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

their gas consumption (e.g., ~20 L/min), their power usage (e.g., 1–2 kW) and their need for

A plasma is an ionized gas [123–131]. The term *plasma* was coined by Langmuir in the 1920's and it is derived from the ancient Greek word *πλάσμα* (plasma), freely translated to something

. Thus, a prerequisite for plasma formation is ionization. Singly-charge ionization (in the form of ion-electron pair formation) is done by detaching an electron from a neutral gaseous atom or molecule. Although there are other ways of detaching an electron (e.g., thermally), one way doing it is by placing a gas between two electrodes and by applying an electric field with a sufficiently field-strength to ionize the gas (**Figure 7**), thus forming an *electrical gas discharge*. Because neutral gaseous atoms or molecules (ordinarily insulators) become ion-electron pairs, they also become (partial) conductors. Partial because to an approximation, conductivity

depends on the degree of ionization (this is important for weakly ionized plasmas).

To obtain electrical gas breakdown, the dielectric strength of the gas must be exceeded. The dielectric strength is the maximum electric field-strength (in V/m) an insulating gas can endure without breaking down into ions and electrons. If there is a sufficiently large field-strength, breakdown of the dielectric strength will cause formation of (typically) a low-current **spark** (i.e., a momentary electrical discharge, an example is electrostatic discharge from static electricity), or formation of a continuous electric-**arc** requiring continuous application of an electric field from an external power supply (**Figure 7**) capable of providing high-current (often

**Conditions for sustaining continuous plasma operation:** Following gas breakdown, there must be continuous application of external power to sustain a plasma. Other criteria include

**Figure 7.** Ideal plasma formed in a gas-tight and pressure-controlled enclosure. The plasma is formed between two conducting plates or electrodes positioned at a distance (or gap) d from each other. For dc operation, pertinent literature

λ3

), and on the average it is quasi-neutral, and for singly ionized gases

) and electrons (with an elec-

D must be > > 1 (this is easy to satisfy

cooling, ICPs are primarily used in a lab.

tron number density n<sup>e</sup>

10 Microfluidics and Nanofluidics

should be consulted [124].

ne ≈ni

**3.1. Some fundamental aspects of plasma science**

"*moldable*". A plasma consists of ions (with ion number density n<sup>i</sup>

in the Amp range). Arcs find applicability in welding of metals.

an electrode distance d that must be > > λD and that ne

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 approximation, to those with gas temperature Tg << Te (electron T). There are scientific implications due 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 fabrication technology?
