Counteracting Marangoni Flow by Faster Adsorption: A Route to Continuous Fabrication of Uniform Polymer Nanofilms

Polymer nanofilms are extensively utilized across aerospace, biomedical, and optical applications. However, their large-scale continuous fabrication often faces challenges related to poor film uniformity and stochastic morphological defects. In this study, we use poly(vinyl alcohol) (PVA) as a model polymer to explore the origin of terraced morphology observed in nanofilm surfaces during continuous production. Through successive dip-coating experiments, we demonstrate that this terraced morphology arises from Marangoni flow, which becomes particularly pronounced when the surface environment undergoes continuous variation during the coating process. Real-time surface tension measurements indicate that, in high-hydrolysis PVA (PVA-97, 97% hydrolyzed), the rates of surface adsorption and molecular rearrangement decrease with successive dip cycles due to strong intramolecular hydrogen bonding. This gives rise to slower surface tension relaxation, which exacerbates surface tension gradients, intensifies Marangoni flow, and ultimately leads to the formation of terraced structures. In contrast, low-hydrolysis PVA (PVA-78, 78% hydrolyzed) maintains rapid adsorption rates across dip cycles, enabling rapid surface tension relaxation and homogeneous surface tension, which results in uniform nanofilms with consistent thickness. By incorporating a small amount of PVA-78 (<5 wt % of the total PVA content), we can counteract Marangoni flow, leveraging its faster adsorption rate to achieve more uniform film formation. This study elucidates the comprehensive contrast in adsorption behavior among structurally almost identical polymers and highlights their significant impact on morphological defects. Furthermore, it presents a practical strategy for producing highly uniform polymer nanofilms in continuous fabrication.