A good article by Una, that covered all of the basics thoroughly. My issue is with the third illustration; it doesn’t show mechanical locking, it simply shows two surfaces that have more surface area than the flat surfaces earlier in the article.
A better depiction of mechanical locking would show concavities, such that the adhesive, once solid, would not be able to be pulled from the surface without breaking. Essentially, the article shows sandpaper where it should be showing velcro.
Thank you for taking the time to read and comment on the article.
Generally you’re correct in what you say, but in the case of the article there are two things that need some clarification. The first was I was thinking of a joint in shear, not tension; and second I was thinking of the mechanical locking of the joint as a whole, not necessarily the adhesive via cohesive failure. Even with those clarifications I may still be incorrect in my labeling, but it’s what we called that sort of joint in my non-metallic materials graduate course, so that term and visual picture have always stuck with me.
I’m sorry I couldn’t see the pictures either so I may have the wrong end of the stick.
The description of the bonding of water sounds like hydrogen bonding rather than Van der Waal’s. Hydrogen bonding is dependent on the dipole produced the greater electronegativity of oxygen compared to hydrogen. The electrons in the bond prefer to spend more time closer to the oxygen. This is very much stronger than VdW. Van der Walls, as I understand it, results from the transient, very small dipoles caused by small changes in electron distribution and can be seen particularly with long chain hydrocarbons and similar molecules with a lot of carbon hydrogen bonds and many opportunities to have temporary dipoles.
I worked many years ago with a biologist called Terry Allen who had a great interest in adhesives in biological systems. He had done energy calculations on the structure of water when trapped near two hydrocarbon or other hydrophobic surfaces. It seems that water trapped between such surfaces assumes an “ice-like” structure. This should be energetically unfavourable at room temperature but in this situation it’s forced to assume it and difficult to disrupt leading to “stickiness”. I’m pretty sure he published on this mechanism for cell-cell and cell-matrix adhesion but I can’t find it right now.
The images are hosted on Una’s server for some reason, and it’s 404ing them. Perhaps it would be better to put them on the SDMB server next time, or GFDL/PD/CC them and put them on Wikimedia Commons.
The reason is because I do not have direct access to the Straight Dope server. The images were only to be temporarily hosted on my server, then moved to the Straight Dope server. Before the Staff had a chance to move them, the SDMB had its crash this weekend and people were too busy putting out other fires to move them. With luck, the files will be on the SD Server shortly.
Follow up question: Given the lack of complete understanding, what is it that chemists do when trying to invent better or newer adhesives? I am assuming they don’t resort to shotgun trial-and-error efforts?
