predetermined volume of liquid in the initial chamber. The liquid in this chamber
then can be released by opening the valve, valving will be discussed in more detail
in further sections (Fig.5.4).
2.3 Mixing
The flow regime of a fluid is described by the Reynolds number
Re¼ρvDh
ηð 5 : 6 Þwhereρis the fluid density,vis the velocity of the fluid,μis the fluid viscosity and
Dhdenotes the hydraulic diameter [ 3 ]. Turbulent flow begins to form as the
Reynolds number surpasses (roughly) 2300. In a typical microfluidic system the
values of each term could beρμ 10 ^5 cm^2 s^1 , Dh 200 μm andv1cms^1 ,
using the above equation this would result in a Reynolds number2 which shows
that laminar flow dominates within typical microchannels [ 3 , 5 ].
Due to the laminar flow regime where advective mixing is absent, mixing
primarily occurs through diffusion, which is the process whereby a concentrated
group of particles spread out over time by means of Brownian motion to distribute
themselves uniformly across a given volume at a time scale
tD¼D^2 h=DFig. 5.4 Image (a) shows a random quantity of liquid for use in a biochemical reaction, this liquid
is required in 10μL quantities for reactions downstream. The spin rate is increased to force all
liquid further from the centre of rotation where it fills all available space on the disk, Image (b)
show the liquid fill the metered chambers and proceed to the overflow chamber. The dimensions of
the metering chambers have been specifically designed to hold only 10μL of liquid (Image (c)),
once all excess liquid has passed to the overflow chamber it is possible to open the valving system
holding back the metered volume by increasing the frequency of rotation to the “Burst Frequency”
of this specific valve
5 The Centrifugal Microfluidic: Lab-on-a-Disc Platform 119