Printable Liquid Metal Foams That Grow When Watered.

Pastes and "foams" containing liquid metal as the continuous phase (LMFs) exhibit metallic properties while displaying paste or putty-like rheological behavior. These properties enable LMFs to be patterned into soft and stretchable electrical and thermal conductors through processes conducted at room temperature, such as printing. The simplest LMFs, featured in this work, are made by stirring liquid metal in air, thereby entraining oxide-lined air "pockets" into the liquid metal. Here, we report that mixing small amounts of water (as low as 1wt%) into such LMFs gives rise to significant foaming by harnessing known reactions that evolve hydrogen and produce oxides. The resulting structures can be ∼4-5x their original volume and possess a fascinating combination of attributes: porosity, electrical conductivity, and responsiveness to environmental conditions. This expansion can be utilized for a type of 4D printing in which patterned conductors "grow", fill cavities, and change shape and density with respect to time. Excessive exposure to water in the long term ultimately consumes the metal in the LMF. However, when exposure to water is controlled, the metallic properties of porous LMFs can be preserved. We characterize the physical changes of water-containing LMFs and explore prospective applications including a "growing" conductor that fills a gap in a circuit and an untethered pressure-driven actuator. We also demonstrate the printability of LMFs and present a printed pattern that changes shape over time. This article is protected by copyright. All rights reserved.