Читать книгу Sticking Together - Steven Abbott - Страница 32

One of the most enduring of adhesion myths is that you should roughen your surface before use to promote adhesion. We have already learned that this is wrong because surface energy adhesion is best when two smooth surfaces come together as intimately as possible. The more you roughen the interface, the harder it gets to ensure intimate contact. Yet the defence of roughening relies on surface energy; it says that if you roughen, you get a larger surface and therefore more surface energy adhesion. You also, they say, get mechanical interlocking between the surfaces, which sounds awesomely helpful.

Both the extra surface area and mechanical interlocking ideas are wrong in an interesting manner. Let's look at a typical rough surface measured by sliding a diamond stylus across the surface and measuring how its tip goes up and down (Figure 3.8). Although the trace in the figure is a simulation from one of my apps, it is realistic and instantly recognized by anyone who has measured a rough surface using this stylus technique.

Figure 3.8 A typical output from a surface roughness measurement device. It looks an amazingly rough surface!

That looks like (and in terms of typical surfaces, actually is) an amazingly rough surface and you can imagine how the adhesive would love to cover all that vast extra surface area. It also suggests how you might create mechanical interlocking between surfaces – imagine the peaks of the top surface stuck down into the valleys of the bottom surface.

To work out how mountainous the surface is, imagine an ant that has to crawl along the smooth equivalent of this surface, along a straight line from left to right. It will crawl exactly 12.5 mm (12 500 µm), the standard scanning distance used for these probes. Now let it walk up and down those mountain peaks and valleys, using ant GPS to measure how far it drags its tired feet. Because I wrote the app that created that image I can look inside and get the exact value. It is 12 506.5 µm, meaning that the ant has only travelled 0.05% further.

We can make sense of this if we look more carefully at that roughness curve. It looks amazingly rough when everything is squashed up – 12 500 µm along the x-axis against 6 µm on the y-axis. As we gradually magnify things along the x-axis, to 1250 µm then to 125 µm and then to slightly more than 12.5 µm we see that the surface is really rather gentle (Figure 3.9).

Figure 3.9 As you expand the scale in the x-direction you see that the roughness is a myth – it is actually a remarkably smooth, gently sloping surface.

So the reason that surface roughening does not help give extra adhesion via extra surface area is that there is no (significant) extra surface area. Similarly, it cannot create mechanical interlocking. If I take the (still exaggerated) graph at 125 µm and invert part of it, clearly those surfaces aren't going to be locked together (Figure 3.10).

Figure 3.10 These two rough surfaces cannot mechanically interlock.

Given that roughening a surface does not help adhesion via extra surface area or interlocking, why is surface roughening so often recommended? Partly out of habit from the myth, partly because a light roughening causes no harm and because many surfaces contain a lot of unstable junk which isn't very well adhered. The junk might be oil, dirt, surface damaged by UV light or maybe some weird hydrated oxides on metals. Rubbing the surface to return it to a clean, solid condition allows the normal surface energy processes to work.

There is one clear exception to my statement that mechanical interlocking is useless. If you apply an adhesive to paper, cardboard, cloth or wood, and if you allow the adhesive to flow the necessary multi-µm lengths to wrap around the fibres, and if the adhesive has significant mechanical strength, then you really can get mechanical interlocking (Figure 3.11). Adhesion science for these systems is much more about getting the flow and mechanical properties right than it is about how things, in general, stick to each other.

Figure 3.11 Mechanical interlocking when the adhesive can flow into and wrap around fibres.