Fluid manipulation on the micro-scale: Basics of fluid behavior in microfluidics

Fluid manipulation on the micro-scale (microfluidics) is bringing new potential applications in a number of fields, including chemistry, biology and medicine. At sub-millimeter channel scale, some phenomena, unimportant at the macroscale, become an important force to consider when designing a microfluidics system. For example, the decrease in fluid mass causes the effects of viscosity to overcome the influence of inertia. Turbulent flow cannot be achieved at any realistic fluid velocity, making mixing a challenging task. The only phenomenon capable of blending liquids at microscale is diffusion and liquid streams can be flowed side-by-side for tens of minutes before they completely fuse together. The decrease in the channel size also leads to an increased surface-to-volume ratio, which increases the importance of surface effects, including adsorption, capillary action and surface wetting and/or electric double layer formation with related electrokinetic phenomena. While rivers cannot flow uphill, a stream of liquid can easily flow up against gravity inside a capillary. Similarly, the formation of electric double layer near the charged surface of a micro-channel or capillary can be applied for electrokinetic actuating. This review summarizes selected physical phenomena related to liquid-based (water solutions) microfluidics as described recently.