Page 33 - Optofluidics Fundamentals, Devices, and Applications

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The details of fabrication using soft lithography can be found else-
where [19]. Here we provide a summary of the processes involved. The
microfluidic channels are designed in a CAD program and printed onto
a high-resolution transparency (~5000 dpi) (or, somewhat less conve-
niently and more expensively, converted into a conventional chrome
mask). This transparency is used as a photomask in 1:1 contact photoli-
thography (usually using SU-8 or PMMA as photoresist) to produce a
master. This master consists of a positive bas-relief of photoresist on a
silicon wafer, and serves as a mold for PDMS. Liquid PDMS prepolymer
is poured over the master and cured for 1 h at 70°C. The PDMS replica is
then peeled from the master and sealed (following plasma oxidation of
the interfaces involved) to a flat PDMS, glass, or silicon surface to form
the microfluidic channels. The overall process takes approximately 24 h.
Figure 2-2 shows a schematic diagram of the procedures involved.

2-5 Characteristics of Flow in Microchannels
A basic understanding of fluid dynamics in microsystems is useful in the
design and development of microfluidic devices. This section summarizes
a few characteristics of flow in microchannels that are important in com-
mon microfluidic components. Comprehensive reviews on the physics of
fluids in microfluidic systems can be found elsewhere [32–34].
In general, as the physical length scale of the system decreases,
gravity becomes less important. Surface forces (surface tension, elec-
trical, van der Waals, and surface roughness) become dominant [33].
Most microfluidic devices are in the micro- or nanoscale range, and
the relative importance of forces typically follows this order: interfa-
cial force >> viscous forces > gravitation ~ inertial force > buoyancy [35].
Most microfluidic devices involve the use of miscible liquids only.
Interfacial tension is therefore usually negligible. Viscous forces dom-
inate, and as a result, the flow is primarily laminar without turbu-
lence; mixing occurs by diffusion only [32]. We will describe laminar
flow and diffusion in more details in the following section.

2-5-1 Laminar Flow
Flow in microchannels is commonly characterized by the Reynolds
number, Re. The Reynolds number describes the tendency of fluid to
develop turbulence. It represents the relative importance of inertia to
viscous dissipation (Re = vlr/μ, where v is the average flow speed, l
is the characteristic length scale of the channel, r is the density of the
fluid and μ is the dynamic viscosity) [32]. For Re much less than 2000,
viscous forces dominate, and the flow is laminar. As Re increases
above 2000, the flow becomes dominated by inertial forces, which
tend to produce instability leading to turbulence.
Since the length scale of microfluidic systems is small (< 500 μm
typically), the flow of fluids in microchannels takes place in the regime