There are many other soft lithography approaches which have been reported in
literature like solvent assisted micromoulding, nanoimprint lithography and dip pen
lithography etc. Where the patterning is carried out at various scales with soft
materials [ 30 ].
3 Microfluidics for Flow Control and Some Novel Effects
The early applications of microfluidic technologies have been mostly in chemical/
biochemical analysis. The field of microfluidics offers numerous capabilities like
rapid sensing and detection of analytes corresponding to limited concentration,
rapidity of performance, easy usage with minimal human intervention, good reso-
lution using very small quantity of sample volume and thus offering solutions to
low cost diagnostics etc. Micro-flows are mostly laminar with values of Reynold’s
no. in the range of less than 1 and this laminarity is an advantage in some of the
typical requirements of sensitive diagnostics like single cell isolation, single cell
manipulation and detection, drug delivery etc. Microfluidics also offers possibilities
to control concentration of sample in both space and time [ 1 ].
The laminar nature of Micro-flows makes molecular diffusion the only means to
facilitate mixing in micro-scaled devices in the absence of convective transverse
fluid motion. For a typical microscale device the diffusion length scale being large
and the diffusion constant being very small for fluids ensures a very high diffusion
time. This coupled with the laminar nature of microflows gives rise to a very
interesting domain where various spatial and temporal strategies are utilized by
various micro-chip designs to micro-scale actuation for enabling a reduced inter-
diffusion length causing the flows to mix vigorously and sometimes in a controlled
manner.
There has been a lot of research on micro-mixing methodologies, which focus
primarily on increasing the contact surface between two streams for diffusion
through multi-lamination strategies. The multi-lamination in flows can be achieved
easily by stacking different streams in parallel, perpendicular, radial, as well as
Fig. 2.8 Solenoid
microvalve (Reproduced
from Singh et al. [ 29 ] with
permission from the
Springer)
44 G. Bhatt et al.