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Metallic inverse opals are porous materials with enhanced mechanical, chemical, thermal, and photonic properties used to improve the performance of many technologies, such as battery electrodes, photonic devices, and heat exchangers. Cracking in the drying opal templates used to fabricate inverse opals, however, is a major hindrance to the use of these materials for practical and fundamental studies. In this work, we conduct desiccation experiments on polystyrene particle opals self-assembled on indium−tin oxide coated substrates to study their fracture mechanisms, which we describe using an energyconservation fracture model. The model incorporates film yielding, particle order, and interfacial friction to explain several experimental observations, including thickness-dependent crack spacings, cracking stresses, and order-dependent crack behavior. Guided by this model, we are the first to fabricate 120 μm thick free-standing metallic inverse opals, which are 4 times thicker than previously reported nonfree-standing metallic inverse opals. Moreover, by controlling cracks, we achieve a crack-free single-crystal domain up to 1.35 mm 2 , the largest ever reported in metallic inverse opals. This work improves our understanding of fracture mechanics in drying particle films, provides guidelines to reduce crack formation in opal templates, and enables the fabrication of free-standing large-area singlecrystal inverse opals.

Healing metallic materials involves high temperatures and large energy inputs. This work demonstrates rapid, effective, low-energy, and roomtemperature healing of metallic materials by using electrochemistry and polymer-coated cellular nickel to mimic the transport-mediated healing of bone. The polymer coating enables selective healing only at the fracture site, electrochemical reactions transport metal ions from a metal source to fractured areas, and the cellular structure of the metal allows facile ion transport to healing sites and effective recovery of strength and toughness when the cellular metal is subjected to three types of damage (scission fracture, tensile failure, and plastic deformation). Using this strategy, samples fractured in tension and by scission recover 100% of their tensile strength in as little as 10 and 4 h of healing. The healing process is stochastic, thus a statistical method is used to quantify and predict the likelihood of achieving target healing strengths based on energy input. This electrochemistry-based approach enables the first demonstration of room-temperature healing of structural metallic materials and requires several orders of magnitude less energy than many previously reported metal healing techniques.

Low-carbon steel is a widely used structural metal that, when fractured, can be repaired with high temperature processes. There are many applications, however, that would benefit from a room-temperature repair process which maintains the steel microstructure and prevents nearby materials and electronics from overheating. This work seeks to enable effective room-temperature healing of steel by understanding how ion transport and electrolyte chemistry influence growth morphology and strength in fractured steel struts repaired with nickel electrodeposition. Experiments and simulations show that pulsed electroplating mitigates diffusion-limited growth to enable smooth and dense nickel deposits that have 4× higher adhesion to steel than nickel deposited by potentiostatic electroplating. By combining pulsed electroplating and electrolyte chemistry selection, fully fractured steel wires could be repaired to achieve up to 69% of their pristine wire strength. Finally, a simple geometric model highlights the advantageous energy and time requirements of electrochemical healing across length scales.

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