Reliable Droplet Formation by controlling oxide layer of Galinstan Liquid Metal in T-junction channels*

Microscale liquid metal droplets are considered to be potential foundational elements for future micro-electro-mechanical systems (MEMS), soft, flexible, and stretchable versions of electrodes, interconnects, and antennas. To precisely control their volume and generation frequency, T-junction channels have been widely used for producing monodispersed droplets by employing continuous and dispersed phases. However, the presence of the formation of an oxide layer on Galinstan can hamper precise size and frequency control when liquid metal is used as the dispersed phase. In high oxygen concentration fluid systems, especially for the continuous phase, the formation of an oxide layer causes liquid droplets to elongate or form tails, leading to unpredictable droplet sizes and oxide debris in the channels. Our research addresses this issue by utilizing low oxygen concentration fluid systems to prevent Galinstan oxide layer formation. Maintaining a low oxygen environment allows for oxide-layer-free liquid metal droplets. We achieved precise control over droplet size and formation frequency through the manipulation of capillary number and flow rate ratio. We also obtained a simple scaling relation to predict the droplet size depending on the flow rate ratio. Additionally, we consider potential applications for hydrodynamic actuation of liquid metal droplets, suggesting the possibility of using droplets as electrodes within dielectric and low oxygen concentration fluid systems.