A bubble curtain model applied in chlorate electrolysis

Equation (4.12) explains how the last column of Table 3 is calculated. The maximum solubility S is
calculated before in Table 2.
P H2 . S = c saturation (4.12)
mol H 2
[bar] = mol H 2
(4.13)
m 3 E,bar H 2 m 3
Table 3 shows that both high saturation pressures and high hydrogen concentrations are needed to
generate bubbles of micrometer size or smaller. The difference in hydrostatic pressure only gives a small
difference of the internal pressure of the bubble. The last column shows the required saturation pressure
of hydrogen in the liquid. A bubble will continue to grow until its internal pressure equals the saturation
pressure. When a full grown bubble moves away from the electrode to places with lower hydrogen
concentration its internal pressure will become too high and the bubble will get smaller by diffusion or
absorption of hydrogen back into the electrolyte until equilibrium by the Young-Laplace equation is found.
If the electrolyte is saturated with hydrogen near the cathode, bubble formation will start at nucleation
points at the electrode surface. [43] This process is called heterogeneous nucleation. It can be
considered as a surface catalysed or assisted nucleation surface. Homogeneous nucleation occurs only
inside the bulk without a surface near and it requires a higher concentration of hydrogen.
The nucleation point is an irregularity; generally a cavity on the cathode surface. Already existing bubbles
can also act as a surface. The smaller the radius of a bubble, the higher the pressure inside must be. For
this reason bubble formation will occur in cavities. The radius here will be much larger than it would be at
a flat surface (see Figure 3). The moment a bubble detaches from the cavity into the bulk, the radius
decreases and the internal pressure increases a lot.
Figure 3: Two bubbles with an equal volume. The bubble in the cavity has a larger radius than the spherical bubble.
Figure 3 shows two bubbles with the same volume. The one still attached to the nucleation point has a
larger radius and therefore also a lower internal pressure and can therefore be formed with lower
saturation concentrations.
Another property that will influence the nucleation is the wetting angle θ (Figure 4). It is determined by the
resultant between adhesive and cohesive forces. A higher wetting angle gives larger radius of the bubble.
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