Life beyond Fritz: On the detachment of electrolytic bubbles

TL;DR

The paper addresses how electrolytic hydrogen bubbles detach from a horizontal electrode, revealing that contact-line hysteresis and dynamic wetting dominate detachment over classical Fritz predictions. By using a transparent, thin-film Pt or Ni electrode, the authors directly observe the contact line and employ the Young-Laplace equation together with a force balance to infer bubble volume and the footprint radius. They demonstrate that spreading versus pinned bubbles depends on acid concentration and surface roughness, with detachment governed by the advancing contact angle rather than a fixed Fritz radius, and they derive a predictive relation $R_{det} = \frac{3}{4}\lambda_c\bigl(\sqrt{2}\sin^2\theta_{rec}\sin\theta^*_\mathrm{adv}\bigr)^{1/3}$ that agrees with measurements. The results have practical implications for designing electrode surfaces and operating conditions to improve bubble removal and electrolysis efficiency, highlighting the limited role of purely electrostatic or Marangoni effects in determining detachment size in this system.

Abstract

We present an experimental study on detachment characteristics of hydrogen bubbles during electrolysis. Using a transparent (Pt or Ni) electrode enables us to directly observe the bubble contact line and bubble size. Based on these quantities we determine other parameters such as the contact angle and volume through solutions of the Young-Laplace equation. We observe bubbles without ('pinned bubbles') and with ('spreading bubbles') contact line spreading, and find that the latter mode becomes more prevalent if the concentration of HClO4 is greater than or equal to 0.1 M. The departure radius for spreading bubbles is found to drastically exceed the value predicted by the well-known formula of W. Fritz (Physik. Zeitschr. 1935, 36, 379-384) for this case. We show that this is related to the contact line hysteresis, which leads to pinning of the contact line after an initial spreading phase at the receding contact angle. The departure mode is then similar to a pinned bubble and occurs once the contact angle reaches the advancing contact angle of the surface. A prediction for the departure radius based on these findings is found to be consistent with the experimental data.

Figures (12)

• Figure 1: Example of a bubble shape obtained from the Young-Laplace (YL) equation (\ref{['eq:YL1']}) at $Bo = 0.3$.
• Figure 2: The concept sketches (insets) and the force evolution for pinned (a) and spreading (b) bubbles. The forces acting on the pinned bubble were computed for a contact radius ($R_{cont}$) of 2µm, while those for the spreading bubble were computed assuming a constant contact angle ($\theta$) of 20$\degree$. The corresponding bubble shapes (labelled by the numbers) including a zoom in on the foot region in the insets are shown in panels (c) (pinned case) and (d) (spreading case).
• Figure 3: The sketch of the experimental setup (a), typical experimental images of pinned (b) and spreading (c) bubbles, and development of the contact angle ($\theta$) for a spreading bubble obtained by different methods (d) are shown as function of time (bottom) and bubble size (top axis). The red half circle in (c) shows the contact area, and blue half circles on (b, c) represent the bubble size. The nucleation site of the pinned bubble is shown by an arrow. The bright areas with diffuse edges in the centre of the bubbles are caused by light passing through the bubble and reaching the camera.
• Figure 4: Atomic forced microscopy (AFM) measurements of a new (a) and used (b) electrode. The color code indicates the height of points on the surface relative to a reference line.
• Figure 5: (a) The spreading bubble fraction as function of the $\ce{HClO4}$ concentration of the electrolytes at new electrode (NE), used electrode (UE), and after roughening the new electrode (AR). (b,c) Dynamic contact angles of drops from liquids with varying acidity measured by sessile drop experiments on a new and a used electrode, respectively. The data point at molar concentration of 0 was measured in deionised water.