachvili, J., Maeda, N. & Akbulut, M. Comment on Reassessment of Solidification in Fluids Confined between Mica Sheets. Langmuir 22 , 2397-2398, doi:10.1021/la051470f (2006). 5. Kienle, D. F. & Kuhl, T. L. Density and Phase State of a Confined Nonpolar Fluid. Phys Rev Lett 117 , 036101, doi:10.1103/ PhysRevLett.117.036101 (2016). 6. Le, T. H. H., Morita, A. & Tanaka, T. Refractive index of nanoconfined water reveals its anomalous physical properties. Nanoscale Horiz 5 , 1016-1024, doi:10.1039/d0nh00180e (2020).
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