Optical mapping of composition in nanoconfined fluids applied to the meniscus geometry of an evaporating picoliter droplet

In confined space, deviations from bulk structure and properties are expected due to additional thermodynamic variables. In particular, composition variations arising from surface interactions may lead to additional phases and altered phase transitions. In this paper, we introduce a non-invasive method for nanoscale composition mapping in confined liquids using the surface force balance (SFB). The method extends conventional SFB analysis from apex measurements to spatially resolved reconstruction of refractive index profiles within confined fluids. The refractive index profiles provide direct access to the position and geometry of nanoconfined fluid interfaces when multiple phases are present. We describe the interferogram analysis in detail and establish its range of validity through two model scenarios. First, measurements in air demonstrate the precision of the method and are consistent with the presence of a nanometric wetting capillary. Second, we analyze dynamic evaporation of a confined heptane droplet down to 0.1 pL volume. The method provides a time-resolved reconstruction of the meniscus geometry throughout the evaporation process, without relying on the Young-Laplace equation. Although evaporation continuously drives the system out of equilibrium, the meniscus remains well described by a catenoidal geometry down to heights of ∼30 nm. At smaller separations, systematic deviations from the catenoidal profile emerge, indicating a crossover from a surface tension-dominated regime to a confinement-dominated regime. Overall, we demonstrate composition profiling as a framework to analyze confinement-induced composition variations and to quantify interfacial thermodynamic effects at the nanoscale. The analysis may be applied to more complex interfacial films and fluids, for example in partially miscible and biomolecular liquid mixtures.