v¼
ρ
32 ηrd^2 ω^2 ð 5 : 4 Þand the volumetric flow rate
Q¼~vA¼
πρ
128 ηrd^4 ω^2 ð 5 : 5 Þwith the cross sectional area of the channel A¼1/4πd^2. As for pressure-driven
systems, centrifugal flow is characterised by a parabolic velocity profile, which
peaks in the centre and vanishes at the wall due to no-slip boundary conditions.
2.2 Metering
Volume metering on the centrifugal microfluidic platform is often based on a
simple principle of centrifugally driven overflow from a chamber with a specific
volume below the bottom edge of an upper outlet channel [ 4 ].
Some microfluidic devices require hydrophilic materials, which aid the flow
against the centrifugal force. This can cause inconsistencies when attempting to
handle very small quantities of liquid; the effect can be suppressed at elevated
rotation frequencies where by the centrifugal force far exceeds interfacial forces
and also flattens the meniscus becomes flat, e.g., to improve accuracy of metering
(Fig.5.3).
The metering chamber is structured with two exits, one will be sealed with a
valve that can be opened upon demand and the other is an unobstructed channel that
leads to a waste chamber. When the sample or reagent is placed into the metered
chamber, the excess will overflow into the waste chamber thereby leaving only the
Fig. 5.3 Image a represents
a metered volume of liquid
within a hydrophilic
microchambers, due to the
effects of capillary action
the liquid creeps up the
sides of the walls. This
bending of the meniscus
causes inconsistencies in
metered volumes, this affect
is reduced by increasing the
frequency of rotation to
create a flat meniscus
(Image (b))
118 B. Henderson et al.