Inertially-induced secondary flow in microchannels

TL;DR

This paper presents a passive method to generate and control strong secondary flows in microchannels using pillar-induced flow symmetry breaking, enabling programmable flow patterns for microfluidic applications.

Contribution

It introduces a novel passive technique leveraging pillar placement to induce and control secondary flows in microchannels, which can be precisely engineered for microfluidic systems.

Findings

Secondary flows are generated by pillar-induced symmetry breaking.

Flow patterns can be engineered by adjusting pillar positions.

The method works over a wide range of flow rates.

Abstract

We report a novel technique to passively create strong secondary flows at moderate to high flow rates in microchannels, accurately control them and finally, due to their deterministic nature, program them into microfluidic platforms. Based on the flow conditions and due to the presence of the pillars in the channel, the flow streamlines will lose their fore-aft symmetry. As a result of this broken symmetry the fluid is pushed away from the pillar at the center of the channel (i.e. central z-plane). As the flow needs to maintain conservation of mass, the fluid will laterally travel in the opposite direction near the top and bottom walls. Therefore, a NET secondary flow will be created in the channel cross-section which is depicted in this video. The main platform is a simple straight channel with posts (i.e. cylindrical pillars - although other pillar cross-sections should also function) placed along the channel. Channel measures were 200 \mum\times50 \mum, with pillars of 100 \mum in diameter. Positioning the pillars in different locations within the cross-section of the channel will result in induction of different secondary flow patterns, which can be carefully engineered. The longitudinal spacing of the pillars is another design parameter (600 \mum spacing was used for this video). The device works over a wide range (moderate to high) flow rates. We used 150 \muL/min in this experiment. The device has 3 inlets where a dye stream is co-flowed between two water streams. In this video, one can see the effect of the net secondary flow created by inertia in the microchannel by visualizing the cross-section of the fluorescently labeled stream. Confocal images are sequentially taken at the inlet and after 49 consecutive pillars.