Ultrasound-controlled stream splitting in a microfluidic coflow

TL;DR

This study demonstrates how an external acoustic field can precisely control stream splitting and droplet formation in microfluidic coflows, enabling programmable manipulation of multiphase flows.

Contribution

It introduces a novel acoustic-driven splitting regime that allows high-capillary-number droplet generation with spatial control, supported by experiments, simulations, and theoretical analysis.

Findings

Acoustic excitation destabilizes the interface, causing reversible flow regimes.

A distinct splitting regime enables droplet formation at specific locations.

Droplet size and residual stream thickness are mainly governed by hydrodynamics.

Abstract

Precise control of multiphase microfluidic flows underpins applications ranging from chemical processing to biomedical diagnostics. We investigate the response of a liquid--liquid coflow in a rectangular microchannel to an externally applied standing acoustic field. Acoustic excitation destabilizes an otherwise stable interface, giving rise to a sequence of reversible interfacial regimes: waviness, splitting, relocation, and stream-droplet breakup. Remarkably, a distinct splitting regime emerges, where a continuous stream partially splits into droplets at tunable locations while retaining a thin residual stream. Unlike conventional droplet breakup, this regime avoids complete disruption of the main flow, enables droplet generation at high capillary numbers, and allows spatial control over droplet formation. Extending across a broad range of capillary numbers, we examine how variations in flow conditions and applied acoustic power influence these regimes. Combining experiments, numerical simulations, and theoretical scaling, we elucidate the mechanisms governing this droplet generation mode and the associated regime transitions. Systematic measurements show that droplet size and residual stream thickness are governed primarily by hydrodynamic parameters, whereas the acoustic field controls the onset and spatial location of the breakup. These results establish a simple avenue for stream splitting and drop generation on-demand in a microfluidic coflow, opening new possibilities for spatially programmable manipulation of multiphase flows.