Generation and Analysis of Axiolitic Liquid-Metal Droplets in a T-Junction Microfluidic Device

Liquid metals like the eutectic gallium-indium (EGaIn) alloys are fantastic materials for producing micro-droplets. In this work, polyethylene glycol (PEG) and deionized (DI) water were chosen as the continuous phase, and EGaIn is the dispersed phase to generate spherical and non-spherical (axiolitic) droplets in a T-junction microfluidic device. Especially for axiolitic droplets, a systematic investigation on the relation of their sizes and flow features was done. A general formula as a function of both the flow rate Q(c) of the continuous phase and the flow-rate ratio R(Q(c)/Q(d)) is given to fit all the experimental data. Furthermore, effects of the viscosity of the continuous phase on the droplet size, size distribution and hydrodynamic regimes are also discussed. When the viscosity gets higher, the general droplet size gets smaller, the zone for generating axiolitic droplets becomes narrower, and the size distribution of the droplets is relatively worse. These are all because the correlation among the viscosity, the oxide layer on the droplet interface and the droplet morphology. The higher the viscosity is, the thinner the oxide layer is, and the less mechanical stability the droplets would have. The results of axiolitic droplets of liquid metals have wide potential applications in microfluidics, electronics, thermal probe and so on, since the axiolitic droplets are anisotropic compared to the spherical.