A Novel Monolithic 3D Printed Axisymmetric Co-Flow Single and Double Emulsion Generator

Droplet microfluidics has been attracting interests of users in diverse fields of study such as cosmetics, pharmaceutical, and food industries due to its versatility in applications.  This vast interest in droplet based microfluidics is rooted in its specific capabilities to encapsulate biological and chemical reagents inside tiny amount of fluids, offering precise control over specific processes by considering efficiency of mass and heat transfer. In addition, droplets can play a significant role in an emerging field of opto-microfluidics. The softness of the droplet surfaces offers a unique structure for trapping light into a so-called Whispering Gallery Mode (WGM). In such a mode, the droplets serve as soft resonators, exhibiting unique optomechanical properties. Over the years, producing a device which is easy-to-use, cost-effective, and with sub-millimeter-droplet generation ability has always been a challenge among innovators in microfluidics.  In our study, we have designed and fabricated a novel droplet generator device which can produce single droplets, and single/multiple droplets in a droplet by implementing monolithic 3D axisymmetric co-flow structure. We used a recent fabrication approach in microfluidics field, additive manufacturing, to fabricate the droplet generator. A commercial, low-cost Stereolithography 3D printer, which is able to offer acceptable transparency, small channel size, and high resolution of printing, produced this device. The device is user-friendly, and any inexpert person can conveniently utilize it. The design of the device is in a “Plug-and-Play” manner, which facilitates the connecting process of tubes to the device, overcoming a traditional issue of microfluidic devices which is fluid leakage. We took deionized water and mineral oil as popular immiscible fluids and tried different combination of generating emulsions, Water in Oil (W/O), Oil in Water (O/W), Oil in Water in Oil (O/W/O), and Water in Oil in Water (W/O/W), and dissolved Fluorescein Sodium Salt in the inner-most fluid to enhance visibility of double emulsion generation and easing the differentiation of  single emulsions from double emulsions with bigger inner core. We also investigated the impacts of change in flow rate of each immiscible fluid, which was used as inner, middle, or outer fluid, in the droplet (emulsion) generation. Also, we evaluated the size of emulsions which was influenced by flow rates and we extended our research to study possibility of generating complex droplet structures involving multiple droplets encapsulated in one outer droplet. We will further extend our work to study novel structures, such as solid shells emulsions. Lastly, computational modeling of emulsification using phase field method has been performed to understand the fluid dynamics of the emulsion generation process. Overall, by using our novel 3D printed monolithic co-flow droplet generator device, generating single emulsion, monodispersed double emulsion, and multiple complex emulsions is now easier than traditional approaches and the device can be readily applicable in industry for many applications.