Hydrophilic nanofiber alignment governs precursor film propagation

• Cellulose nanofiber (CNF) alignment controls precursor-film propagation distance ahead of the macroscopic contact line. • Precursor films extend ∼100 μm in highly aligned CNF regions but only ∼70 μm in less aligned areas. • Where multiple CNFs overlap, water is wicked upward from the precursor film, forming microdroplets that merge with the advancing contact line and enhance transport. Liquid spreading on surfaces with micro- and nanoscale structures is crucial for a variety of engineering applications—including wick-enabled heat pipes, drug delivery systems, and fuel cells. However, the underlying nanoscale interactions governing wicking remain poorly understood. In this study, by employing a straightforward measurement system that integrates atomic force microscopy (AFM) with coherence scanning interferometry (CSI), we captured the local interactions between nanofibers and the spreading liquid with nanoscale spatial resolution. Consequently, we demonstrated that the horizontal alignment of cellulose nanofibers (CNFs) influences precursor film propagation: in regions with high alignment, the precursor film advanced steadily, reaching an effective propagation distance of up to approximately 100 μm. In contrast, over areas with low alignment, the film’s progression suppressed, limiting its maximum extension to approximately 70 μm. In addition, in areas where CNFs exhibit relatively large diameters or increased thickness due to overlapping deposition, capillary forces draw water from the precursor film, leading to the formation of microdroplets. These microdroplets coalesce with the macroscopic contact line, inducing its deformation toward the direction of contact line motion. Our findings demonstrate the importance of nanoscale surface morphology in governing macroscopic liquid behavior and provide fundamental insights for optimizing surface designs in advanced liquid-transport devices.