Bubble propagation on a rail: a concept for sorting bubbles by size

We demonstrate experimentally that the introduction of a rail, a small height constriction, within the

cross-section of a rectangular channel could be used as a robust passive sorting device in two-phase

fluid flows. Single air bubbles carried within silicone oil are generally transported on one side of the rail.

However, for flow rates marginally larger than a critical value, a narrow band of bubble sizes can

propagate (stably) over the rail, while bubbles of other sizes segregate to the side of the rail. The width

of this band of bubble sizes increases with flow rate and the size of the most stable bubble can be

tuned by varying the rail width. We present a complementary theoretical analysis based on a depth-averaged

theory, which is in qualitative agreement with the experiments. The theoretical study reveals

that the mechanism relies on a non-trivial interaction between capillary and viscous forces that is fully

dynamic, rather than being a simple modification of capillary static solutions.

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