Sticking Together. Steven Abbott

a couple of pieces of rubber attached to my hands I could cling, gecko-like to the window of the lecture theatre. My request to do this was turned down: “It's not that we're against science demos; it's just that we don't fancy the headline ‘Mad professor falls from 14th floor window trying to be a gecko’”.

3.2 WHY DON'T WE HAVE LOTS OF GECKO ADHESIVE TAPES? ACTUALLY, WE DO

      When the first scientific studies of gecko adhesion revealed the simple elegance of the mechanism there was a rush of popular articles (they still appear from time to time) saying that soon we would have gecko tapes giving us “chemical free” adhesion. A moment's thought would have saved a lot of wasted research effort.

      Most of us, most of the time, do not want an adhesive that is trivially broken with a crack or a pull in the right direction. Most of us, most of the time, do not have super-smooth, super-clean surfaces to stick to, so pure surface energy will not work very well.

      https://youtu.be/kiFDCfR817Y An example of the dangers of this type of adhesion is the Two Strong Men video which I made many years ago, but is still striking. It shows that two strong men (we weren't acting, we were genuinely trying our hardest) cannot pull apart two smooth sheets of rubber. Yet a young girl can peel them apart with no effort.

      The really clever part about the gecko's foot is the hierarchy of mechanisms to ensure good contact: toe, lamellae, setae, spatulas. Any piece of gecko tape which lacked this hierarchy might be OK on glass but relatively useless on most non-smooth surfaces.

      I once argued with a famous gecko adhesive scientist, saying that even if you could make a gecko tape (which happens to be a difficult challenge) it would be of little use. To my surprise he entirely agreed with me: “But we have found that there are some medical applications where they are willing to pay the high price because they require both the strengths and the limitations of our tape”. I was delighted with his reply.

      In general it is impossible to get good surface energy contact between surfaces, so gecko-style adhesion is relatively rare. One example is called, in my home, cling film; Wikipedia tells us that it is called variously “plastic wrap, cling film, shrink wrap, Saran wrap, cling wrap, food wrap, or pliofilm”. These films of polyvinylchloride, polyvinylidene chloride or, more usually these days, PE, are super smooth and cling together via pure surface energy. You can easily get a model gecko (or, in my case, Spider-Man) to stick to a pane of glass via a little rectangle of cling film which in turn (this is cheating, of course) is stuck to the gecko's foot (Spider-Man's hand) via a cocktail stick (Figure 3.3).

      Figure 3.3 Spider-Man clinging on with 10 cm² of clingfilm.

      https:/youtu.be/zAG2CICZT-s

      https://youtu.be/dxD9cXtI2nc

      Cling film provides a lot of science in a simple format. If you take a sheet and squeeze it carefully against another sheet, you find that the adhesion is quite impressive. But if you make the slightest mistake (as I usually do) and get a wrinkle, the effective adhesion reduces considerably. A bit of dirt or a crumb of food reduces the adhesion still further. In the clingfilm video we measure the peel strength from pure surface energy using cling film, though I would have got a more accurate number if I had eliminated the static that added some extra attraction.

      A rather more serious variant of cling film has a formal name “self-amalgamating tape” and a number of informal ones, including “F4 tape”. Why F4? One of the more successful fighter jets in history was the F4 Phantom. Missions are very tough on aircraft and they would often need a temporary fix to get the plane back in the air quickly. Some cables might need to be bundled back together and insulated, or a leaking pipe might need to be fixed. For many of these jobs, duct tape was not the right thing (even if much of the rest of the plane was held together with it). The tape that did the job for the F4 (and many others, but the name remained) was a smooth, relatively soft rubber or silicone tape with a rough release liner. You wrapped the tape around the area of interest and, because it was smooth, it stuck together instantly. If you made a mistake you could easily peel it back and try again. It was easy to smooth out air bubbles and wrinkles. So far it is just a soft cling film. The trick was that the rubber or silicone had some low molecular weight polymer that could slowly migrate through the system, causing the boundaries to blur and to make adhesion perfect because there was no longer a crack to fail. Being soft it could withstand vibrations and the stresses and strains of active duty.

      When it was time to do a full repair, the F4 tape, which had withstood tough combat missions, could be carefully cut along its length and easily peeled off.

      Another example of surface energy adhesion that many of us use are some types of hooks that stick to relatively smooth surfaces such as glass, tiles or smooth paint and can then cope with loads of 1–2 kg yet are readily removed and re-stuck. Although they really can hold 1–2 kg, you need to pre-clean the surface carefully and have to attach heavy loads delicately, otherwise the sudden jolt creates a peel force. As we shall see, the resistance to the peel force (peeling away from the wall) is much lower than the shear (if the hook is on a wall) or butt pull (hook on the ceiling), so the peel allows you to easily remove and, in principle, reposition the hook. The problem with these gecko-style hooks is that the slightest bit of dirt between the two surfaces creates a crack path and most of the adhesion is lost. If they fail for any reason, such as a sudden shock or vibration, then they fail completely. If you read the feedback pages of a large online store you find equal numbers of “these are wonderful” and “these are terrible” stories, with the latter revealing examples of, say, holiday wreathes completely smashed when the hook's adhesion suddenly failed.

      An alternative removable hook system is a variation on pressure sensitive adhesive tapes so is discussed in Chapter 6. Although the mechanism is very different, the online world is equally divided. When they work they work, when they fail they fail suddenly, which in a way is not surprising because they are designed to be easy to remove.

      There is a further class of surface-energy-only adhesives that we throw away without a second thought, even though creating them is hugely difficult. These are the protective laminates we see on a new smartphone screen or many smooth shiny parts of other electronic devices such as TVs. Often, they are just a thin film of smooth, low density (meaning “not too crystalline”) polyethylene. By being smooth they make good surface contact and the adhesion can be adequate. A standard low density PE is too crystalline to be able to adapt to small degrees of surface roughness, so its adhesion is too unreliable. By deliberately adding defects (carbon side chains) to the main polymer chain the PE can go from low density to ultra-low density which has very low crystallinity and is more forgiving to the surface, allowing higher adhesion. What if you need even higher adhesion? No problem: throw in a non-polymer that makes the polymer structure even weaker, providing even better contact with the surface, generating strong adhesion. The additive plasticizes the polymer, making it more flowable. Hopefully you can see the potential downside to these low-grade polymers with plasticizing additives: there is the risk of leaving a residue on the surface.

      For these protective laminates, finding the right balance between good adhesion and leaving a residue is not so hard in a lab. The real world is much harsher. A protective laminate might sit on a surface for a year and during that time some of the junk in the polymer might accumulate at the surface, leaving a residue and an angry customer. It is a frustration for everyone who creates or requires protective laminates that the right balance of clarity, cost, adhesion and long-term resistance to leaving a residue is so hard to find. Customers never notice the 99% of the laminates that do their job perfectly and get irate with the 1% of laminates that have come loose, are too