Interfacial Rheology of Gallium-Based Liquid Metals

Abstract

Gallium and its alloys react with oxygen to form a native oxide that encapsulates the liquid metal with a solid "skin". The viscoelasticity of this skin is leveraged in applications such as soft electronics, 3D printing, and components for microfluidic devices. In these applications, rheological characterization of the oxide skin is paramount for understanding and controlling liquid metals. Here, we provide a direct comparison of the viscoelastic properties for gallium-based liquid metals and illustrate the effect of different subphases and addition of a dopant on the elastic nature of the oxide skin. The du Nouÿ ring method is used to investigate the interfacial rheology of oxide skins formed by galliumbased liquid metal alloys. The results show that the oxide layer on gallium, eutectic gallium−indium, and Galinstan are viscoelastic with a yield stress. Furthermore, the storage modulus of the oxide layer is affected by exposure to water or when small amounts of aluminum dopant are added to the liquid metals. The former scenario decreases the interfacial storage modulus of the gallium by 35−85% while the latter increases the interfacial storage modulus by 25−45%. The presence of water also changes the chemical composition of the oxide skin. Scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy suggest that a microstructural evolution of the interface occurs when aluminum preferentially migrates from the bulk to the surface. These studies provide guidance on selecting liquid metals as well as simple methods to optimize their rheological behavior for future applications.

A general approach to composites containing nonmetallic fillers and liquid gallium

“…Furthermore, we could not mix large particles when they had hierarchical surface features; mesoporous silica and activated alumina with particle sizes up to 200 m were nonmixable; however, regular, nonporous powdered silica is mixable. We It has been observed that when the volume of liquid gallium is small, the behavior of the oxide skin dominates the rheological properties (5,40), showing viscoelastic behavior with a high yield stress (41). We thus propose that GalPs form as shown schematically in figs.…”

Liquid Metal Direct Write and 3D Printing: A Review

“…It also avoids the need for shear when printing from a nozzle. As noted previously, liquid metals are low viscosity fluids, having a viscosity on the order of ≈2mPa s. [24,26,81] This should let them be easily aerosolized or jetted, however the extremely large surface tension and presence of a surface oxide complicate using liquid metal in droplet dispensing systems. In brief, it limits the nozzle size.…”

“…In fact, this behavior is seen with mercury, which does not rapidly form a native surface oxide, [24] or with EGaIn if the oxide layer is removed. [25] The oxide skin will yield beyond a critical surface stress (≈0.3-0.5 N m −1 ) [5,24,26,27] but otherwise is stable and prevents the metal from flowing in a way that increases area. Similar to surface tension, the pressure required to rupture the surface is simply the critical surface stress multiplied by the curvature of the surface (2/R in the case of a spherical drop, where R is the radius of the drop).…”

“…Bulk liquid metal is a low viscosity (water-like), Newtonian fluid which only obtains a non-Newtonian rheology due to the presence of the solid native oxide on its surface. [24,26,48,49] The low viscosity metal can flow readily within its oxide. However, printing the metal requires dispensing it from a nozzle in a way that increases the surface area, which means the oxide must rupture (and rapidly reform) during printing.…”

Ink‐Based Additive Nanomanufacturing of Functional Materials for Human‐Integrated Smart Wearables

“…[319][320][321] Liquid metals usually have high surface tensions, and an oxide layer occurs on the surface of liquid metal droplets once it is exposed to air. [322,323] DIW and spray coating have been applied to deposit liquid metal electrodes. [324][325][326][327][328][329][330] In addition to the noncontact methods, liquid metal can also be patterned by the contact method.…”