116 V. Bazargan and B. Stoeber
human intervention and reduces the risk of sample loss. Another advantage of
the compactness of mocro TAS is faster completion of a sequence of laboratory
processes. In addition, the time required for many analytical steps and chemical
reactions scale favorably at small length scales, as will be described in more detail
further below. The small size and reduced power consumption of microTAS makes
them attractive for portable analytical instruments for the biomedical field or for
environmental monitoring and control.
The transport and precise control of small volumes of fluid on a microfluidic
chip requires adequate flow control devices such as pumps, valves and mixers
that provide the required performance and are easily integrated. In addition,
these flow control devices should not compromise the biological samples through
electrical or mechanical interventions; furthermore, the biocompatibility of the
flow environment such as the channel walls is a concern, and correspondingly,
biocompatibility of materials is an important field of research for the microTAS
community. The following discussion will concentrate on flow control mecha-
nisms for microfluidic devices with an emphasis on thermally responsive fluids
for microflow manipulation.
6.2 TRANSPORT IN MICROFLUIDIC CHANNELS
Typical dimensions L of the height and width of microfluidic channels range
from several microns to several 100s of microns. Aqueous liquids that are most
commonly transported in microfluidic systems have a density ρ = 1000 kg/m
3
and a shear viscosity μ =10
3
Pa · s. With a typical flow velocity U = 1 mm/s,
flow in microchannels is characterized by a very low Reynolds number
Re = ρ LU/ μ 10
2
<< 1. The Reynolds number relates inertial forces and
viscous forces of a fluid. A low Reynolds number corresponds to high viscous
forces relative to the inertia forces in a flow. Consequently, microfluidic flow
is dominated by viscous forces, in most cases inertia can be neglected.
1,2
The
flow regime in microfluidic channels can therefore be considered laminar in most
cases
3
; while turbulence is not impossible in microfluidic devices, it is rather
uncommon.
2,4,5
The P´eclet number, Pe = LU/ D , is a dimensionless number that describes the
ratio of convection to diffusion, and it can be defined either for molecular diffusion
Pe
m
or for thermal diffusion Pe
T
, using the molecular diffusion constant D or the
thermal diffusivity α, respectively. The Peclet number can also be interpreted as
the ratio of a characteristic diffusion time scale to a characteristic convection time
scale. The molecular diffusion coefficient is low especially for liquids, such as
D =6· 10
10
m
2
/s for molecular diffusion of sodium phosphate in water.
6
The
associated P´eclet number Pe 100 indicates high convective effects compared
to molecular diffusion. However, in general there is no characteristic order of
magnitude for the P´eclet number in microfluidic devices.
7
In absence of turbulence
in microfluidic systems as mentioned above, molecular diffusion is important
SO13997_text.indd 124SO13997_text.indd 124 26/01/2011 3:50 PM26/01/2011 3:50 PM

Get Biomaterials for MEMS now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.