Electrochemical Bubble Generation

Fiquid pumping or dispensing was achieved on a Si-Pyrex chip based on electrochemical generation of gas bubbles (H2 and 02) at Pt electrodes [309,391]. To ensure accuracy in the dispensing volume, an AC impedance measurement (at 200 kHz) of gas volume using interdigital electrodes was adopted. This method is 

Microfluidic Lab-on-a-chip for Chemical/Biological Analysis and Discovery 

FIGURE 3.4 CCD images of 100 pM rhodamine B in water obtained at the intersection of a side-arm channel with a main channel on a microchip, applying a positive potential at the top channel relative to the side arm and using (a) all native glass surfaces, and (b) a native glass main channel and a linear polyacrylamide surface coated side-arm channel [816]. Reprinted with permission from the American Chemical Society. 

FIGURE 3.5 Electropherograms showing the separation of a cation (TRITC-Lys-Lys), a neutral (TRITC-Aig), and an anion (TRITC-Gly) at detection points 0.05 cm down each arm of the T-intersection [387]. Reprinted with permission from the American Chemical Society. 

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FIGURE 3.8 Proposed dual liquid dosing system layout, geometry, and electrical connections based on electrochemical bubble generation [309]. Reprinted with permission from the Institute of Physics Publishing. 

In the Westinghouse electrochemical step, hydrogen is released at the cathode interface. In the numerical study, bubble generation is assumed to be localized in the first row of fluid cells neighbouring the electrode. In this special zone analogue to a boundary layer, the rate of gas production is assumed equal to the rate of the reduction process. It is modelled by the source term S2 of the dispersed phase mass-balance equation, assuming a 100% Faradic yield. 

The experiment done by Kellogg [70] in 1949 confirms these values. He describes how the gas film surface behaves like a vibrating structure (see Fig. 4.11). Kellogg also stressed that once the gas film is formed, no further bubble generation can be observed. He concluded that the electrochemical reactions must now take place at the gas-electrolyte interface. 

Nonmechanical pumping. Micropumps in this class are usually continuous and include the use of effects such as electrochemical displacement (bubble generation), thermal expansion, electrohydrodynamics, capillarity, and evaporation forces. The most commonly used nonmechanical pumping method is based on electrokinetic flow. In comparison with mechanical micropumps, field-induced flow is advantageous as it acts as both a valve and a pump, enabling both the direction and the magnitude of the flow to be controlled. 

In the cathodic scan in cyclic voltammetry, gas bubble generation was seen around 0.9 and 0.2 V, and the cathodic current increased simultaneously. This gas generation was considered to be Hj generation by electrochemical reduction of moisture in the bath. 

Porosity through thin dielectric films on metallic substrates may be measured by corrosion (liquid gas), selective chemical dissolution (electrographic printing - solution analysis), electrochemical decoration, anodic current measurement, gas bubble generation (electrolytic), liquid crystal (electric field) effects, and absorption (dyes - liquid or gaseous radioactive material). 

Whilst there may be different origins for the variety of these effects, one well-characterised consequence of ultrasonic irradiation is the generation and subsequent collapse of cavitation bubbles within the electrolyte medium and near to the electrode surface of the electrochemical cell. The electrode surface causes asymmetrical collapse of a bubble which in turn leads to the formation of a high velocity jet of liquid... 

Electrochemically generated bubbles were also used as microfluidic valves (see Figure 3.32). The valves closed when the bubbles inflated, and vice versa [458]. 

Electrochemical generation of gas bubbles has also been employed for dosing precise nanoliter amounts of liquid [391]. 

A final example of electrochemical kinetics will consider a return of the Pt WE from before but now exposed to a neutral (pH 7.2) solution into which oxygen is bubbled. The kinetics of the oxygen reduction reaction (ORR) will be studied. The data generated might appear as shown in Fig. 24. The reversible potential for the ORR in pH 7 solution, according to the Nernst equation, is... 

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