Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 117
for mixing, which can become rather difficult for long diffusion time scales.
Depending on the particular application purely diffusive mixing might be either
desired, especially where controlled mixing or separation of the species is needed,
or not sufficient, when rapid mixing is required. Several rapid mixing strategies
have been proposed for microfluidic devices that are effective despite the diffusion
process being relatively slow.
For the thermal P´eclet number heat transfer is analogous to mass transfer. With
a much larger thermal diffusivity than molecular diffusivity, heat diffusion is often
more significant than convection so that heat diffusion can be considered very
effective over the typical short length scales of microfluidic devices. With a thermal
diffusivity of water α =1.41· 10
/ s, a typical time scale t L
/ α =0.7ms
for heat diffusion can be achieved across a 10 μm deep channel.
6.3.1 Microvalve Principles
As described in Section 6.2, fluids are expected to behave quite differently in
microfluidic systems as compared to large scale systems because the different
forces governing flow scale differently. Therefore it is difficult to transfer flow
control concepts from macroscopic systems to microscale flow devices. This
led to the development of new flow control strategies for small-scale systems
as conventional flow control concepts cannot be simply miniaturized. At the
same time, microfluidics allows taking advantage of forces and effects that are
negligible in large scale systems, thus opening new opportunities for innovation.
The category of microvalves that are closest to their large scale counterparts simply
obstruct a fluid conduit by moving a solid into the flow path; some of the actuation
methods including electrostatic, piezoelectric, pneumatic, thermopneumatic, and
electromagnetic actuation are more effective at the microscale than in large-scale
Other examples include, passive micro check valves
that use drag
forces rather than inertial forces as well as microflow control strategies based on
surface tension.
A common disadvantage of many of these designs is the require-
ment for complicated fabrication schemes, unacceptable leakage rates, complex
external actuation, and the risk of irreversible blockage of the valve.
A different approach to valve actuation has been achieved using environmen-
tally responsive polymers that undergo a volumetric or phase change in response
to changes in temperature or concentrations. These concepts lead to fast actuator
response taking advantage of the relatively fast propagation of temperature or
concentration changes on the microscale that was discussed in Section 6.2. In
one of the first examples of this approach, Fr´echet and co-workers
grafted a
temperature-sensitive polymer (poly N-isopropylacrylamide) to the pore surfaces
of a porous polymer monolith in a microchannel. As the device temperature
transitioned through the lower critical solution temperature, the grafted polymer
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