Description
In droplet microfluidics two immiscible fluids, usually an aqueous phase and an oil phase, are brought together at specially designed channel junctions to generate controlled emulsions. For biological applications, the emulsions consist of water-in-oil droplets stabilized by surfactants. Each droplet acts as an isolated microreactor, allowing the stable separation of chemicals, biomolecules and cells, while simultaneously reducing reagent consumption and enabling higher-throughput experimentation compared to bulk methods [1]. Despite these advantages, droplet-based microfluidic technologies remain limited in their ability to scale up droplet production to meet the demands of biotechnological and biomedical applications. Current systems typically achieve droplet generation rates on the order of 12–15 kHz [2, 3], whereas many applications require throughputs in the MHz range [4].
The most commonly employed droplet generation strategies are shear-based and include T-junction, co-flow, and flow-focusing geometries. In contrast, droplet generation via step emulsification relies on Rayleigh–Plateau instability: droplets form as the dispersed phase flows through a long, shallow microchannel and subsequently expands into a deeper and wider reservoir [5]. This approach represents a promising solution, as it enables highly monodisperse and automated droplet production with significantly lower material consumption compared to shear-based techniques.
In this work, we present an exploratory study of step emulsification for high-throughput droplet generation. A microfluidic device was designed and fabricated using a combination of multi-layer photolithography, soft lithography and 3D printing. The device incorporates 60 nozzles to produce droplets in parallel. Water-in-oil emulsions were generated under controlled flow conditions and device performance was characterized by measuring droplet diameter and production rate.
Consistent with previous studies on step emulsification [6], droplet diameter is primarily governed by nozzle geometry and remains largely independent of the flow rate over the investigated range. The generated droplets exhibit excellent monodispersity, with a coefficient of variation below 3%, and demonstrate stable formation across operating conditions. These results highlight the robustness and scalability of step emulsification for producing uniform droplets in high-throughput biological applications that require to analyse large volumes of droplets efficiently.
[1] Helen Song, Delai L. Chen, and Rustem F. Ismagilov. “Reactions in droplets in microfluidic channels”. In: Angewandte chemie international edition 45.44 (2006), pp. 7336–7356.
[2] Levent Yobas et al. “High-performance flow-focusing geometry for spontaneous generation of monodispersed droplets”. In: Lab on a Chip 6.8 (2006), pp. 1073–1079.
[3] Xiaonan Xu et al. “High aspect ratio induced spontaneous generation of monodisperse picolitre droplets for digital PCR”. In: Biomicrofluidics 12.1 (2018).
[4] Christian Holtze. “Large-scale droplet production in microfluidic devices – an industrial perspective”. In: Journal of Physics D: Applied Physics 46.11 (2013), p. 114008
[5] Dangla, Rémi, et al. "The physical mechanisms of step emulsification." Journal of Physics D: Applied Physics 46.11 (2013): 114003.
[6] Zhi Shi et al. “Step emulsification in microfluidic droplet generation: mechanisms and structures”. In: Chemical Communications 56.64 (2020), pp. 9056–9066.