Page 41 - Optofluidics Fundamentals, Devices, and Applications

22 Cha pte r T w o

become sufficient to generate a transverse flow (“Dean flow”) [32]
across the streams. This transverse flow increases the contact area
between the streams, and enables more efficient mixing of the liquids.
Active mixers have also been developed for enhancing mixing:
rotary mixers, where solutions to be mixed are actively pumped peri-
staltically in a circulating loop [48]; mixers based on electrowetting [49],
nonlinear electrokinetic effects [50,51], and acoustic streaming [52].
These systems are usually complicated to fabricate; however, recently,
a simple, portable, hand-powered mixer has been developed that
exploits the introduction and movement of bubbles in microchannels
to mix the continuous fluids [53].

2-6-4 Diluters for Generating Concentration
Gradients in Microchannels
Gradients in the concentration of solutions are important in many bio-
logical and chemical processes, such as chemotaxis and nerve growth
cone guidance. Various forms of diffusion-based dilution microfluidic
devices have been developed to generate concentration gradients. The
general design consists of two inlets, one for the reagent to be diluted,
and the other for the diluting agent or buffer, leading into a network of
multistep fluid-dividers [54] (Fig. 2-7). Mixers are usually incorporated
to ensure the complete mixing of the reagent and the buffer. The ratio
of fluidic resistance in the branches determines the ratio of volumetric
flow of the reagent and the buffer in each branch, which in turn deter-
mines the output concentration. The fluidic resistance can be increased
by increasing the length of the channel, or by decreasing the cross-sec-
tional area of the channel. Different schemes have been developed to
generate linear and logarithmic gradients [54–62].
2-6-5 Local Heaters and Electromagnets
Incorporation of metals into microfluidic systems for applications
such as on-chip heating and magnetic sorting usually require more
complicated procedures as the materials and the fabrication processes
are different from those of microfluidic channels, which are polymer-
based. A simple method—microsolidics—has been developed to fabri-
cate complex metallic structures by injecting liquid solder into micro-
fluidic channels, and allowing the solder to cool and solidify [63,64].
The general procedure consists of five steps (Fig. 2-8a):

1. Fabrication of microfluidic channels in PDMS.
2. Plasma oxidation and silanization of the inside surfaces of
the microchannels with 3-mercaptopropyltrimethoxysilane
(0.1 M solution in acetonitrile). This reduces the surface free
energy of the channel surface, and allows the solders (such
as In100, or 100% Indium) to wet the channel wall.
3. Injection of molten solder into the channels by applying a
vacuum to draw metal into the channels.