A 27 cm long straight square microchannel in viscoelastic fluid for submicron- sized particle and bacteria focusing

Particle focusing within a flow cell is an essential step in performing flow cytometry and fluorescence-activated cell sorting (FACS). Viscoelastic particle focusing, in which particles suspended in a synthetic polymer solution migrate laterally against the main flow direction, has attracted considerable attention because it enables particle focusing without any external force. In this work, we demonstrate a viscoelastic flow focusing device that enables simple and robust focusing of submicron-sized particles in the channel center by optimizing operating conditions such as channel length, flow rate and PEO (poly(ethylene oxide)) concentration. The device was fabricated using a common soft lithography technique for the polydimethylsiloxane (PDMS) channel, which has a width of 50 µm and a height of 50 µm with a channel length of 27 cm. The focusing performance was first demonstrated using submicron-sized polystyrene (PS) beads ranging from 870 nm to 50 nm and then using biological particles such as E. coli bacteria to demonstrate the biological feasibility of the device. The PS beads, which ranged in diameter from 870 nm to 100 nm, were focused to the center of the channel, achieving over 90% of the focusing efficiency for down to 510 nm beads, and 62% of focusing efficiency even in 100nm sized bead. The device also was able to align a bacterial suspension in the center of the channel at flow rates up to 30 µL/min, demonstrating its biological relevance. The developed viscoelastic flow focusing device was able to align submicron particles within a narrow flow stream in a highly robust manner, validating its use as a flow focusing platform for high throughput and accurate flow cytometry of submicron objects.