2 Centrifugal Microfluidics
One of the main challenges for improving the efficiency and reliability of the
resulting diagnostic device is flow control on the microscale. On the centrifugal
microfluidics, the inertial pseudo forces that occur in a rotating reference frame are
combined with a purpose-built network of multi-scale channels, chambers and
obstacles to pump and manipulate fluids towards through a sequence of laboratory
unit operations (LUOs). These LUOs include metering, mixing and particle sepa-
ration [ 4 ]. The centrifugal field depends on the mass density of the fluid and the
distance of the liquid volume from the centre of rotation. The centrifugal field
squares with the frequency of rotation [ 4 ] and so provides a wide range of available
force strengths. The disc cartridge functions without connection to an external
pump and can be loaded at atmospheric pressure using a simple pipette. Thus
there are few or no‘world to chip’interface limitations; similarly the disc cartridges
are often relatively cheap and disposable and thus particularly suitable for diag-
nostics. The inherent capability to centrifuge samples makes the platform particu-
larly suitable for blood processing and also for assays based around particle
sedimentation and cell handling [ 29 ].
2.1 Centrifugal Hydrodynamics
Within the centrifugal microfluidic platform there are three forces acting on a fluid
during rotation an angular spin frequency ω, there are three (pseudo) forces
(densities).
Centrifugal Force
fω¼ρrω^2 ð 5 : 1 ÞEuler Force fE¼ρrdω
dt
ð 5 : 2 ÞCoriolis Force fC¼ 2 ρωv ð 5 : 3 Þacting on the fluidρis the mass density of the liquid,ris the radial distance from the
centre of rotation,ωis the frequency of rotation and v is the fluid velocity in the
plane of the disc (Fig.5.1).
The effect of changes in these variables is visualised below in Fig.5.2.
The three forces can be changed by the frequency of rotation to achieve a
specific mean flow velocity [ 4 ]. Given a fluid plug within a straight channel
(segment) with round cross section of diameterd,liquid plug lengthΔr,mean
radial positionr, at a set angular frequency of rotationω, we obtain the mean
velocity
116 B. Henderson et al.