**3. Why plasmas?**

**Figure 4.** (a) Mask; (b) mask on c-Si chip, coin has been added for size; (c) chemically etched c-Si chip (serving as a stamp), the meandering pattern is protruding from the surface of the chip; (d) imprint generated by pressing the stamp and the polymeric chip together by placing them in a hydraulic press and by applying pressure at room temperature; (e) imprinted sample-well on a polymer chip shown under magnification; and (f), part of an imprinted meandering channel

**Figure 5.** (a) Imprinted channel on a polymeric chip (60x magnification), (b) sample-well (60x magnification) and (c) Venturi micropump with no moving parts and no electrical power requirements fabricated by imprinting (coin included

shown under magnification. For (d) and (e) and (f) different polymeric materials were used.

for size) [73].

8 Microfluidics and Nanofluidics

There are four states of matter: gases, liquids, solids and plasmas [123–131]. To generalize, atmospheric pressure plasmas are ionized gases that are either hot or cold (about room temperature or somewhat above it). Plasmas occur in nature, for example those found in inter-stellar space, in the ionosphere, in auroras and in lightening. There are also artificially-generated plasmas that are being used in many every-day-life applications. Neon signs and fluorescent lights in which low-pressure plasmas are formed either in Neon (Ne) gas or in Argon (Ar) gas) are two such examples. Other examples include plasmas employed for device fabrication by the semiconductor industry or for materials synthesis in nanoscience and nanotechnology [129–131]. **It has been estimated that over 50%** of whatever goes inside any electronic device (e.g., a tablet, a smartphone, TV) is fabricated using a low-pressure plasma.

**Conventional-scale** (or **lab-scale**) atmospheric pressure plasmas are widely used in **chemical analysis**, primarily in the form of atmospheric-pressure, **6000–10,000 K hot** Inductively Coupled Plasmas or ICPs [132]. Due to their size and weight (e.g., in the few 100's of pounds), their gas consumption (e.g., ~20 L/min), their power usage (e.g., 1–2 kW) and their need for cooling, ICPs are primarily used in a lab.

for the plasmas of interest to this work), where λD is the Debye length [133–137]. These will be

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

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

11

For microplasmas formed inside fluidic microchannels, in addition to gas breakdown and to continuous application of power, a microplasma must be formed in a constrained

Arbitrarily defined, microplasmas are those with **one critical dimension** in the micro-meter (μm) or in the sub-milli-meter regime [138, 139]. The words *"critical dimension"* (i.e., one dimension such as channel depth or width or radius) are important here: an **atmospheric** pressure microplasma in a microfluidic channel can range in length from μm to a 10's of mm, as long as its critical dimension fits the definition above. But as the critical dimension is reduced to submm and depending on operating conditions, atmospheric pressure plasmas transition from **thermal and 10,000°C hot** (e.g., lab-scale ICP [132]) to **non-thermal and cold** [133–139] (e.g., microplasmas). They also transition from equilibrium to non-equilibrium (to an approxima-

to these transitions (e.g., for nanomaterials synthesis) and for excitation mechanisms (e.g., for chemical analysis). In terms of technology-implications, cold plasmas enable use of inexpensive polymeric substrates that do not melt because microplasmas are cold and they do not require cooling; and they allow use of inexpensive 3D printing technology for fabrication.

**Why miniaturize atmospheric-pressure plasmas?** Operation at (or near) atmospheric-pressure is preferred because it obviates the need for heavy-weight and power-consuming vacuum pumps. By reducing weight and power consumption, atmospheric-pressure operation enables microplasma portability for chemical analysis *on-site* (i.e., in the field). By bringing a microplasma-based instrument to the field, microplasmas are expected to cause a paradigm shift in classical chemical analysis in which samples are collected in the field and are brought to a lab for analysis [140–148]. Due to plasma miniaturization, a number of questions arise. For example, how small **can** microplasmas be made? And, how small **analytical** microplasmas **should** be made? From a technology perspective, what is the minimum voltage required to ignite and sustain a microplasma? Would substrates tolerate the required high voltage? And, what is the preferred fab-

A plasma (**Figure 7**, regardless of its size) consists of two plasma sheaths (located in the vicinity of two electrodes bathed in a gas-of-interest in a gas-tight container) and a bulk plasma [133–137]. Shielding (or damping or screening) of the electric field arises from the presence of charged species in the plasma and from the unequal mobility of ions and electrons in the vicinity of the electrodes. Inside the plasma sheath, macroscopic electrical neutrality is likely not maintained. But outside of it (labeled bulk plasma in **Figure 7**), macroscopic neutrality is maintained and (time-averaged) electron and ion fluxes are roughly equal. Thus (on a time-

(electron T). There are scientific implications due

(for singly charged species). The distance (or thickness) a

<< Te

**3.2. Scaling of lab-size, ambient-pressure plasmas to microplasmas**

**3.3. How small atmospheric-pressure microplasmas can be made?**

≈n<sup>i</sup>

sheath screens electric fields is called the *Debye length* (λD), given by Eq. 1.

briefly discussed later in this section.

tion, to those with gas temperature Tg

rication technology?

average and per unit-volume), n<sup>e</sup>

microchannel.
