  In this work, we report the phenomenon of formation of particle aggregates in
the form of thin slender strings when a polyacrylamide (PAM) solution, laden
with polystyrene (PS) particles is introduced into a microfluidic device
containing an array of micropillars. PAM and dilute solution of PS beads is
introduced into the microfluidic channel through two separate inlets and
localized particle aggregation is found to occur under certain conditions. The
particle aggregates initially have a string-like morphology that remain
tethered at their ends to the micropillar walls, while the rest of the
structure remains suspended in the fluid medium. It is this morphology that
inspired us to name these structures streamers. The flow regime under which
streamer formation is observed is quantified using through a phase diagram. We
discuss the streamer formation time-scales and also show that streamer
formation is likely the result of flocculation of the PS beads. These streamers
can serve as excellent model systems to study a biological phenomenon by the
same name.
