to represent those actually used in the study (see text). It can be seen that by stopping the etch at various times (9– 81 m are shown) a range of half angles can be produced. It is possible to produce essentially straight-sided conical pits, steeply curved pits or round-ended pits, but these geo-metries cannot be achieved independently of the half angle. By using a controlled, collimated radiation source, it is possible in principle to produce pits where the axis intersects the polymer surface at different angles, giving openings of different sizes and geometries (e.g., ellipses of various eccentricities), but this was not performed in the study referred to in the text.

doi:10.1201/9780203021262-156

Chapter

to represent those actually used in the study (see text). It can be seen that by stopping the etch at various times (9– 81 m are shown) a range of half angles can be produced. It is possible to produce essentially straight-sided conical pits, steeply curved pits or round-ended pits, but these geo-metries cannot be achieved independently of the half angle. By using a controlled, collimated radiation source, it is possible in principle to produce pits where the axis intersects the polymer surface at different angles, giving openings of different sizes and geometries (e.g., ellipses of various eccentricities), but this was not performed in the study referred to in the text.

Chapter

to represent those actually used in the study (see text). It can be seen that by stopping the etch at various times (9– 81 m are shown) a range of half angles can be produced. It is possible to produce essentially straight-sided conical pits, steeply curved pits or round-ended pits, but these geo-metries cannot be achieved independently of the half angle. By using a controlled, collimated radiation source, it is possible in principle to produce pits where the axis intersects the polymer surface at different angles, giving openings of different sizes and geometries (e.g., ellipses of various eccentricities), but this was not performed in the study referred to in the text.

ABSTRACT

Volanschi et al. [39] have performed elegant experiments with cavities of well-defined geometry. An inverted square pyramidal cavity of accurately known size (depth and halfangle) was made by etching silicon. Electrochem-ical bubbles were generated in this site, and were observed with an optical system giving a magnification of about X400. Overpotential and capacitance measurements were made, and this allowed studies of individual bubble events. The results led the authors to postulate what they call the “concen-trator” effect: essentially, that the concentration of liberated electrolytic gas species is greater at the base of the cavity than elsewhere. Given that such sites are already favored for nucleation, this added effect could go some way to explaining the much lower than expected supersaturation needed to cause electrolytic bubble nucleation.