Visualization of orange peel defects in coatings using a lipophilic spiropyran

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Abstract
Understanding and controlling interfacial dynamics during coating evaporation is crucial for suppressing surface defects, yet direct visualization of these nonuniform processes remains challenging. Here we introduce spiropyran derivative, NSA‐SP , as a probe that enables real‐time optical mapping of thermal field distribution, capillary‐driven flows, and interfacial adsorption during film drying. Owing to its high solubility in ester‐based formulations and ultrafast photoisomerization, NSA‐SP converts local physicochemical variations into a spatiotemporally resolved optical response, allowing direct visualization of fluid‐dynamic instabilities associated with orange‐peel defects. The concentration‐dependent optical response exhibits biexponential behavior, reflecting sequential saturation of Gibbs surface excess at interfaces of distinct thermodynamic character. Preferential adsorption at the solid–liquid interface dominates at low concentrations, followed by redistribution to the gas–liquid interface at higher loadings. By quantitatively linking optical observables to interfacial thermodynamics, this approach enables calculation of the optimal leveling‐agent dosage and provides a predictive framework for formulation design.
Funding Information
  • National Natural Science Foundation of China (22408233)
  • National Natural Science Foundation of China (22308220)
  • National Natural Science Foundation of China (22090011)
  • National Natural Science Foundation of China (22525803)
  • Basic and Applied Basic Research Foundation of Guangdong Province (2025A1515012197)
  • Basic and Applied Basic Research Foundation of Guangdong Province (2026A1515030049)

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