An overview of Ga-based liquid metal research at NeptunLab: from microactuators to heavy metal ion sensors (Invited Paper)

Gallium-based liquid metals are interesting materials due to their high surface tensions, low viscosities, and high electrical conductivities comparable to classical solid metals. They have been used for several applications in microelectromechanical systems (MEMS) and liquid metal microfluidics (LMMF) including micro-actuators and sensors. However, when it comes to the electrochemical actuation of LMs, in particular Galinstan, two main challenges must be tackled: 1) Generation of reproducible Galinstan plugs with controllable size and interspacing, and 2) Tendency to alloy with the most common metals used for electrodes such as gold (Au), platinum (Pt), titanium (Ti), nickel (Ni), and tungsten–titanium (WTi). At NeptunLab, we designed an on-chip liquid metal plug generator for generating reproducible and scalable sub-millimeter plugs. Using constricted channels based on Laplace pressure, we generated reproducible plugs in terms of size and interspacing compared to pulse width modulation (PWM)-based generation in straight channels. To avoid the second challenge, i.e., alloying, we carried out in situ electrodeposition of electrically conductive polypyrrole (PPy) on the electrodes to provide a conductive alloying-barrier layer between Galinstan and electrodes. While Galinstan alloying was a challenge in contact with electrodes, its amalgamation with heavy metal ions (HMI) like lead was an advantage for developing HMI sensors. Common droplet-based LM HMI sensors showed limitations in scalability and the homogeneity of the surface. To overcome these problems, we introduced tungsten oxide (WO)-Galinstan electrodes fabricated via photolithography and galvanic replacement (GR) in a tungsten salt solution. The in-house fabricated electrodes demonstrated enhanced sensitivity in comparison with electrodes structured from pure Galinstan and detected lead at concentrations down to 0.1 mmol·L−1. Further details of this abstract in gallium-based liquid metal microfluidics for applications in microactuators and sensors will be discussed in the presentation.