Page 44 - Optofluidics Fundamentals, Devices, and Applications

Basic Micr ofluidic and Soft Lithographic Techniques 25

solder coil (In100, height = 80 μm, width = 800 μm, length = 12 cm)
wrapped around a central microfluidic channel (height = 80 μm, width
= 800 μm, length = 3 cm). This device was fabricated using a procedure
similar to that used to fabricate a “basket-weave” microstructure: three
layers of PDMS containing microfluidic channels were aligned, bonded
together, and mounted to a glass slide to form a multilayer network of
microfluidic channels. The network was composed of two channels: a
central microfluidic channel and a “coil channel” that passed through
all three microfluidic layers to surround the central channel. Solder
was injected into the coil channel and cooled to form the microheater.
To characterize the microheater, electrical currents (I = 0−600 mA,
at 100 mA intervals) were applied through the wire while deionized
water flowed through the central channel (flow rate, Q = 100 μL/min).
As the current passing through the solder coil increased, the tempera-
ture of the fluid passing through the microfluidic channel increased up
to 40°C as a result of Joule heating.
Microsolidics simplifies the incorporation of metals into microfluidic
channels, but it also has several limitations. This method can only be
used with metals and alloys with a low melting point (generally < 300°C)
and affinity for the surface of the channel wall. These low-melting-point
solders are usually more expensive than commonly used solders, and
some (those containing Pb or Cd) are not biocompatible. In addition, the
wire must be fabricated as a loop; this method cannot be used to fill
“dead-end” channels. Lastly, it is currently difficult to use this process to
fabricate wires with cross-sectional dimensions less than 10 μm.

2-6-6 Bubble and Droplet Generator
We have focused primarily on miscible systems so far. The use of
immiscible fluids for the formation of emulsions and foams in
microfluidic systems is also interesting, and has undergone rapid
development in recent years. The controlled formation of microscale,
individual fluid segments allow compartmentalized biochemical
reactions and analyses using small volumes of reagents. It has also
been shown that droplet and bubble-based microfluidics can perform
simple Boolean logic functions [65,66].
There are several ways to generate droplets and bubbles in micro-
fluidic systems; details are reviewed elsewhere [67]. Here we describe
two common methods that depend on the geometry of the channel to
control the generation of droplets and bubbles: the flow-focusing
device and the T-junction.

Flow-Focusing Device
Figure 2-9a and 2-9b illustrates the flow-focusing device [68–70]. Gas
and liquid meet upstream from the orifice at the junction of the three
inlet channels. The pressure drop along the axis of the device forces
the tip of the gas stream into the orifice. Here the thread breaks and