Enhancement of mixing inside ionic liquid droplets through various micro-channels design

- Incorporated ionic liquid (IL) into droplet flow system as the droplet phase.
- Intensified mixing inside IL droplets through microfluidic channel design.
- Visualized mixing patterns inside IL droplets in various micro-channels.
- Investigated the effects of flow rates and droplet size on mixing efficiency quantitatively.
- Disclosed the mechanism of mixing intensification inside IL droplets in various micro-channels.

Ionic liquid (IL, [C4mim][BF4]) was introduced into micro-channels to form IL droplets and mixing performance inside the droplets was investigated via micro-LIF (laser induced fluorescence) technique. Experiments showed that mixing inside IL droplets generated in a simple cross-junction straight channel was very poor, due to the high viscosity and small diffusion coefficient of IL. To enhance the mixing inside IL droplets, two methods were then proposed. One was to change the way of droplet formation, which was achieved in T-junction and Y-inlet channels. The other was to change the way of droplet movement along the channel, which was achieved in meandering channel and deforming channel. Combing these two methods led to the design of Y-deforming channel, which resulted in excellent mixing inside IL droplets. Because of the extremely slow diffusion rate in IL, the circulation patterns and mixing characteristics within IL droplets were observed clearly in each type of micro-channels. Besides, mixing efficiency was defined and calculated, and its dependence on flow rates and droplet size were discussed in according channels. Based on the qualitative and quantitative analyses, mechanism of the mixing intensification brought by each micro-channel was clarified, and the core was to break the symmetrical circulation pattern inside IL droplets and introduce new vortices to enhance mixing.