  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.
