Centrifugal microfluidics has been recognized as a
promising pumping method in microfluidics because of its
simplicity, easiness of automation, and parallel processing.
However, the patterning of stripe flow in centrifugal
microfluidics is challenging because a fluid is significantly
affected by the Coriolis force, which produces an intrinsic
secondary flow. This paper reports a technical and design
strategy for centrifugal microfluidics called “density-gradi-
ent-assisted centrifugal microfluidics.” The flow behavior is
observed with the presence of a density gradient and without a
density gradient in two concentrically traveling phase flows.
As a result, clear stripe flow pattern is observed with a density
difference of 0.05 g/cm 3 between water and a percoll solution
at a flow rate of 11.8 μl/s (7 ml/10 min) and spinning speed of
3000 rpm. In contrast, without a density gradient, it is neces-
sary to reduce the flow rate and spinning speed to 0.1 μl/s and
1000 rpm, respectively. This paper also presents the use of a
density gradient to assist in focusing resin (polystyrene) par-
ticles on the boundary of a stripe flow pattern that consists of
water and percoll with different densities. Moreover, the
density-based separation and sorting of particles in a mixed
particle suspension is demonstrated. Polystyrene is selectively
focused on the boundary, but silica particles are separated

• Density-gradient-assisted centrifugal microfluidics: an approach to continuous-mode particle separation