Droplet-based or digital microfluidics is an emerging
field that is capturing the attention of researchers worldwide.
While continuous-flow systems are adequate for many well-defined
and simple biochemical applications, they lack the scalability
and flexibility required for more complex fluid manipulations.
Droplets are micro to pico litre fluid volumes separated
usually by oil within a capillary or microfluidic channel.
As each droplet is completely isolated, it can act as
individual nanoscale reaction vessel for experimentation.
Using various microchannel geometries, droplet volume
can be adjusted, chemical concentration of droplets can
be regulated, and a series of droplets can be sorted.
Droplet-based microfluidics hence is ideal for chemical
or biochemical analyses, high-throughput screening, and
synthesis of nanoparticles. When droplets are considered
as nanoscale reaction vessels, their size uniformity
and frequency become important. Since droplets need
to be introduced at a precise location in definite size
and volume, rapid and steady generation of droplets
can be a challenge.
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Figure
1: rapid and steady generation of water-in-oil droplets
in silicon-based microchannel |
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IME researchers recently demonstrated
a high performance flow-focusing microfluidic device
for the spontaneous formation of droplets (Fig. 1).
The design integrates a 3-D circular constriction directly
inside a flow-focusing geometry, as opposed to the many
2-D microchannel devices currently known. IME’s
device is simple to fabricate, using a microfabrication
process based on planar silicon technology.
Forming a circular constriction inside a planar microchannel
is not a trivial affair. Several researchers demonstrated
circular constrictions for droplet formation but through
non-planar fabrication methods. Although some of these
devices revealed the fundamental science of the droplet
formation, they do not have the advantages, such as
microfluidic integration and batch manufacturing, offered
by the planar lithography technique.
The microfluidic device, described in Fig. 2, involves
two types of microchannel configuration: a T-shaped
profile (T-channel) for focusing flow and a pipette-shaped
profile (pipette-channel) for collecting the generated
droplets. A circular integrated constriction beneath
the surface links the two microchannel configurations.
The circular constriction geometry is a characteristic
of the layout angles shown and isotropic etching profile
of silicon.
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Figure
2 The microfluidic device: (a) planar view, (b)
close-up view of the device centre encircled, and
(c) cross-section profile along AA' showing the
integrated circular orifice beneath the surface |
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In order to generate droplets, the dispersed phase
(D) and continuous phase (C) were delivered into the
channels using a dedicated syringe pump for each liquid
in the directions assigned in Fig. 2(a). All three streams
were forced together into the pipette-channel and droplets
were generated through the circular constriction.
IME’s novel design offers improved performance,
in terms of rapid and controlled generation of droplets
(steady and high frequency) at a precise location on
chip. The geometry is expected to require a reduced
mechanical input to generate monodispersed droplets
(i.e. all droplets having the same diameter); and the
design can be easily transformed into truly axisymmetric
flow-focusing geometry simply by tailoring the etching
profile, without additional lithography step.
Made of an anodically bonded silicon–glass pair,
the device can withstand chemical reactions involving
organic solvents, elevated temperatures, and relatively
large pressures. It therefore offers an ideal platform
for on-chip synthesis of inorganic nanocrystals. It
is also useful for lab-on-a-chip applications requiring
consistent and steady streaming of monodispersed droplets.
The device has been reported in the Lab-on-a-Chip
Journal website.
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