A micro-needle induced strategy for preparation of monodisperse liquid metal droplets in glass capillary microfluidics

Abstract

Monodisperse micro-sized liquid metal droplets have received considerable attention for developing flexible electronics, microfluidics actuators and reconfigurable devices. Herein we report an innovative and efficient strategy for large-scale preparation of Galinstan liquid metal microdroplets with controllable sizes using a micro-needle induced glass capillary microfluidic device. By inserting a stainless steel micro-needle into the inner liquid metal phase in the glass capillary, the hydrodynamic instability of the liquid metal stream is significantly suppressed to guarantee steady fluid flow before the liquid metal is pinched off by the outer phase flow, giving rise to a stable generation of monodisperse liquid metal microdroplets. The microdroplet size dependence on the flow ratio of the continuous and dispersed-phase is investigated experimentally. A theoretical framework based on the Plateau-Rayleigh instability is proposed to explain the advantage of the micro-needle induced strategy. This strategy has great potential for the generation of high interfacial tension liquid metal emulsions.

Numerical Simulation and Experimental Validation of Liquid Metal Droplet Formation in a Co-Flowing Capillary Microfluidic Device

“…The weight ratio of glycerol and 5% PVA aqueous solution was 10:2.9. Therefore, the dynamic viscosity for the two-phase fluids were defined as 0.044 Pa•s and 0.002 Pa•s at room temperature [31], respectively. The interfacial tension between the liquid metal and the glycerol aqueous solution was 0.534 N/m [28].…”

“…The two inner capillaries were concentric aligned with axial spacing of 100 µm, and the metallic needle was inserted into the inlet capillary along the axis of the orifice, with a transverse distance between the micro-needle tip and the center of the capillary orifice of 175 µm. The detailed dimensions and fabrication processes of the micro-needle induced co-flowing glass capillary microfluidic device could be obtained in our previous published paper [31]. The glycerol and liquid metal were supplies separately by two microsyringe pumps (Harvard Apparatus, Holliston, MA, USA) equipped with gastight precision glass syringes (Hamilton, Bonaduz, Switzerland).…”

“…Microfluidic technology provides fast, high-throughput and better control over the droplet size distribution, which are useful and highly potential in various applications in many fields such as biochemical and materials science [23][24][25]. Typically, 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]. When the two neighboring immiscible fluids coming across a small orifice in the microchannel, the flow-focusing based microfluidic chip split the bulk liquid metal into microspheres with the continuous phase fluid pinching off the dispersed phase liquid metal.…”

“…Meanwhile, the strategy of placing wire inside the inlet microchannel was introduced to improve the droplet uniformity and provide better control over the droplet size [35][36][37]. To further improve the monodispersity of generated liquid metal microspheres, we therefore 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 steel micro-needle was inserted into the inner liquid metal phase in the glass capillary [31]. The experimental investigation showed that highly uniform liquid metal microdroplets can be obtained.…”

Lightweight Liquid Metal Entity

“…So far, there still lacks a generalized way to make as desired liquid metal based materials with even lighter density according to various needs. Fortunately, recent research has found that the existence of the oxide is of great help on the combination of eGaIn with other materials, especially silica, [ 40,41 ] metal particles, [ 42,43 ] and polymers. [ 44,45 ] Therefore, we found that the promising answer might lie in the development of composite materials.…”