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Introduction
Fluid flow is a ubiquitous aspect of microfluidic systems. Flow mechanisms, depending on the types of driving forces, can generally be categorized into active pumping mechanisms and passive pumpless mechanisms. Active pumping mechanisms for microfluidics involve driving fluid via an external power source/field or actuators1. Common active pumps for microfluidics include syringe pumps2, 3–4, peristaltic pumps5,6, piezoelectric pumps7, 8–9, and magnetic pumps10, 11–12. While active pumping methods generally provide precise control on the port pressures or flow patterns, they typically require external off-chip controllers, which limits their use for applications where many independent pumps are needed in parallel. Interfacing with external controllers can also introduce bubbles to fluidic channels, and void volumes associated with tubing connections lead to waste of expensive reagents. Some efforts have attempted to mitigate these issues by integrating the pumps onto chips13, 14, 15–16, but those pumps still require complex manufacturing procedures and external power sources.
Passive flow techniques, meanwhile, do not require external power sources or actuators. Devices configured for passive flow techniques take advantage of the potential energy of the fluid itself, including osmotic potential energy for osmosis-driven flow17, 18–19, surface energy for surface tension-driven flow20, 21–22 and capillary-driven flow23, 24, 25, 26, 27–28, and gravitational potential energy for gravity-driven flow29, 30, 31, 32, 33, 34, 35, 36, 37–38. Those techniques are self-operational without external controls, and are generally more compact and less bubble-prone than the active pumping mechanisms. The elimination of bulky power sources allows easy integration of those flow mechanisms on chip, which reduces the device footprint and decreases the difficulty of fluid handling.
Because of those advantages, passive flow techniques are commonly used in lab-on-a-chip or point-of-care microfluidic systems1. That said, they bring their own set of challenges. For example, osmosis-driven flow requires a semi-permeable membrane1, and does not have flexibility in fluid concentrations. Surface tension-driven flow and capillary-driven flow depend on the device material and fluid properties, which limits the device’s adaptability to new systems and flexibility to generate self-regulated flows. On the other hand, gravity-driven flow is appealing in its simplicity in both fabrication and operation, because the device needs...