This is a follow-up question to a parallel thread “What keeps epon sections on copper grids”, in which it was mentioned many times that sections are hydrophobic. How so? As I understand, if sections were hydrophobic, they should try to detach themselves from the water surface, curve and float in such a way that to have minimal contact with water. In reality we see perfect wetting of a freshly cut surface of a section: in full contact with water, the section is floating nicely and adheres to the hydrophilic surface of a grid.
There is no doubt about the hydrophobocity of the ultra-thin epoxy sections! That's the reason why the sections float on water surface instead of going down. For instance, even formvar is hydrophobic and we make its film to float on water surface to coat the grids. However, I don't know the physics involved in floating of hydrophobic substances on water surface!
Dear Vladimir, dear Karumanchi, I think this is really a complicated question due to the fact (IMHO) that we are mixing up properties of "resins" [which can be hydrophilic, i.e. meaning they collect water and do swell for example (e.g. polymer foams), and hydrophobic, which means , Epoxide resins are ...on the other hand "properties on water surfaces" are described or taken into consideration...so this might be not the same, I guess... (comparison en gros: Steel (naturally hydrophobic I guess) like from a ship is NOT at all but nevertheless it water and will "swim" (due to another natural force than surface tension...) . As for now I am thinking some day about this and let you know what my opinion with this is...(:-)), best regards and wishes
While mixing with water is connected with hydrophilic properties, it is not important for many solid materials. For example, many metals are hydrophilic but they do not absorb water. Classical test for hydrophilicity/hydrophobicity is a measurement of a contact angle, i.e. measurement of an angle between surface of water droplet and surface of solid specimen. If an angle is greater than 90 degrees than surface is hydrophobic. So, I put a droplet of water on my old resin block and took a look at it with my near naked eyes (I had my glasses on). This near scientific experiment convinced me that the angle was a bit less than 90 degrees, so block surface was a bit hydrophilic. It explains why we have sections floating in our boats: hydrophilicity is good enough for wetting of a bottom surface of a section (so it can spread on the water) and not good enough to let water go on the upper surface (contact angle should be really small to let water on the top, around an edge of a section).
Dear Vlad,
you are right as usual. I wanted to leave a little bit "secret" to my thoughts -which included also thinking of the "contact angle" of a water droplet on surfaces. As you certainly know (and I have faced sometimes such problem), especially in older epoxy-EPON-resin blocks - depending on the quality of embedding (infiltration as well as polymerization) it is possible that there exosts a "sucking" of water into the block surface and below....this merely perhaps an effect of poor polymerization and maybe also micro-cracks....an effect which is tricky to overcome when one has to section again such blocks (wetting of the block surface when touching the knife edge and therefore not able to get good sections - at least most of them not free of wrinkles/wrinkle formation). Thanks for your posting! Best regards and wishes, Wolfgang
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