Predicting Drop Arrival Sequence Based on Starting Position*

In droplet microfluidics, micro-droplets are used as biochemical reactors. Each of these droplet reactors can contain different reactions. Predicting drop sequence as they flow in a microfluidic system is important for tracking the content of the drops. We find that at low capillary numbers, when up to 4 rows of concentrated emulsion flow together and converge into a single channel, the order that these drops enter the constriction can be predicted. Furthermore, the drops’ arrival sequence can be predicted based on their starting position in the straight channel. We demonstrate the robustness of the system by showing that the drop order sequence can recover from small defects in the emulsion. Practically, our results enable drop tracking based on drop starting position rather than chemical labeling, and can lead to significant cost reduction in droplet assays.