Abstract
A two-phase flow axisymmetric numerical model was proposed to understand liquid metal droplet formation in a co-flowing capillary microfluidics device based on a phase field model. The droplet detachment processes were observed in the experiment and are in good agreement with the simulation method. The effects of the viscosities and flowrates of the continuous phase fluid, interfacial tension as well as the wetting property of the metallic needle against the bulk liquid metal on the droplet formation and production rate were numerically investigated. It was found that the droplet diameter decreased with the increment of the viscosities and flowrates of the outer phase carrier fluid. The dispersed phase fluid with high interfacial tension tended to prolong the time for equilibrium between the viscous drag force and interfacial tension on the liquid–liquid fluid surface, delaying the droplet to be pinched off from the capillary orifice and causing large droplet diameter. Finally, the wetting performance of the metallic needle against the liquid metal was explored. The result indicate that the droplet diameter became less dependent on the contact angle while the size distribution of the liquid metal droplet was affected by their wetting performance. A more hydrophilic wetting performance were expected to prepare liquid metal droplet with more monodispersity. The numerical model and simulation results provide the feasibility of predicting the droplet formation with a high surface tension in a glass capillary microfluidic device.
Highlights
With so many extraordinary physical and chemical properties, such as low melting point, low vapor pressure, high electrical/thermal conductivity and high surface tension, the liquid metals are very useful and highly potential in soft and stretchable electronics [1], heat transfer management devices [2] and functional composites [3], e.g., ultra-soft and compliant electrodes [4], interconnects [5], electrochemical sensors [6], three-dimensional printing [7,8], smart actuators and shape-memory alloys [9]
Liquid metal microdroplets with a symmetrical spatial structure are useful in developing novel microfluidics engineering devices, advanced functional electronics
Flow-focusing and co-flowing droplet formation mechanism-based droplet microfluidics have been developed for the preparation of uniform-sized liquid metal microspheres [26,27,28,29,30,31,32,33]
Summary
With so many extraordinary physical and chemical properties, such as low melting point, low vapor pressure, high electrical/thermal conductivity and high surface tension, the liquid metals are very useful and highly potential in soft and stretchable electronics [1], heat transfer management devices [2] and functional composites [3], e.g., ultra-soft and compliant electrodes [4], interconnects [5], electrochemical sensors [6], three-dimensional printing [7,8], smart actuators and shape-memory alloys [9]. Dickey et al experimentally studied the formation of galinstan liquid metal microdroplets with high viscosity continuous phase fluid pinching off the bulk liquid metal They investigated the influence of flowrates ratios, outer continuous phase fluid viscosities, chip geometries, and interfacial tension on the droplet size [30]. Flow-focusing-based droplet technology can produce high interfacial tension liquid metal microdroplets with a relative high monodispersity, the fabrication procedures of commonly used polydimethylsiloxane (PDMS) microchannel [34] are complicated and the continuous phase organic solvents may sometimes inflate the soft PDMS microchannel, which may influence the droplet size distribution when the flowrates of the injecting flows are relatively high. To further improve the monodispersity of generated liquid metal microspheres, we previously put forward a micro-needle induced strategy for the fabrication of liquid metal droplets in a co-flowing capillary microfluidic device in which a stainless. The effects of the interfacial tension, wetting properties of the micro-needle, and the viscosities and flowrate of the continuous phases on the droplet size were systematically studied
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