The main finding of these experiments was that EZ size depends on the shape of the nucleating surface. We found that the smaller the bead, the smaller the EZ. This was seen whether the bead was of the hydroxide type or the chloride type ().
Surface curvature evidently impacts EZ size, and the question is why this is so. The EZ is a relatively more ordered zone of water adjacent to some nucleating surface. Envisioning growth of such an ordered zone adjacent to a flat surface is straightforward. However, convex surfaces imply ray-like divergence, and a possibility is that any such divergence has negative impact on size because the growing domains disconnect. That would explain why smaller curvature radii gave smaller EZ size. And it would also explain why flat surfaces produce EZs larger than those found here.
A question that arises is the EZ size value adjacent to smaller particles including those not easily measurable by ordinary optical microscopy. The EZ is a feature common to many if not all hydrophilic surfaces [4
]. If so, then they would also occur adjacent to very small particles and molecules. Generally the water-containing zones adjacent to such surfaces are considered as hydration and presumed to extend no more than several molecular layers from the surface. But the extensive EZs noted here and elsewhere imply that such ‘hydration’ layers could be larger and perhaps much larger.
The results obtained here cannot answer the question of what happens with small curvatures but they provide a clue. If the trend follows, then smaller scale nucleators should produce correspondingly small EZs. Extrapolating the graph of to zero radius gives a small but non-zero EZ size, although the zone of extrapolation would seem too large to be reliable.
On the other hand, indirect evidence implies that the extrapolation may be valid down at least to the micron scale. The data of show EZ values roughly half of the respective bead radius. For a 1-μm sphere, that would imply an EZ on the order of 0.5 μm. Experiments on Brownian dynamics [5
] demonstrate an EZ on the micron to sub-micron scale, hence within the range expected. Thus, the extrapolation of would seem to hold at least down to the micrometer scale. Whether it holds down to the molecular scale remains to be determined.
Notable features of the results are the multiple EZs, tails and swirls (–). These features were found with the hydroxide beads but not the chloride beads, implying that liberated protons might play a role [6
]. The multiple EZs might arise from proton abundance: if protons link micro-spheres by the like-likes-like mechanism [7
], then the linked microspheres form a gel-like entity, which could itself nucleate another EZ. Hence multiple EZs would appear with time. Such multiple zones have previously been seen adjacent to flat surfaces; hence they are not necessarily features associated with curvature. They are quite common.
Swirls formed from a hydroxide bead at 25 min.
The tails may arise as extensions of the EZ. Such extensions arise commonly in EZs adjacent to flat Nafion surfaces. The ordinary EZ typically reaches a terminal width, but continue their growth with time by projecting narrow extensions normal to the plane of the EZ. Such zones can project up to 1 m or farther [8
]. When even low-level flow is present parallel to the nucleating surface these extensions will deflect, yielding the swirls that were commonly seen in the hydroxide beads. Again, if the protons, or proton gradients, are responsible for any such flows, then this might explain why the swirls appear much more routinely with the hydroxide beads than with the chloride beads.
In conclusion, the results have shown that surfaces with smaller radii of curvature generate smaller EZs. EZs are by no means constant in size. They depend on various factors, not the least of which is the characteristic dimension of the nucleating surface.