**1.1. Microfluidics as a science**

**Microfluidics** has been defined [1–17] as the study of the behavior of fluids (or whatever is in them, e.g., colloids, discrete nanoparticles or individual cells), **in micro or in sub-millimeter channels** or around microstructures. Although microchannels can be relatively long (e.g., several 10's of mm), they are still called microchannels as long as one **critical dimension** (e.g., channel-width or channel-depth or tube radius) is in the micro scale. Microfluidic channels can be used for example to confine or to guide or to mix or to manipulate fluids.

• **The science of scaling as applied to microfluidics:** a number of physical properties of fluids change as size gets smaller [1–47], to quote *"smaller brings new capability"* [31]. These changes are often non-linear and have been discussed in books [1–17] and in journal papers [18–29]. A non-exhaustive list of size-dependent phenomena and effects is outlined below.

**Figure 1.** Examples of an approximate scale of things. The boundaries between micro and nanofluidics and between micro and millifluidics are fuzzy. In many cases, the strict definition adopted by the National Science Foundation (NSF) of the US for nano as anything with one critical dimension ≤100 nm is not strictly adhered to, thus there is a gap between 100 nm and 1 μm. Similar arguments apply to the NSF definition for micro (defined as one with a critical dimension between 1 and 100 μm). In many cases, the micro-scale is arbitrarily widened to ~1 mm and sometimes slightly more. The term millifluidics has recently been used for channels (or structures) with one critical dimension of a few mm.


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 channels.
