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

Abstract: The micromixing process in microfluidic devices is of central importance in quite a few applications, with microfluidic studies on carbon capture and environmental monitoring being two examples. High surface area to volume ratio in microscale flows outweighs the role of interfacial tension manipulation by means of introducing a secondary phase to the main flow in order to augment the typically diffusion-dominant mass transfer operation. In this paper, we introduce an easily integrable continuous flow micromixing scheme implemented on a simple Y-type microchannel based on thermally excited intrinsic air within microchannel. Thanks to the direct contact between the sputtered thin platinum microheater and the co-flowing streams, trapped air in the liquid and/or tiny crevices is employed to generate elongated bubbles with a millisecond lifespan, consuming less than 0.4 W of power and thereby enabling homogeneous mixing. 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 uL/min. Interpreting the experimental results, supplemented by scaling arguments, we quantitatively describe the ephemeral role of emerging elongated bubbles in the micromixing process in which the Marangoni as well as Weber numbers are recognized as the characteristic local non-dimensional groups while the enhancing role of tiny daughter microbubbles on the micromixing downstream of the microchannel can be reflected in the Peclet and the Capillary numbers.