This work establishes that static contact angles for gallium-based liquid metals have no utility despite the continued and common use of such angles in the literature. In the presence of oxygen, these metals rapidly form a thin (∼1−3 nm) surface oxide "skin" that adheres to many surfaces and mechanically impedes its flow. This property is problematic for contact angle measurements, which presume the ability of liquids to flow freely to adopt shapes that minimize the interfacial energy. We show here that advancing angles for a metal are always high (>140°)even on substrates to which it adheresbecause the solid native oxide must rupture in tension to advance the contact line. The advancing angle for the metal depends subtly on the substrate surface chemistry but does not vary strongly with hydrophobicity of the substrate. During receding measurements, the metal droplet initially sags as the liquid withdraws from the "sac" formed by the skin and thus the contact area with the substrate initially increases despite its volumetric recession. The oxide pins at the perimeter of the deflated "sac" on all the surfaces are tested, except for certain rough surfaces. With additional withdrawal of the liquid metal, the pinned angle gets smaller until eventually the oxide "sac" collapses. Thus, static contact angles can be manipulated mechanically from 0°to >140°due to hysteresis and are therefore uninformative. We also provide recommendations and best practices for wetting experiments, which may find use in applications that use these alloys such as soft electronics, composites, and microfluidics.

This review highlights the unique techniques for patterning liquid metals containing gallium (e.g., eutectic gallium indium, EGaIn). These techniques are enabled by two unique attributes of these liquids relative to solid metals: 1) The fluidity of the metal allows it to be injected, sprayed, and generally dispensed. 2) The solid native oxide shell allows the metal to adhere to surfaces and be shaped in ways that would normally be prohibited due to surface tension. The ability to shape liquid metals into non‐spherical structures such as wires, antennas, and electrodes can enable fluidic metallic conductors for stretchable electronics, soft robotics, e‐skins, and wearables. The key properties of these metals with a focus on methods to pattern liquid metals into soft or stretchable devices are summari.

Soft materials tend to be highly permeable to gases, making it difficult to create stretchable hermetic seals. With the integration of spacers, we demonstrate the use of liquid metals, which show both metallic and fluidic properties, as stretchable hermetic seals. Such soft seals are used in both a stretchable battery and a stretchable heat transfer system that involve volatile fluids, including water and organic fluids. The capacity retention of the battery was ~72.5% after 500 cycles, and the sealed heat transfer system showed an increased thermal conductivity of approximately 309 watts per meter-kelvin while strained and heated. Furthermore, with the incorporation of a signal transmission window, we demonstrated wireless communication through such seals. This work provides a route to create stretchable yet hermetic packaging design solutions for soft devices.

The fracturing and incorporation of liquid gallium surface oxides during shear mixing in air enables the stabilization of air bubbles within gallium which leads to the formation of a room-temperature liquid metal foam.

Metastable poly(phthalaldehyde) (PPHA) can be triggered to depolymerize under visible light by incorporation of photosensitive compounds, such as a photoacid generator (PAG), which can generate a strong acid in situ. However, photosensitive compounds can be thermally unstable and have limited shelf life, causing inadvertent device triggering. It can also be difficult to fabricate components that are photosensitive because special lighting conditions are needed. In this paper, nonphotosensitive PPHA films were formed and made photosensitive at the point of use. This improved the material shelf life and manufacturability by adding a second, PAG‐containing layer to the original nonphotosensitive layer at an optimal point before use. The catalytic photoacid was generated rapidly by exposure of the PAG‐containing layer to radiation. Depolymerization of PPHA via the acid catalyst was followed by diffusion of the acid into the nonphotosensitive layer causing it to depolymerize. Diffusion of the photoacid into the nonphotosensitive medium was quantified at various temperatures. Photoacid diffusion in a liquid, moving‐front caused depolymerization of the nonphotosensitive PPHA layer. The fabricated bilayer structure allowed for better stability of the structural material using PPHA while still achieving transience.

Poly(phthalaldehyde) (PPHA) can be used as a structural material in transient devices and photo‐catalytically depolymerized at the end of device life by the use of a photo‐acid generator (PAG). However, device degradation requires the presence of a radiation source at the end of device mission. It has been found that the onset of PPHA depolymerization after PAG photo‐exposure can be delayed by incorporation of a particular weak bases in the PPHA/PAG mixture. This method of delayed PPHA depolymerization allows for PAG activation prior to or during device deployment when the device is under full user control. The basicity of specific lactams and amides was found to slow the PPHA depolymerization, giving the transient device a longer but finite mission lifetime. The weak base reacts with the photo‐generated strong acid to form a weak conjugate acid, which reacts more slowly with PPHA to extend the onset of PPHA depolymerization. The addition of a molar excess of specific lactams or amides, with respect to PAG, maintains PPHA stability and mechanical properties for more than 80 minutes after photo‐exposure at room temperature. The amide or lactam mediated acid activation of PPHA follows first‐order kinetics. The time delay of PPHA depolymerization can allow for prelaunch photo‐exposure and eliminates the need for postmission photo‐exposure where reliable light‐sources may not be available.

Amorphous metal oxides (AMO) are a class of semiconducting materials that show promising application in optoelectronics because of their high carrier mobility and optical transparency. By alloying with other metallic species and regulating the oxygen vacancies, the carrier mobility, and the optical bandgap energy of AMOs can be modified. This customizability not only broadens the operating window of AMOs in optoelectronics but also further enables other applications, such as digital memory devices and thin-film-transistors. Typically, AMO thin films are obtained by conventional chemical or physical vapor deposition; however, these processes generally require undesirable toxic gas precursors, a vacuum environment, and a long processing time. Gallium-based liquid metals (LMs) – a class of metals that exist as liquid at or near room temperature – naturally forms an ultrathin layer of AMO (~3nm) on their surface under ambient conditions. Herein, we propose a method to harness this feature to continuously deposit gallium oxide (GaOx) and gallium indium oxide (GaInOx) traces with their host LMs at or near ambient conditions.