Bubble-assisted micromixing via thermally excited intrinsic air within microfluidic systems

Abstract

The micromixing process in small-scale systems is of central importance in many applications where high surface-area-to-volume ratio outweighs the influence of interfacial tension manipulation achieved by introducing a secondary phase to the main flow to enhance the otherwise diffusion-dominant mass transfer operation. In this paper, we introduce an easily integrable continuous flow micromixing scheme implemented on a Y-shape microchannel based on thermally generated bubbles. The generation of Bretherton-like bubbles with millisecond lifespans, followed by their breakup into tiny spherical daughter bubbles moving downstream, is considered a major reason in augmenting the micromixing process. The performance of the micromixer is characterized in terms of the mixing index (MI), and it is shown that immediately ahead of the microheater, the MI exceeds 95% for side-by-side flowing of water at different overall flowrates, namely 4, 10, and 20 µL/min. The emergence of bubbles can be attributed, on one hand, to trapped air in the liquid and/or tiny crevices—particularly in water, where the formation of bubbles may appear at temperatures below normal boiling point. On the other hand, it can also result from superheating beyond the equilibrium boiling temperature, especially in mixtures of water and glycerol. Finally, we quantitatively describe the ephemeral role of emerging elongated bubbles in the micromixing process by extracting their growth rates and nucleation period, and by drawing an analogy with bubble generation in a T-junction microfluidic device.