High throughput particle focusing
In viscoelastic flows through curving microchannels, the following
dominant forces affect particle focusing: lift force
(FL), elastic force (FE), and a
curvature-induced Dean’s drag force (FD). The combined
interaction results in particles migration and focusing. While previous
work from our group and others have mainly focused on particle migration
towards an equilibrium position using either inertial and
elasto-inertial microfluidics, in this work we show that it is possible
to pre-position particles at the inlet and keep the particles fully
focused throughout the channel length strictly based on particle size.
To this end, for the first time, we show that particles above a certain
size cut-off, prepositioned at the lateral equilibrium position will
remain fully focused throughout the channel, independent of length, in
flow through spiral microchannels. Smaller particles are affected
differently by the dominant forces and entrapped into the Dean vortices
and continuously migrate away from the equilibrium position. By
optimizing the spiral geometry and length, we show it is possible to
fully migrate the smaller particles close to the inner wall for
efficient separation By. Experimentally, by simply introducing a
viscoelastic sheath buffer (PEO) into a two-inlet spiral microchannel,
the blood will be pinched toward the outer wall. The pre-positioned
blood cells will remain fully focused while smaller bacteria readily
migrate towards the inner wall and can be extracted. Fig.1 shows a
schematic illustration of how the method works. A sheath fluid pinches
the sample containing large and small particles to occupy a narrow
stream at the outer wall of the inlet. As the hydrodynamic forces are
developed, the larger particles remain focused due to the balance
between the three main forces while the smaller particles are trapped in
the Dean vortices and differentially migrate away from the outer wall.
Finally, the smaller particles reach the inner wall and can then be
separated. Note, the total flow rate (sample + sheath) is extremely
high, in the range of mL/min. Furthermore, the focusing phenomena differ
from inertial microfluidics, where the focusing point is toward the
inner wall laterally and at two vertically focusing points as a result
of balance between FL and FD. We
evaluated several parameters that influence the particle focusing
behavior, preposition of the particle at the inlet, flow rate and
particle size. To this end, we first used microparticles to examine the
focusing phenomena before evaluating the spiral for bacteria separation.