Liquid Bridges Mediate Crack Initiation, Propagation, and Coalescence to Suppress Film Failure within Flexible Electronics.

Abstract

Recent flexible electronics with conformal interfaces between devices and human bodies are prone to receive circuit failure caused by uncontrollable cracking during physiological movements. A structural engineering strategy is reported that utilizes capillary-stabilized liquid bridges to spontaneously mediate crack initiation, propagation, and coalescence for film reinforcement. Specifically, rigid nanowire array are decorated onto flexible polydimethylsiloxane substrates and the nanoscale gaps between the nanowires are filled with non-volatile ionic liquids to form well-regulated meniscus. Using metal films as a model, it is found that stretchability of an Au film deposited on this meniscus exceeds that of its flat counterpart (180 vs 30%). In-situ optical observations and fluid dynamics analyses show that liquid bridge forces mechanically hinder the propagating of crack fronts and simultaneously initiate new cracks in different locations, leading to dispersed small cracks at strains below 80%. This scattering of cracks prevents the concentrated propagation and merging of cracks into penetrating fractures, with effective electrical percolation of Au films even under a high strain of 160%, which contrasts sharply with the counterparts without liquids where penetrating cracks occur at a small strain of ≈10%. Results indicate fluid mechanics as a versatile approach to reprogram film cracking for high-performance electronics.