The ancient wisdom found in iron gall ink guides this work to a simple but advanced solution to the molecular engineering of fluidic interfaces. The Fe(II)–tannin coordination complex, a precursor of the iron gall ink, transforms into interface‐active Fe(III)–tannin species, by oxygen molecules, which form a self‐assembled layer at the fluidic interface spontaneously but still controllably. Kinetic studies show that the oxidation rate is directed by the counteranion of Fe(II) precursor salts, and FeCl₂ is found to be more effective than FeSO₄—an ingredient of iron gall ink—in the interfacial‐film fabrication. The optimized protocol leads to the formation of micrometer‐thick, free‐standing films at the air–water interface by continuously generating Fe(III)–tannic acid complexes in situ. The durable films formed are transferable, self‐healable, pliable, and postfunctionalizable, and are hardened further by transfer to the basic buffer. This O₂‐instructed film formation can be applied to other fluidic interfaces that have high O₂ level, demonstrated by emulsion stabilization and concurrent capsule formation at the oil–water interface with no aid of surfactants. The system, inspired by the iron gall ink, provides new vistas on interface engineering and related materials science.

中文翻译：

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再谈铁胆油墨：用于流体界面工程的Fe（II）-单宁络合物的原位氧化

铁胆油墨中发现的古老智慧将这项工作引导到了一种简单而先进的解决方案，用于流体界面的分子工程。Fe（II）-单宁配位络合物（铁胆油墨的前体）通过氧分子转变为界面活性的Fe（III）-单宁物种，它们在流体界面上自发形成自组装层，但仍然可控。动力学研究表明，氧化速率是由Fe（II）前体盐的抗衡离子控制的，并且发现FeCl ₂比FeSO ₄更有效（铁胆油墨的一种成分）在界面膜制造中。优化的方案通过连续不断地原位生成Fe（III）-鞣酸络合物，导致在空气-水界面处形成微米厚的独立膜。形成的耐用膜具有可转移性，自修复性，柔韧性和后功能性，并且可以通过转移到基本缓冲液中进一步硬化。这种由O ₂指示的成膜作用可以应用于其他具有高O ₂含量的流体界面，这可以通过乳液稳定化和同时在油水界面上形成胶囊而无需使用表面活性剂来证明。该系统受铁胆油墨的启发，为界面工程和相关材料科学提供了新的视野。