Stretchable Liquid Metal Films with High Surface Area and Strain Invariant Resistance

Abstract

In this work, we sought to create stretchable LM electrodes with high surface area, but ended up observing an additional useful property: strain-invariant resistance. Electrodes with high surface area are useful for capacitors, [15] sensors, [16] and certain energy-harvesting devices. [17] Due to surface tension, liquids naturally form spherical or hemispherical shapes, which minimize both surface energy and surface area. Gallium-based liquid metals form a native oxide 'skin' that allows LM to be shaped and patterned into nonspherical shapes. For example, LM can be stencil printed, 3D printed, or injected into microfluidic channels to form nonspherical shapes. [18] Although these approaches increase the surface area per volume of the metal relative to a sphere, they only do so modestly.The area of LM can be drastically increased by breaking it into small droplets (the diameter of which depends on the conditions, but sizes can range from 10 nm to hundreds of microns). [19] The presence of this native oxide skin on the surface allows for the formation of stable liquid metal particles that do not spontaneously coalesce or merge together when the diameters of the particles are sufficiently small. Consequently, this oxide layer causes films of liquid metal particles (LMPs) to be electrically insulating. Yet, removing the oxide layer would cause the particles to coalesce into larger droplets. Thus, it is challenging to create high surface area, conductive liquid metal films.Several approaches have been reported in the literature to percolate droplets of liquid metal within a film to create conductive pathways. Perhaps the simplest approach is to 'mechanically sinter' the particles, which involves physically pressing films of droplets together. [20] Mechanical sintering can be applied to encased [1] or exposed [20] films of particles or to particles dispersed in elastomer [21] (so-called liquid metal elastomers). Peeling films of liquid metal elastomers can also generate sufficient stress to percolate particles. [22] Particles can also be percolated using laser heating [1,23] by capillary forces, [24] or by using dielectrophoretic forces. [25] Particles covalently linked together can sinter when placed on a stretchable substrate; the stress from elongation causes the particles to rupture and merge. [26] Several of these approaches result in percolated conductors with resistance that is nearly invariant with strain.Inspired by this prior work, [26] we speculated that we might achieve a similar result by placing particles without any covalent linkages onto a tacky, stretchable substrate, and utilize This paper reports soft and stretchable films of liquid metal particles (LMPs) that offer high surface area and high conductivity. Liquid metals (LM) based on gallium are compelling conductors for soft and stretchable devices, yet it is difficult to make electrodes of LM with high surface area due to the tendency of liquids to minimize their surface area. To form films of percolated particles with high surface area, LMPs are f...

Thiol Coordination Softens Liquid Metal Particles To Improve On-Demand Conductivity

“…The mechanical sensitivity of EGaIn particles formed in alcohol can be improved by coordinating surfactants like silanes and thiols while the particle network is strained or pressure-sintered. ,,− In these same studies, electrical resistance measurements corroborate the bulk mechanical actuation. For instance, the thiolation of the EGaIn particles in ethanol leads to a ∼50% reduction in the pressure needed to rupture a bed or film of the liquid metal capsules .…”

“…35 In another study, instead of treating EGaIn particles with acid after sonication, trace amounts of acid are introduced during sonication and the resulting particle films display high conductivity while maintaining a high surface area. 36 These two studies suggest that the addition of acid, whether during or after particle formation, is sufficient to remove the acid layer and allow for thiol complexation with the liquid metal itself. Interestingly, our experimental method, in which acid is introduced immediately before sonication, corroborates that the introduction of acid removes the oxide.…”

“…24 More recently, stretchable EGaIn particle films prepared by thiolation after acidification in isopropanol have also shown tremendous electrical response at low and high strain percentages. 36 Probing the response of individual particles instead of bulk clusters may provide the resolution needed to observe the core− shell mechanical properties. By using atomic force microscopy (AFM), Lear and co-workers uncover that the force required to rupture individual thiolated EGaIn nanoparticles is up to 45% greater than unfunctionalized EGaIn prepared in ethanol.…”

Highly Stable Liquid Metal Conductors with Superior Electrical Stability and Tough Interface Bonding for Stretchable Electronics

“…However, for the sample by direct writing, the interface is a remarkable plane between the LM and the substrate (Figure b). The presence of the oxide layer tends to adhere to many different substrate surfaces, resulting in solid–solid contact between the LM and the solid substrate and providing strong adhesion. , Additionally, the area of LM could be significantly increased by breaking it into small droplets . After the polymer is poured onto the LMPs, more oxide layers are in contact with the polymer substrate, thereby enhancing the adhesive strength by increasing the contact area between the LM and the polymer …”

“…34,35 Additionally, the area of LM could be significantly increased by breaking it into small droplets. 36 After the polymer is poured onto the LMPs, more oxide layers are in contact with the polymer substrate, thereby enhancing the adhesive strength by increasing the contact area between the LM and the polymer. 13 Figure 3c shows a fluid− solid interfacial adhesive force−distance curve when a LM droplet is separated from the surface of the embedded structure (red line) and the on-surface structure (blue line), respectively.…”