An atomically smooth container: Can the native oxide promote supercooling of liquid gallium?

Metals tend to supercool-that is, they freeze at temperatures below their melting points. In general, supercooling is less favorable when liquids are in contact with nucleation sites such as rough surfaces. Interestingly, bulk gallium (Ga) can significantly supercool, even when it is in contact with heterogeneous surfaces that could provide nucleation sites. We hypothesized that the native oxide on Ga provides an atomically smooth interface that prevents Ga from directly contacting surfaces, and thereby promotes supercooling. Although many metals form surface oxides, Ga is a convenient metal for studying supercooling because its melting point of 29.8°C is near room temperature. Using differential scanning calorimetry (DSC), we show that freezing of Ga with the oxide occurs at a lower temperature (-15.6 ± 3.5°C) than without the oxide (6.9 ± 2.0°C when the oxide is removed by HCl). We also demonstrate that the oxide enhances supercooling via macroscopic observations of freezing. These findings explain why Ga supercools and have implications for emerging applications of Ga that rely on it staying in the liquid state.

Are Contact Angle Measurements Useful for Oxide-Coated Liquid Metals?

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.

Metallophobic Coatings to Enable Shape Reconfigurable Liquid Metal Inside 3D Printed Plastics.

Liquid metals adhere to most surfaces despite their high surface tension due to the presence of a native gallium oxide layer. The ability to change the shape of functional fluids within a three-dimensional (3D) printed part with respect to time is a type of four-dimensional printing, yet surface adhesion limits the ability to pump liquid metals in and out of cavities and channels without leaving residue. Rough surfaces prevent adhesion, but most methods to roughen surfaces are difficult or impossible to apply on the interior of parts. Here, we show that silica particles suspended in an appropriate solvent can be injected inside cavities to coat the walls. This technique creates a transparent, nanoscopically rough (10-100 nm scale) coating that prevents adhesion of liquid metals on various 3D printed plastics and commercial polymers. Liquid metals roll and even bounce off treated surfaces (the latter occurs even when dropped from heights as high as 70 cm). Moreover, the coating can be removed locally by laser ablation to create selective wetting regions for metal patterning on the exterior of plastics. To demonstrate the utility of the coating, liquid metals were dynamically actuated inside a 3D printed channel or chamber without pinning the oxide, thereby demonstrating electrical circuits that can be reconfigured repeatably.

Ultrastretchable Elastic Shape Memory Fibers with Electrical Conductivity.

Herein, elastomeric fibers that have shape memory properties due to the presence of a gallium core that can undergo phase transition from solid to liquid in response to mild heating are described. The gallium is injected into the core of a hollow fiber formed by melt processing. This approach provides a straightforward method to create shape memory properties from any hollow elastic fiber. Solidifying the core changes the effective fiber modulus from 4 to 1253 MPa. This increase in stiffness can preserve the fiber in a deformed shape. The elastic energy stored in the polymer shell during deformation drives the fiber to relax back to its original geometry upon melting the solid gallium core, allowing for shape memory. Although waxes are used previously for this purpose, the use of gallium is compelling because of its metallic electrical and thermal conductivity. In addition, the use of a rigid metallic core provides perfect fixity of the shape memory fiber. Notably, the use of gallium-with a melting point above room temperature but below body temperature-allows the user to melt and deform local regions of the fiber by hand and thereby tune the effective modulus and shape of the fiber.

Materials tactile logic via innervated soft thermochromic elastomers.

Conventional machines rely on rigid, centralized electronic components to make decisions, which limits complexity and scaling. Here, we show that decision making can be realized on the material-level without relying on semiconductor-based logic. Inspired by the distributed decision making that exists in the arms of an octopus, we present a completely soft, stretchable silicone composite doped with thermochromic pigments and innervated with liquid metal. The ability to deform the liquid metal couples geometric changes to Joule heating, thus enabling tunable thermo-mechanochromic sensing of touch and strain. In more complex circuits, deformation of the metal can redistribute electrical energy to distal portions of the network in a way that converts analog tactile 'inputs' into digital colorimetric 'outputs'. Using the material itself as the active player in the decision making process offers possibilities for creating entirely soft devices that respond locally to environmental interactions or act as embedded sensors for feedback loops.

Toughening stretchable fibers via serial fracturing of a metallic core.

Tough, biological materials (e.g., collagen or titin) protect tissues from irreversible damage caused by external loads. Mimicking these protective properties is important in packaging and in emerging applications such as durable electronic skins and soft robotics. This paper reports the formation of tough, metamaterial-like core-shell fibers that maintain stress at the fracture strength of a metal throughout the strain of an elastomer. The shell experiences localized strain enhancements that cause the higher modulus core to fracture repeatedly, increasing the energy dissipated during extension. Normally, fractures are catastrophic. However, in this architecture, the fractures are localized to the core. In addition to dissipating energy, the metallic core provides electrical conductivity and enables repair of the fractured core for repeated use. The fibers are 2.5 times tougher than titin and hold more than 15,000 times their own weight for a period 100 times longer than a hollow elastomeric fiber.

Patterning and Reversible Actuation of Liquid Gallium Alloys by Preventing Adhesion on Rough Surfaces.

This work reports a simple approach to form, study, and utilize rough coatings that prevent the adhesion of gallium-based liquid metal alloys. Typically, liquids with large interfacial tension do not wet nonreactive surfaces, regardless of surface topography. However, these alloys form a surface oxide "skin" that adheres to many substrates, even those with low surface energy. This work reports a simple approach to render closed channels and surfaces, including soft materials, to be "oxide-phobic" via spray-coating (NeverWet, which is commercially available and inexpensive). Surface spectroscopic techniques and metrology tools elucidate the coatings to comprise silica nanoparticles grafted with silicones that exhibit dual length scales of roughness. Although prior work shows the importance of surface roughness in preventing adhesion, the present work confirms that both hydrophobic and hydrophilic rough surfaces prevent oxide adhesion. Furthermore, the coating enables reversible actuation through submillimeter closed channels to form a reconfigurable antenna in the gigahertz range without the need for corrosive acids or bases that remove the oxide. In addition, the coating enables open surface patterning of conductive traces of liquid metal. This shows it is possible to actuate liquid metals in air without leaving neither metal nor oxide residue on surfaces to enable reconfigurable electronics, microfluidics, and soft electrodes.

Electrically reconfigurable terahertz signal processing devices using liquid metal components.

Many applications of terahertz (THz) technology require the ability to actively manipulate a free space THz beam. Yet, although there have been many reports on the development of devices for THz signal processing, few of these include the possibility of electrical control of the functionality, and novel ideas are needed for active and reconfigurable THz devices. Here, we introduce a new approach, based on the integration of electrically actuated liquid metal components in THz waveguides. This versatile platform offers many possibilities for control of THz spectral content, wave fron"ts, polarization, and power flow. We demonstrate two illustrative examples: the first active power-splitting switch, and the first channel add-drop filter. We show that both of these devices can be used to electrically switch THz communication signals while preserving the information in a high bit-rate-modulated data stream.

Correction: Electrowetting without external voltage using paint-on electrodes.

Correction for 'Electrowetting without external voltage using paint-on electrodes' by Collin B. Eaker et al., Lab Chip, 2017, DOI: .

Electrowetting without external voltage using paint-on electrodes.

Electrowetting uses voltage to manipulate small volumes of fluid for applications including lab-on-a-chip and optical devices. To avoid electrochemical reactions, a dielectric often separates the fluid from the electrode, which has the undesired effect of adding processing steps while increasing the voltage necessary for electrowetting. We present a new method to dramatically reduce the complexity of electrode and dielectric fabrication while enabling multiple performance advances. This method relies on a self-oxidizing paint-on liquid-metal electrode that can be fabricated in minutes on rigid, rough, or even elastic substrates, enabling low operation voltages (<1 V), and self-healing upon dielectric breakdown. Furthermore, due to the non-negligible 'potential of zero charge', electrowetting occurs by simply short circuiting the electrodes. This work opens up new application spaces for electrowetting (e.g. stretchable substrates, soft and injectable electrodes) while achieving large changes in contact angle without the need for an external power supply.

Liquid-Metal Microdroplets Formed Dynamically with Electrical Control of Size and Rate.

Liquid metal co-injected with electrolyte through a microfluidic flow-focusing orifice forms droplets with diameters and production frequencies controlled in real time by voltage. Applying voltage to the liquid metal controls the interfacial tension via a combination of electrochemistry and electrocapillarity. This simple and effective method can instantaneously tune the size of the microdroplets, which has applications in composites, catalysts, and microsystems.

Steering liquid metal flow in microchannels using low voltages.

Liquid metals based on gallium, such as eutectic gallium indium (EGaIn) and Galinstan, have been integrated as static components in microfluidic systems for a wide range of applications including soft electrodes, pumps, and stretchable electronics. However, there is also a possibility to continuously pump liquid metal into microchannels to create shape reconfigurable metallic structures. Enabling this concept necessitates a simple method to control dynamically the path the metal takes through branched microchannels with multiple outlets. This paper demonstrates a novel method for controlling the directional flow of EGaIn liquid metal in complex microfluidic networks by simply applying a low voltage to the metal. According to the polarity of the voltage applied between the inlet and an outlet, two distinct mechanisms can occur. The voltage can lower the interfacial tension of the metal via electrocapillarity to facilitate the flow of the metal towards outlets containing counter electrodes. Alternatively, the voltage can drive surface oxidation of the metal to form a mechanical impediment that redirects the movement of the metal towards alternative pathways. Thus, the method can be employed like a 'valve' to direct the pathway chosen by the metal without mechanical moving parts. The paper elucidates the operating mechanisms of this valving system and demonstrates proof-of-concept control over the flow of liquid metal towards single or multiple directions simultaneously. This method provides a simple route to direct the flow of liquid metal for applications in microfluidics, optics, electronics, and microelectromechanical systems.