Time‐Resolved Simultaneous Mapping of Thickness and Nanoparticle Concentration in Nanofluid Thin Films via Imaging Ellipsometry and Deep‐UV Reflectance Imaging

ABSTRACT Quantifying the spatiotemporal distribution of nanoparticles within dynamic liquid films is fundamental for optimizing solution processing, functional coatings, and self‐assembly. However, simultaneously resolving liquid‐film thickness and local particle concentration across nanometer‐to‐micrometer scales remains a significant challenge. This study establishes a dual‐channel metrology framework that integrates phase‐shifting imaging ellipsometry with deep‐ultraviolet reflectance imaging. Geometric thickness effects are decoupled from intrinsic material extinction, enabling label‐free simultaneous mapping of film thickness and nanoparticle mass fraction. The method is demonstrated by examining the wetting dynamics of ‐laden ‐alkane droplets on a silicon substrate, revealing distinct internal transport behaviors: superspreading heptane nanofluid droplets exhibit significant particle accumulation near the contact line, whereas nonane nanofluid droplets, following Tanner's law, display a particle‐depleted zone. Validated by mass conservation, this approach provides direct access to internal dynamics within thin films, offering a versatile tool for analyzing complex transport phenomena in thin‐film flows.