**4. Microplasma formation inside fluidic channels**

The key idea behind microplasma miniaturization [138] is to obtain analytical performance about equal to that of lab-scale ICP-optical emission spectrometry (ICP-OES) systems [132] but by using self-igniting, low-power, low-cost, small-size, light-weight, continuous-flow and low gas-consumption (e.g., 250 mL/min) atmospheric-pressure microplasmas. The expectation is that such microplasmas can be used for *"taking part of the lab to the sample"* types of applications [140–142].

Based on these ideas, we fabricated and tested a variety of battery-operated, atmospheric pressure, self-igniting, mm-length microplasmas in fluidic channels [143–156]. Due to their mm-length, plasma sheath and Debye length are not of a concern. In addition to being "cold", their high surface area-to-volume ratio further facilitates heat dissipation, thus facilitating use of polymeric substrates and 3D-printing fabrication methods. Example microplasmas fabricated in a variety of substrates will be discussed next.

#### **4.1. Microplasmas in fluidic channels on amorphous substrates**

electrode material (e.g., work function) and it assumes that gas breakdown is predominantly a function of electron emission from the electrodes. In short, the two key variables in this equation are pressure (p) and inter-electrode distance (d). The product of p times d is often

Paschen's law applies to electrical discharges formed at low-pressures. In high-vacuum or at high pressures (e.g., atmospheric), Paschen's law fails ([139] and references herein). There are also deviations from the behavior predicted by Eq. 2 when kHz or MHz ac voltages are used or when μm inter-electrode distances (or gaps d) are employed [139]. Undeniably, there are limits to applicability of Paschen's law. Despite of these limitations, Paschen's law **(presumably, the only choice**) can be used to obtain rough estimates of the magnitude of the voltage required to ignite (or initiate) an atmospheric pressure plasma. Thus it can be used as an aid in the design of appropriate power supplies. For instance, when the electrodes are made from Iron (Fe) and the inter-electrode distance d is 2.8 mm, and the discharge gas is

breakdown (or for microplasma ignition) is about 6000 V. As the inter-electrode distance d

sustain a microplasma lower voltages are typically required. An example is the ballast used

The key idea behind microplasma miniaturization [138] is to obtain analytical performance about equal to that of lab-scale ICP-optical emission spectrometry (ICP-OES) systems [132] but by using self-igniting, low-power, low-cost, small-size, light-weight, continuous-flow and low gas-consumption (e.g., 250 mL/min) atmospheric-pressure microplasmas. The expectation is that such microplasmas can be used for *"taking part of the lab to the sample"* types of

) required for gas

drops to about 2400 V, and

drops to about 1400 V. It should be emphasized that

is not always necessary and that (once ignited), to

called "**pd scaling**." An example of a Paschen curve is shown in **Figure 9**.

**Figure 9.** Paschen curve for argon gas and for a 2.8 mm inter-electrode gap (d) as a function of pd.

Argon (Ar) at (or near) atmospheric pressure, the minimum voltage (V<sup>b</sup>

decreases from 2.8 to 1 mm (and by keeping all else constant), V<sup>b</sup>

**4. Microplasma formation inside fluidic channels**

when d further decreases to 0.5 mm, V<sup>b</sup>

in fluorescent lights.

14 Microfluidics and Nanofluidics

applications [140–142].

gas breakdown at the minimum voltage V<sup>b</sup>

For microplasmas formed inside a **microfluidic channel on a chip**, a dual substrate approach was used (**Figure 10**). Briefly, cleanroom-technologies (**Figure 2**) were employed to define and to sputter-deposit Au electrodes E1 and E2 (**Figure 10a**). Holes were drilled for the inlet and the outlet. On the bottom wafer, a chemically etched microchannel was formed. The top and bottom wafers (**Figure 10a** and **b**) were aligned so that the central part of the etched channel matched the protruding part of electrodes E1 and E2. Then the wafers were bonded together (**Figure 10c**) [143] and glass-tubes were affixed to the inlet and outlet holes (**Figure 10d**). The inlet was connected to a gas-supply (Ar-3%H2 ) that was used as the microplasma gas and as the sample-introduction carrier-gas. Upon application of electrical power, the microplasma selfignited, it was formed between electrodes E1 and E2 and was sustained by continuous application of electrical power (~10 W). To avoid electrode breakage, a high-voltage ac [143] was used.

**Figure 10.** (a) Top chip showing electrodes E1 and E2, (b) bottom chip showing the etched microchannel, (c) the top and bottom chips bonded together (the microplasma was formed between electrodes E1 and E2, and (d) an *"angle"* view of the two bonded chips.

## **4.2. Postage stamp-sized microplasmas on polymeric substrates**

To reduce ownership, operation and fabrication costs, we developed and evaluated a variety of microplasmas on polymeric substrates (e.g., **Figures 11** and **12**) [144–146]. Although a critical microplasma dimension was in μm-meter regime (**Figure 11**), these microplasmas were formed inside **millifluidic channels** (e.g., ~2 mm wide). This was done for rapid prototyping [109] and to avoid accidental contact of the microplasma with the channel-walls (important during testing). Once prototypes were produced, channel width was never revisited. Although polymeric substrates have high dielectric strength, to address poor transmis-

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

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

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3D-printing [109–122] was accomplished using a 3D-printer (~\$1000) to rapidly prototype 3D-chips in a few hours (or less), thus obviating the need for cleanrooms and lithography. We used 3D-printing to fabricate hybrid chips (fitted with a quartz plate and needle electrodes) for microplasma formation in millifluidic channels [146, 147, 149]. An example is

The nanoscale [157–194] is a natural extension of the microscale (**Figure 1**) and it is defined as the science, technology and application of transport phenomena and of fluid-flow in channels ≤100 nm or around nano-size objects [158, 170]. This is not universally accepted, many consider nano-size as anything with one critical dimension ≤1 μm. The range between 100 nm and 1 μm is sometimes referred to as *"extended nanofluidics"* [181]. Nanofluidics is not new,

In nanofluidics, size (or scale) is important, likely more so than in microfluidics. For instance, at the nano-scale many dimensions of molecules are of similar size as the nano-fluidic channels that constrain them (**Figure 1**). A few scientific questions that being addressed include: How do properties of individual atoms, ions or molecules, manifest themselves as they are confined in spaces (roughly) of their own size? Would quantum effects become important [173]? Since pressure is not used to force fluids through nanochannels, should electrokinetic flow be preferred? And, as surface-to-volume ratio increases significantly (over microchannels), what is the effect of surface-charge on ions or molecules confined in nanochannels? What is the effect of surface roughness on fluid-flow? And, how do surfaces interact with ions or molecules so close to them? What are the best surface modification approaches? What is the effect of van der Waals forces and of the electric double layer (EDL) at the nm-scale? Some questions arising from technology include: how would one introduce very small volumes of analytical samples into nanofluidic channels? To facilitate discussion, assume a cylindrical nanochannel with 100 nm diameter and 1 μm length. In this case, the volume is 100 **a**tto **L**iter (**aL**). How would one introduce an aL volume sample into a nanochannel without evaporation of some of the analyte or of the solvent? Due to the infinitesimal volumes used, would single atom, ion or molecule measurement techniques be essential? In support of this, it has been estimated that in a liquid the volume of a cube with dimensions 100 nm by 100 nm by 100 nm, there are only ~6 analytes when the concentration of the analyte is 1 μm [160]. Would sample separation, pre-concentration and use of highly-sensitive detection techniques (e.g.,

sion of polymers in the UV, the channels were fitted with a quartz plate (**Figure 11b**).

**4.3. Millifluidic channels in 3D-printed chips for microplasmas**

shown in **Figure 12**.

**5. Nanofluidics**

although the name is [159, 160].

**5.1. The science of nanofluidics**

laser induced fluorescence or LIF) become essential?

**Figure 11.** (a) Postage stamp-sized polymeric 3D-chips and (b) microplasma formed between electrodes E1 and E2. Depending on operating conditions, microplasmas with diameters of (b) ~750 μm, (c) ~400 μm and (d) ~200 μm were formed. A 1 cent coin was included for size, the microplasma fit inside the letter a of the coin.

**Figure 12.** 3D printed microplasma on a hybrid 3D-chip formed between electrodes E1 and E2 (coin has been included for size, the microplasma fit inside the letter a of the 1 cent coin).

(important during testing). Once prototypes were produced, channel width was never revisited. Although polymeric substrates have high dielectric strength, to address poor transmission of polymers in the UV, the channels were fitted with a quartz plate (**Figure 11b**).
