**1. Science, technology and advantages of microfluidics**

As is the case in many fields of scientific research, the field of microfluidics has three main components: a science, a technology and an applications component.

• **The length-cube relationship:** the geometrical scale of length varies linearly but volume varies as length-to-the-power-of-three. As a consequence, volume changes rapidly as length decreases. Typical volumes of fluids in microfluidic channels range between nano-liter (nL) and femtoliter (fL). At the μm-scale, some properties of fluids change (as compared to a **mesoscale**, arbitrarily defined as the intermediate scale between the micro-scale and the macro-scale). Example properties that dominate at a micro-scale and that are different than those observed at the meso and macroscales include dominance of laminar-flow, diffusiondominated mixing and capillary action. To highlight one such effect, a counter-intuitive example (from an every-day scale point of view) involves two parallel-flowing fluid-streams that come into contact in a microchannel. Since there are no eddy currents or turbulence (due to laminar flow), the only mixing that occurs is a result of slow-occurring diffusion at the interface between the two fluid-flows. Since there is no bulk mixing, mixture-separations

Microfluidics and Nanofluidics: Science, Fabrication Technology (From Cleanrooms to 3D...

http://dx.doi.org/10.5772/intechopen.74426

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• **The square-cube law:** states that volume increases faster than surface area. In microfluidics, fluid-flows in microchannels are influenced or controlled or are a function of surface area (e.g., surface tension) while others (e.g., weight) are a function of volume. Typically, in microfluidics there are no gravity effects but dominance of surface tension and of interface effects.

• **Examples of other phenomena influenced by size and expressed by dimensionless numbers:** these include laminar flow expressed by the Reynolds number; surface tension expressed by the Bond number; transient thermal effects expressed by the Fourier number; viscous heating by the Brinkman number; and fluid compressibility by the Mach number.

• As a result of channel-size, microfluidics enables one to probe individually whatever it is in a fluid constrained in a microchannel (e.g., a single cell), thus providing additional avenues for scientific inquiry and discovery (important especially in the bio-analytical sciences).

Overall, the relevant literature [1–47] describes efforts at exploring and understanding the Physics of flow-related phenomena. Developments enabled by microfluidics will be highlighted in this chapter, with emphasis on ionized gases (e.g., Paschen's law for electrical gas breakdown; plasma sheaths and the Debye length) as applied to microplasmas formed inside fluidic

**Microfluidics** refers to a variety of approaches that enable exploitation of the phenomena mentioned above by fabricating microfluidic channels on a variety of substrates. For instance, on crystalline Silicon (of c-Si) wafers, on amorphous glass or on polymeric substrates. Due to the advantages of confining flow in microfluidic channels, several fabrication technologies have been developed and tested and will be briefly reviewed. These technologies are often collectively called micro Total Analysis Systems (μTAS) or Lab-on-a-Chip (LoC) or Micro Electro Mechanical Systems (MEMS). Microfluidics or whatever acronym is used to describe it, has attracted significant attention in books [1–17] and in journals [18–29]. While in the topic of publications, older references have been purposely included in this chapter followed by some recent publications. Where possible, the citations in the reference list have been grouped either according

in microchannels are faster and have shorter separation times.

channels.

**1.2. Microfluidics as a technology**

For microfluidics, a common thread between all of these components is that they are microsized, so size will be briefly discussed first. The dimensions shown in **Figure 1** are approximate because size of naturally-occurring objects (and of some manufactured-things) varies, for example the diameter of a human hair is between 50 and 100 μm; the diameter of the tip of a rollerball pen is between fine, medium and bold (e.g., between 0.5 and 0.7 mm); and of a 1 cent coin with its diameter varying slightly depending on the jurisdiction the penny was minted (typically around 20 mm or somewhat more).
