Micromixing strategy for under-oil open sessile droplets driven by Faraday waves

Under-oil open sessile droplets (UOSD), a microfluidic system in which aqueous droplets are confined by an immiscible oil layer, integrate the inherent advantages of both open and closed systems. These include free accessibility, high flexibility, and allowance for gas exchange, while preventing evaporation and contamination, making them ideal for separated biological and chemical reaction units. However, the overlay oil barrier, though effective in protecting droplets from external interference, also poses challenges for micromixing within the droplets. Developing an efficient micromixing strategy for UOSD is therefore essential, to overcome the slow diffusion limit. In this study, we present a contactless and highly efficient micromixing strategy for this immiscible phase system. Mixing in our proposed approach is induced by generating Faraday waves at the oil-air interface, which periodically deform the lower droplets and drive internal convective flows. We document the substrate oscillations, the resulting nonlinear standing waves, and the associated droplet vibrations. The improvement in the rate of both solid-phase reaction (a 5.86-fold improvement compared to static) and liquid-phase mixing (completion within 15 s), attributed to the micromixing effect, is demonstrated. Additionally, we validated effectiveness of the under-oil droplet micromixer as a nucleic acid extraction unit, which is superior to the conventional tube-based method. The Faraday wave-assisted micromixing strategy, with its high controllability and robust performance, provides a promising tool for the operation of under-oil biological and chemical microreactors.