We use microparticle image velocimetry and numerical simulations to study the vortex evolution in liquid-liquid plug flows in a $400\mu \mathrm{m}$ T-junction microchannel. Experiments are conducted using the $\left[\mathrm{C}4\mathrm{mim}\right]\left[{\mathrm{T}}_2\mathrm{fN}\right]$ ionic liquid as the continuous phase and a glycerol-water blend as the dispersed phase, giving a dispersed to continuous phase viscosity ratio, $\lambda$, of 0.117. The range of mixture velocities studied is $0.002-0.03$ m/s, which corresponds to a capillary number range of ${\text{Ca}}_p\sim 0.0075-0.16$. We characterize the vortical structures based on geometry, vorticity, and circulation times within the plugs. Good agreement is found between the experimental data and the numerical predictions in terms of plug length, film thickness, and circulation patterns. The plug length is found to decrease while the film thickness increases with ${\text{Ca}}_p$. Three pairs of vortices are formed within the plug in a reference frame moving with the steady plug speed. The pairs comprise a main pair that occupies the central region of the plug and two secondary pairs located at the plug front and rear. The secondary vortices diminishment with ${\text{Ca}}_p$ is quantified based on their geometry and vorticity, subsequently correlated to the critical film thickness for vortex loss based on the predictions of Balestra et al. [Microfluid. Nanofluid.22, 67 (2018)]. It is found that the front and rear secondary vortex pairs diminish at different rates depending on ${\text{Ca}}_p$. With increasing ${\text{Ca}}_p$, based on their size, the rear vortex pair is lost before the front one, while, based on vorticity, only the front vortex pair is lost.

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• Microfluidics
• Multiphase flows
• Vortex dynamics