Active Control of Inertial Focusing Positions and Particle Separations Enabled by Velocity Profile Tuning with Coflow Systems

Inertial
microfluidics has drawn much attention not only for its
diverse applications but also for counterintuitive new fluid dynamic
behaviors. Inertial focusing positions are determined by two lift
forces, that is, shear gradient and wall-induced lift forces, that
are generally known to be opposite in direction in the flow through
a channel. However, the direction of shear gradient lift force can
be reversed if velocity profiles are shaped properly. We used coflows
of two liquids with different viscosities to produce complex velocity
profiles that lead to inflection point focusing and alteration of
inertial focusing positions; the number and the locations of focusing
positions could be actively controlled by tuning flow rates and viscosities
of the liquids. Interestingly, 3-inlet coflow systems showed focusing
mode switching between inflection point focusing and channel face
focusing depending on Reynolds number and particle size. The focusing
mode switching occurred at a specific size threshold, which was easily
adjustable with the viscosity ratio of the coflows. This property
led to different-sized particles focusing at completely different
focusing positions and resulted in highly efficient particle separation
of which the separation threshold was tunable. Passive separation
techniques, including inertial microfluidics, generally have a limitation
in the control of separation parameters. Coflow systems can provide
a simple and versatile platform for active tuning of velocity profiles
and subsequent inertial focusing characteristics, which was demonstrated
by active control of the focusing mode using viscosity ratio tuning
and temperature changes of the coflows.