Micro-scale liquid metal droplets have been hailed as the potential key building blocks of future micro-electro-mechanical systems (MEMS). However, most of the current liquid metal enabled systems involve millimeter-scale droplets, which are manually injected onto the desired locations of the system. Despite simplicity, this method is impractical for patterning large arrays or complex systems based on micro-scale droplets. Techniques that can generate and process uniform liquid metal droplets on demand would be greatly preferred. The purpose of my PhD research is to develop microfluidic systems capable of continuous generation of micro scale liquid metal droplets in highly viscous liquids such as glycerol, and transfer them into secondary liquids such as sodium hydroxide (NaOH) hydrodynamically. A comprehensive set of experiments utilising high-speed imaging and computational fluid dynamics (CFD) simulations are conducted to investigate the dynamics of liquid metal droplets with single and double liquid-liquid interfaces while transitioning into the secondary liquids. The results indicate that the transition of continuously generated liquid metal droplets from glycerol into NaOH occur under the combined effect of hydrodynamic lift force (due to different viscosities of glycerol and NaOH) and surface tension gradient force (due to different interfacial tensions between the droplet and the liquids). Transition of droplets is quite ordered and predictable in the presence of a single liquid-liquid interface unlike the system with double liquid-liquid interface where the dynamics of droplets depends on the shape of the glycerol core (which in turn depend on the flow rate of the NaOH), and the transition can become disordered, semi-ordered or ordered. Moreover, the direction of transitioning droplets can be hydrodynamically controlled by mismatching the flow rates of the two NaOH streams in a double liquid-liquid interface system. Minimum flow rate mismatch of 150 µl/min between the two NaOH streams is required to change the direction of the droplets where droplets transit into the stream with higher flow rate. This platform offers continuous and selective hydrodynamic transfer of micro scale Galinstan droplets into NaOH stream, which can be integrated into other microfluidic platforms to enable liquid metal droplet based systems for a variety of applications in microfluidics, MEMS, soft electronics and reconfigurable devices.
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