Bubble adherence region

In the adherence region the bubbles adhere to the electrode surface and grow. The typical thickness of this layer is the mean bubble diameter. During the growing phase if the bubbles have mechanical contact, they coalesce [111]. 

Figure 3.8 Schematic representation of the bubble layer structure (according to Janssen [67] and Boissonneau and Byrne [12], with kind permission from Springer Science+Business Media). Three regions axe identified the adherence region, the bubble diffusion region, and the bulk region.

Note that, in this description, the shadowing of the surface by the bubbles in the adherence region was not considered. With increasing bubble coverage fraction 6, the resistance in the diffusion region ff will also increase. A possible ansatz is to write (3.44) as ... 

Figure 3.12 Percolation model of the bubble adherence region the lateral active electrode surface ft is subdivided into a lattice of size L. Sites are occupied with a mean occupation probability p. Neighbouring bubbles are assumed to coalesce and build a new larger bubble [123].

The remainder of this section is dedicated to the description of percolation theory and how it can be applied to model the bubble adherence region. Providing a complete description of the percolation theory is beyond the scope of this book. Only the important concepts required for our purpose will be presented. The interested reader can find more information in specialised textbooks. For example, the book by Stauffer and Aharony [101] provides an excellent introduction to the subject. 

Table 3.5 summarises the main definitions from percolation theory and their interpretation in terms of bubbles in the adherence region. The main idea is,... 

Table 3.5 Main Definitions of the Percolation Model and Their Interpretation as Bubbles in the Adherence Region [123]...

The evolution of bubbles on a gas evolving electrode is a dynamic process. As described in previous sections, percolation theory can help to quantify the bubble size distribution in the adherence region if the mean bubble coverage fraction 9 is known. In order to derive an equation describing the evolution of 6 for gas evolving electrodes, we will write a mass balance equation between the amount of gas produced and the quantity of gas leaving the adherence region [125,129]. Faraday s law computes VG, the amount of gas produced per unit time at an electrode ... 

Figure 3.16 Geometry of two coalescing bubbles in the adherence region in the case of a (a) flat and (b) cylindrical electrode.

After the run, a gel slice containing the RNA region is cut out with a scalpel or razor blade. The gel slice is put into the electrophoresis cell perpendicularly to the long axis, and set into the cell in a manner similar to that described above. The new acrylamide solution (20%) is poured into the cell and allowed to polymerise around and below the 10% strip. Because the 20% gel adheres tenaciously to the Perspex apparatus, it was found necessary to coat with fluorocarbon both the slot former and a region of about 2 cm around the two sides and bottom of the coolant plates that are in direct contact with the gel. Unless the cell is treated in this way, it is very difficult to remove the slot former after polymerisation or to dismantle the apparatus after the run. While the 20% gel polymerises, coolant is circulated to prevent the accumulation of air bubbles between the gel and the plates. Electrophoresis in the second dimension is carried out as in the first, but over a period of 17 hr at the same voltage. 

In addition, the membrane is prone to develop salt blisters in (1) the gas zone in the anolyte compartment, (2) gasket areas, and (3) regions where gas bubbles adhere to the membrane. The term salt blister describes the deposition of salt in the membrane because of a localized increase in the salt concentration. Complete flooding of the electrolyzers and proper gasket assembly prevent the formation of salt blisters. Gas bubble adhesion is suppressed by pressuring the cell (to lower the void fraction and gas... 

Figure 2 shows fracture surfaces for bonds between zinc and rubber-toughened epoxy resin (see Toughened adhesives). In Fig. 2(a), there is a region of cohesive failure within the resin the sites of bubbles, which may have initiated the fracture, can be seen. In Fig. 2(b), a piece of resin is seen adhering to what appears to be the bare zinc substrate. 

---