Coriolis-Induced Flow Control in Centrifugal Microfluidics
Jens Ducrée, Thomas Glatzel, Thilo Brenner and Roland Zengerle
IMTEK – Institute of Microsystem Technology, University of Freiburg,
Georges-Köhler-Allee 103, D-79110 Freiburg, Germany,
email: [email protected], Tel: +49 / 761 / 203-7320, FAX: -7322
Flow control in microfluidic systems faces two major problems: the implementation of active
components and the fabrication of complex microstructures to enable fast mixing under
strictly laminar conditions. We present novel flow-control elements which take advantage of
the transversal Coriolis pseudo-force experienced by flows in simple microchannels on fast
spinning disks (Fig.1 left). These unique microfluidic components represent fundamental
building blocks of our novel “lab-on-a-disk” platform [1]. They can also nicely supplement
recently developed systems based on centrifugally controlled hydrophobic microfluidics [2,3].
Figure 2 (left) displays a simulation (CFDRC-ACE+) of the switching of flow between the
two symmetric outlets of an inverse Y-structure. The diagram shows the ratio between the
flow rates in the two outlet channels as a function of the angular speed ω. Towards growing
ω, the current increasingly diverted into one of the outlet. Far above the switching frequency
of about 250 rad s-1, the flow is almost completely switched into the right channel. The
switching frequency was experimentally confirmed by optical inspection of the filling heights
in reservoirs collecting the liquid from the two outlets (Fig.2 right). Effects resulting from the
acceleration of the disk, which would resemble the Coriolis induced switching, are ruled out
by an upstream chamber which overflows well after steady conditions have been reached in
the switch structure.
The implementation of fast mixing under strictly laminar conditions remains a major
challenge for miniaturized fluidic systems. As the integration of active components is not
feasible with the majority of applications, passive guide structures are used to shape incoming
flows into thin lamellae. For the rearrangement of flow, e.g. from an initial AB pattern into an
alternating ABAB pattern, 3-dimensional microstructures are required in a pressure-driven
flow. In addition, to minimize the flow resistance of pressure-driven mixers, high-aspect-ratio
channels have to be fabricated.
Figure 3 demonstrates how the same multilamination effect can be achieved by means of the
Coriolis force FCoriolis in a simple, 2-dimensional network of three microchannels in parallel.
Two adjacent flows A and B, each featuring a width d, enter a common inlet channel. The
flows are split into three parallel channels in a pattern |A|AB|B|. Due to the transversal
convection induced by FCoriolis (Fig.1), the AB pattern in the central channel is reversed at a
certain downstream position. At the point where an |A|BA|B| pattern is established, the three
parallel flows rejoin to align in an |ABAB| pattern. The initial diffusion length d has been cut
by a factor of two, such that the time for diffusive mixing tD ~ d 2 is reduced by a factor of 4.
Note that the high-aspect ratio “fins” in Fig. 3 are not necessary for the multilamination
process and can therefore be replaced by broader guide structures to ease their fabrication.
References
[1] www.bio-disk.com
[2] M. J. Madou and G. J. Kellogg, “LabCD: A centrifuge-based microfluidic platform for
diagnostics,” in Proceedings of SPIE, vol. 3259, 1998, pp. 80–93.
[3] G. Ekstrand, C. Holmquist, A. E. Örlefors, B. Hellman, A. Larsson, and P. Andersson,
“Microfluidics in a rotating CD,” in Micro Total Analysis Systems. Kluwer Academic, The
Netherlands, 2000, pp. 311–314.
Fig. 1 (left) Example of microstructured “Bio-Disk” featuring microfluidic (metering) structures.
(right) The flow of velocity v through a channel on a disk rotating at an angular frequency ω is
governed by the centrifugal force Fω ~ ω2 and, in the reference frame rotating with the disk, by the
Coriolis force FCoriolis ~ ω x v. Due to the parabolic flow profile induced by Fω , the transversal field
FCoriolis(ω) is inhomogeneous over a cross section of the channel to induce a transversal flow pattern
v(FCoriolis).
Fig. 2. Coriolis Switch. The flow is driven by the centrifugal force Fω down the radial channel.
Within the split, flow is distributed by the transversal Coriolis force FCoriolis into the symmetric
outlets according to the magnitude and direction of the vector ω. (left) Fluid dynamic simulation
of the switch. The diagram shows a measurement of the ratio of flow rates through the outlets as
a function of the angular speed ω. (right) The picture displays the experimentally observed
switching in the right-hand outlet reservoir. The ratio of the flow rates is evaluated from the
final filling heights.
Fig. 3. Lamination structure. The Coriolis-induced hydrodynamic convection on a rotating disk
(Fig.1) can be applied to rearrange two incoming flows A and B in an ABAB pattern. To this
end, the flow is partitioned into three parallel channels. In the central channel, transversal
convection leads to an almost complete reversal of the flow pattern (AB → BA) at the point of
reunification. In the resulting ABAB pattern, the diffusion length for laminar mixing are cut
down by a factor of two, thus accelerating mixing by a factor of 22=4.