Self-sustained microdroplet generation using 3D-printed fluidic cell

Liquid-walled microdroplet-based optical sensors have great potential for applications that require minimal samples (e.g., a few microliters) while maintaining quality factors greater than 108. However, their practical use has been constrained by evaporation over time. To overcome this limitation, we developed a 3D-printed fluidic cell capable of reliably generating microdroplets on plasma-treated microspheres fabricated on silica posts. The silica post is secured in a custom-built fiber holder extended through the fluid reservoir. A high voltage source is used to generate plasma that runs through the microsphere and silica post, making their surfaces hydrophilic. After plasma treatment, a lower voltage is applied to electrostatically pulls fluid over the silica post from the reservoir to the microsphere, forming a nanoscale liquid bridge. The fluid turns into a droplet on the microsphere as it experiences a high surface tension. Once a droplet is formed, it can survive until the reservoir is depleted, even when the voltage source is turned off, as the hydrophilic nature of the microsphere and silica post facilitates a continuous liquid flow. Additionally, the silica post that holds the droplet in air extends a few hundred micrometers from the reservoir, supporting both free-space and fiber-based light coupling. We anticipate that this technique of generating repeatable and self-sustained microdroplets will find numerous applications in biosensing and chemical reaction kinetics.