Shear‐Driven Direct‐Write Printing of Room‐Temperature Gallium‐Based Liquid Metal Alloys

Abstract

Gallium-based metal alloys have high electrical conductivity in the liquid state at room temperature. These liquid metal conductors inspire unique electronic applications such as reconfigurable circuits and stretchable components with extremely high strain tolerance. Previously, liquid metals have been successfully patterned via direct-writing, yielding metallically conductive features on-demand at room temperature that do not require post-processing, down to a resolution of %10 μm. While most direct-write processes extrude materials from a nozzle via pressure or volumetric displacement, liquid metal is instead printed here by a shear-driven mechanism that occurs when the oxide-coated meniscus of the metal adheres to the printing substrate and is "pulled" from the nozzle at pressures that are well-below that needed to extrude the metal in the absence of shear. Herein, the key operating parameters that enable shear-driven printing of liquid metals including dispensing pressure, choice of substrate, print height, the surrounding environmental conditions, and the speed and acceleration of the print head are elucidated. A guide to the best practices as well as limitations for implementing shear-driven printing of liquid metals at room temperature is provided in these studies.

Liquid Metal Direct Write and 3D Printing: A Review

“…Such a test is necessary since conventional contact angle measurements are complicated by the fact liquid metals form a solid-oxide on their surface. [56] The strategy is to form a liquid metal meniscus at the tip of a nozzle and then repeatedly contact the surface of the substrate in question in ≈100 different places. A "good" substrate will form a small cone or "peck" of liquid metal on the surface after each contact (for a total of ≈100 cones), while a "bad" substrate will leave behind little or no metal on the surface due to poor adhesion.…”

“…The first approach is shear driven direct writing. [56] As shown in Equation 1, the pressure can be controlled by applying pressure (or vacuum) in the "head space" above the metal in the nozzle, combined with the hydrostatic pressure from the height of metal in the nozzle. In principle, the pressure could also be controlled using Lorentz force (running current through the metal while applying a large magnetic field normal to the nozzle).…”

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

“…[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. For example, Datta et al applied the squeeze-printing process to deposit the thin-film liquid metal.…”

Electrowetting‐Assisted Selective Printing of Liquid Metal

“…Factors such as substrate surface energy, roughness, and adsorbed water from ambient humidity have drastic effects on this interfacial adhesion and therefore the printability of liquid metal. Additionally, even when printing on an ideal substrate, the printing parameters that determine the trace dimensions and print probability (e.g., print height, pressure, tip diameter) are not independent, oftentimes making it difficult to achieve an optimized print geometry …”