Flexible Microfluidic Devices for Tunable Formation of Double Emulsion.

Double emulsions are highly structured dispersion systems that generate double-layered droplets. Double emulsions offer an effective platform for encapsulating liquid samples. Multilayer protection, controlled release of encapsulated materials, and stability make double emulsions superior to single emulsions in handling sensitive liquid samples. This technology is widely used in biology, food technology, cosmetics, and environmental sciences. Microfluidic emulsification is a promising method for producing highly monodisperse double-emulsion droplets with a high encapsulation efficiency. Well-controlled adjustment of the core size and shell thickness is critical for applications of double emulsions. Changing the flow rates of the fluid phases is the most straightforward method to control the emulsion sizes. However, monodisperse double-emulsions can only be generated within a small range of flow rates. Thus, producing monodisperse double emulsions with a wide size range without changing the device design or drastically altering the fluid properties is challenging. Here, we demonstrate a facile method to generate monodisperse double-emulsion droplets with tunable core size and shell thickness without changing the flow rates of the fluid phases. To address this challenge, we developed a proof-of-concept flexible and stretchable microfluidic device capable of controlling core size, shell thickness, and generation frequency by adjusting channel dimensions and stretching the microfluidic device. We incorporated three stretching cases to assess the feasibility of controlling the generation process of the double emulsion. We demonstrated that stretching increases the core size and shell thickness and decreases the generation frequency. Experimental results showed an ∼84% increase in core volume and an ∼23% increase in shell volume by applying ∼16% device strain. This innovative approach significantly advances the field of droplet-based microfluidics, providing on-site, real-time tunability for the generation of double-emulsion droplets with high precision and reproducibility.