An optical method for monitoring the character of air-water interface by the self-assembly behavior of nano-colloidal

This study presents an optical method for the direct visualization and quantification of air-water (AW) interface properties, which serve as a critical subphase governing the self-assembly behavior of nanoparticles. By employing the needle tip flow (NTF) technique, we fabricated two-dimensional photonic crystals (2D PCs) at the AW interface. The structural order of these 2D PCs, reflected in their Debye ring diffraction patterns, is highly sensitive to subphase conditions such as temperature, ionic strength, and pH. A key innovation of our work is the integration of this optical setup with an automated image analysis workflow using Image J software, which objectively quantifies the Debye ring morphology. This approach minimizes human error and demonstrates strong agreement with scanning electron microscopy (SEM) validation. We established quantitative correlations between Debye ring width and subphase properties: a linear increase with temperature from 15 to 45 °C (R² = 0.9607) and a similar positive correlation with NaCl concentration (0.0001–0.1 g/L). This method provides a cost-effective, accessible platform for in-situ interfacial characterization.