Visualization of turbid two-fluid flows inside microfluidic conduits

Frequency domain optical coherence tomography (OCT) with phase-resolved algorithm is presented to perform high-resolution (8 mu m), cross-sectional imaging of structure and velocity in turbid gas-liquid slug flow inside a microtube and turbid liquid-liquid flow inside chaotic mixromixers: a barrier embedded Kenics micromixer and a Kenics micromixer. Slug flow, the most common flow regime in microfluidic gas-liquid two-phase flow, consists of trails of bubbles separated by liquid slugs flowing concurrently and provides significant radial heat and mass transfer. Since interfacial transports are proportional to the interfacial area between two phases, interfacial area concentration defined by interfacial area per unit mixture volume is an important parameter in a biochip with turbid biofluids. All the en face image techniques, such as light microscopy, experience errors in the interfacial area concentration measurement due to light refraction. In addition, they have overlapping depth inforination from the layers within a laser sheet or a depth of focus of the objective lens. OCT, however, can provide accurate interfacial area concentration in a microtube because it is a cross-sectional imaging technique which dispenses with the refraction correction in the radial direction. Simultaneously, OCT can measure bubble velocity and velocity field inside liquid slugs. The radial liquid velocity was quantified without refraction correction. Two toroidal vortices per liquid slug were visualized which is the essential mechanism for radial heat and mass transfers. The barrier embedded Kenics micromixer and Kenics micromixer are high performance chaotic micromixers with complex three-dimensional geometry. Conventional techniques cannot visualize a cross-sectional mixing pattern in such a complex micromixer. OCT, however, can image the pattern and, thereby, show mixing mechanisras. The barrier embedded Kenics micromixer was proven to have a higher performance by comparison mixing patterns of two micromixers.