Droplet array is widely applied in single cell analysis, drug screening, protein crystallization, etc. This work proposes and validates a method for rapid formation of uniform droplet array based on microwell confined droplets electro-coalescence of screen-printed emulsion droplets, namely electro-coalescence droplet array (ECDA). The electro-coalescence of droplets is according to the polarization induced electrostatic and dielectrophoretic forces, and the dielectrowetting effect. The photolithographically fabricated microwells are highly regular and reproducible, ensuring identical volume and physical confinement to achieve uniform droplet array, and meanwhile the microwell isolation protects the paired water droplets from further fusion and broadens its feasibility to different fluidic systems. Under optimized conditions, a droplet array with an average diameter of 85 µm and a throughput of 10 6 in a 10 cm × 10 cm chip can be achieved within 5 s at 120 Vpp and 50 kHz. This ECDA chip is validated for various microwell geometries and functional materials. The optimized ECDA are successfully applied for digital viable bacteria counting, showing comparable results to the plate culture counting. Such an ECDA chip, as a digitizable and high-throughput platform, presents excellent potential for high-throughput screening, analysis, absolute quantification, etc.
Xiaoming Luo,Haoran Yin,Haipeng Yan,Xin Huang,Donghai Yang,Limin He
In this study, the electrocoalescence characteristic of two drops with different surfactant concentrations suspended in oil is revealed. Different from the symmetrical coalescence of identical drops, the asymmetrical liquid bridge is induced by the capillary force, which involves the interfacial tension gradient between the drops. In addition, a mushroom-like jet is formed during asymmetrical coalescence, causing mixing of drops. A characteristic time considering the interfacial tension gradient is proposed to describe the asymmetrical liquid bridge evolution. The mushroom-like jet is found to be driven by the pressure difference between drops, which has a positive correlation with the interfacial tension gradient. Moreover, it is found that the electrocoalescence time is shortened as the interfacial tension increases. This study indicates that increasing the interfacial tension gradient will enhance the jet flow rate and reduce the coalescence time, which is of significance to improve the mixing efficiency of the drops.
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