Microfluidics refers to devices and methods for controlling and manipulating fluid flows with length scales less than a millimeter. Fluid manipulation on the micro-scale brings new potential applications in several fields, including chemistry, biology, and medicine. The fluid phenomena that dominate liquids at this length scale are measurably different from those that dominate at the macroscale.
The micron structure feature of microfluidics significantly increases the ratio of area to volume in a fluid environment. This change results in a series of surface-related unique effects in a microfluidic system. These unique effects include laminar flow effect, surface tension and capillary effect, fast heat conduction effect and diffusion effect, and/or electric double layer formation with related electrokinetic phenomena. Most of these effects make the performance of the microfluidic analytical system significantly exceeds the analysis macroscopical system.
Fig.1 Diffusion. (Beebe, 2002)
When a fluidic system is scaled down to the point where cross-section reaches sub-millimeter dimensions, inertia begins to lose its influence on the liquid flow. Viscous forces, on the other hand, decrease much less significantly. Mass of the liquid at the micro-scale decreases so much that viscosity dominates inertia, resulting in predictable laminar flow. The Reynolds number (Re) is a quantity that describes the ratio of inertial to viscous forces in a fluid. Re is almost always in the laminar flow regime for microfluidic systems, allowing for highly predictable fluid dynamics. Molecular transport also changes dramatically at this scale because convective mixing does not occur, enabling predictable diffusion kinetics.
Surface tension describes the tendency of a fluid on a surface to reduce its free energy by contracting at the surface-air interface. Interfacial tension is a similar phenomenon but is generally applied to two immiscible fluids (oil and water). When shrinking the size of a fluidic system, the surface conditions become an important parameter. The ratio between surface and volume gets higher as dimensions of the system decrease. A high surface-area-to-volume ratio means that surface effects play a much bigger role at the microscale.
Capillary action describes the movement of a fluid through a narrow constriction, such as a narrow tube or porous material. In a micrometer-scale channel, capillary action dictates the fluid flow and manipulates fluids in many applications.
Electrokinetic phenomena belong to the most important surface-related effects at the micro-scale. Electrokinetic flows are classified into the following four types: electrophoresis, electroosmosis, streaming potential, and sedimental potential. Upon applying an external electric field, the hydrated ions start electromigrating and dragging the surrounding liquid with it. In a capillary or micro-channel, this tug is sufficient to actuate the movement of the whole bulk of the liquid. Although electrokinetic phenomena are quite sensitive to the buffer and sample composition, their use may be useful in some practical applications, especially for electroosmotic pumping in devices for fluid delivery, chromatography, and electrophoresis.
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