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arxiv: 2606.24809 · v1 · pith:KVMSZYFCnew · submitted 2026-06-23 · ❄️ cond-mat.soft · cond-mat.stat-mech

Optical mapping of phases and phase boundaries in nanoconfined fluids

Lauriane Pierrot Deseilligny , Susan Perkin This is my paper

Pith reviewed 2026-06-25 21:48 UTC · model grok-4.3

classification ❄️ cond-mat.soft cond-mat.stat-mech
keywords nanoconfined fluidssurface force balancerefractive index profilesphase boundariesmeniscus geometryevaporationoptical mapping
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The pith

The surface force balance can be used to reconstruct refractive index profiles and thereby map phases and interfaces in nanoconfined fluids.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper introduces an optical method based on the surface force balance to create spatially resolved maps of refractive index in liquids confined between surfaces. These maps allow direct determination of where different phases meet and how their boundaries are shaped at the nanoscale. Sympathetic readers would care because surface interactions in tight spaces can create new phases or change transitions, and seeing the composition variations directly helps understand those effects. The authors test the approach first on air gaps to spot tiny wetting features, then on an evaporating heptane drop to watch the interface evolve in real time.

Core claim

The method extends conventional SFB analysis from apex measurements to spatially resolved reconstruction of refractive index profiles within confined fluids. When multiple phases are present, the refractive index profiles provide direct access to the position and geometry of the nanoconfined fluid interfaces. We describe the interferogram analysis in detail and establish its range of validity through two model scenarios. Measurements in air demonstrate the precision of the method and allow detection of a nanometric wetting capillary. Analysis of dynamic evaporation of a confined heptane droplet provides a time-resolved reconstruction of the meniscus geometry, which remains catenoidal down to

What carries the argument

Spatially resolved reconstruction of refractive index profiles from SFB interferograms

If this is right

  • The method detects a nanometric wetting capillary in air.
  • It provides time-resolved reconstruction of the meniscus geometry during evaporation of a 0.1 pL heptane droplet.
  • The meniscus remains catenoidal down to heights of approximately 80 nm.
  • Deviations from the catenoidal profile emerge at smaller separations, signaling a shift from surface tension to confinement dominated regime.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • This mapping could be used to study confinement effects in a wider range of fluid mixtures or under varying temperatures and pressures.
  • Direct geometric data on interfaces may enable measurement of nanoscale line energies or wetting energies.
  • The approach might be adapted to observe phase separation dynamics in real time in other confined geometries.

Load-bearing premise

The interferogram analysis accurately reconstructs refractive index profiles from SFB measurements assuming that optical effects are dominated by composition variations without significant contributions from scattering or non-equilibrium dynamics.

What would settle it

An independent measurement of the meniscus shape at separations below 80 nm that differs from the reconstructed catenoidal profile with deviations would show the reconstruction is not accurate.

Figures

Figures reproduced from arXiv: 2606.24809 by Lauriane Pierrot Deseilligny, Susan Perkin.

Figure 1
Figure 1. Figure 1: FIG. 1. Schematic of the SFB. White light passes through the inter [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Example FECO interferograms corresponding to different [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Graphical solution to equations 5 and 6 applied to the con [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. (a) Refractive index and separation profiles in air extracted from the intercepts as in figure 3, applied to different apex separations [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Schematic of the model evolution of [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Three-dimensional schematic of the heptane annulus wetting on a sphere and a plane. The sphere–plane confinement constrains the [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. (a): Refractive index as a function of separation for varying time points during the evaporation of the droplet. At large separations the [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Fitting of the meniscus evaporation surface [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Evolution of the annulus volume and evaporation rate during confined heptane evaporation. The volume is calculated from the [PITH_FULL_IMAGE:figures/full_fig_p008_9.png] view at source ↗
read the original abstract

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. Here, 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. When multiple phases are present, the refractive index profiles provide direct access to the position and geometry of the nanoconfined fluid interfaces. 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 allow detection of a nanometric wetting capillary. Second, we analyse 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. Although evaporation continuously drives the system out of equilibrium, the meniscus remains well described by a catenoidal geometry down to heights of approximately 80 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 analyse confinement-induced composition variations and to quantify interfacial thermodynamic effects at the nanoscale.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 0 minor

Summary. The manuscript introduces a non-invasive method using the surface force balance (SFB) for nanoscale composition mapping in confined liquids via spatially resolved reconstruction of refractive index profiles from interferograms. This extends conventional apex-only SFB analysis to enable direct access to the position and geometry of nanoconfined fluid interfaces when multiple phases arise from surface interactions. Validity is established via two model scenarios: air measurements detecting a nanometric wetting capillary, and time-resolved analysis of dynamic heptane droplet evaporation showing a catenoidal meniscus down to ~80 nm with deviations at smaller separations indicating a crossover to confinement-dominated regime.

Significance. If validated, the approach would offer a useful extension of SFB capabilities for probing confinement-induced phase behavior and interfacial effects at the nanoscale, with potential for dynamic studies as shown in the evaporation case. The explicit note of the ~80 nm crossover provides a falsifiable observation that could guide further tests of surface-tension vs. confinement regimes.

major comments (2)
  1. [Abstract] Abstract: The two model scenarios (air and evaporating heptane) test simple single-component geometries but omit multi-phase composition gradients arising from surface interactions (e.g., binary mixtures or preferential enrichment), which is the central advertised application for locating phase boundaries. This leaves the key assumption—that interferogram inversion is dominated by composition-driven RI changes without confounding from scattering or non-equilibrium effects—untested precisely where the method claims new access.
  2. [Abstract] Abstract: No quantitative data, error bars, fitting details, or full methods are provided for the interferogram analysis or the two scenarios, so the claimed precision, range of validity, and reconstruction accuracy cannot be assessed.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their review and constructive comments. We address each major comment below, indicating revisions where appropriate. The model scenarios validate the reconstruction method, which is generalizable to composition mapping.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The two model scenarios (air and evaporating heptane) test simple single-component geometries but omit multi-phase composition gradients arising from surface interactions (e.g., binary mixtures or preferential enrichment), which is the central advertised application for locating phase boundaries. This leaves the key assumption—that interferogram inversion is dominated by composition-driven RI changes without confounding from scattering or non-equilibrium effects—untested precisely where the method claims new access.

    Authors: The scenarios establish the validity and precision of the spatially resolved RI reconstruction using controlled single-component cases with known geometries. The air case quantifies detection of nanometric features, while heptane evaporation tests performance under dynamic non-equilibrium conditions, confirming the catenoid description to ~80 nm. The underlying inversion is based on RI, which directly maps composition variations regardless of single- or multi-component nature. We have added a discussion paragraph explaining extension to binary mixtures and addressing potential scattering or non-equilibrium confounders. revision: partial

  2. Referee: [Abstract] Abstract: No quantitative data, error bars, fitting details, or full methods are provided for the interferogram analysis or the two scenarios, so the claimed precision, range of validity, and reconstruction accuracy cannot be assessed.

    Authors: The full manuscript details the interferogram analysis and reconstruction in the Methods section, with quantitative fitting results, precision estimates, and scenario data presented in Results and figures. We have revised the abstract to incorporate key quantitative elements (e.g., the 80 nm crossover) and added explicit references to the Methods and supplementary information containing error bars and fitting procedures. revision: yes

Circularity Check

0 steps flagged

No circularity: method description and independent validation scenarios

full rationale

The paper introduces an interferogram analysis method for reconstructing refractive index profiles from SFB measurements and validates it on two separate model systems (air wetting capillary and dynamic heptane evaporation). No load-bearing step reduces by construction to a fitted input, self-citation, or renamed ansatz; the central reconstruction procedure is presented as an extension of conventional SFB optics with explicit range-of-validity checks on independent geometries. The derivation chain remains self-contained against external optical benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Based on abstract only; no explicit free parameters, axioms, or invented entities are stated. The method relies on standard optical interferometry and SFB principles assumed valid in the model scenarios.

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Reference graph

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