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

TL;DR

This paper introduces a passive, rail-based method for sorting bubbles by size in two-phase flows, demonstrating experimental and theoretical insights into size-dependent bubble propagation.

Contribution

The study presents a novel passive sorting technique using a rail in a channel, supported by experimental results and a depth-averaged theoretical model.

Findings

Bubbles of specific sizes can be stably transported over the rail.

Flow rate and rail width control the size band of stable bubbles.

Theoretical analysis aligns qualitatively with experimental observations.

Abstract

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.