A Reynolds number of the primary flow in a microchannel is usually less than 100 and one cannot expect turbulent mixing. In order to enhance mixing efficiency, time-periodic or geometrical-periodic perturbations have been suggested. All of them result in an increased complexity in the secondary flow. However, there is no experimental method to visualize the secondary flow and the associated mixing pattern in a microchannel because of difficulties in high-resolution, noninvasive, cross-sectional imaging. We reported the observation of secondary flow and convective two-liquid mixing in a meandering microchannel using spectral-domain Doppler optical coherence tomography. We observed an increased efficiency of mixing in liquid slugs when air was injected to produce a bubble-train flow.