Sticking Together. Steven Abbott
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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.
3.7 SOLDERING
Another form of liquid glue is molten solder. If you solder badly (as I have done rather too many times), the result is a pure surface adhesion bond that can look OK and which gives adequate strength if you give it a vertical pull, but which fails when there is a peel force. Good soldering is more than surface adhesion. First, it requires the use of a “flux” that dissolves and/or “reduces” the oxide contaminants on the surface to produce the native metal. Then the metals in the solder need to join up with the metal (typically copper) to create a metallic bond, i.e. where electrons are shared across all the metal atoms. This will still not provide a tough joint because the electrons are shared unequally across dissimilar metals and there is a clear boundary where stresses can concentrate and a crack can propagate. The trick is that the molten tin in the solder is able to dissolve (not melt!) a few of the surface atoms from the copper. This means that the boundary is tin/tin-copper/copper which, by not being a sharp boundary, is more able to cope with crack stresses.
Brazing is somewhat similar. Two high melting point components (usually steel) are separated by a narrow gap. Molten metal (typically copper) with a lower melting point is made to flow into the gap. On cooling there is a solid metal bond. As with solder, there is a need for a flux to remove the oxides from the metal surfaces. The use of lower melting point metals means that there is less thermal strain on the components, and the brazed area can be larger than a welded joint, leading to a more distributed set of stresses. The bonding metal is of higher melting point and modulus than the necessarily more gentle solder, resulting in a bond that is likely to be more durable than soldering.
3.8 WHAT HAVE WE LEARNED SO FAR?
If you are a gecko, surface energy adhesion is ideal. If you want repositionable hooks on your wall, surface energy adhesion can sometimes work. If you want to fix something at home and you aren't going to put unexpected peel loads onto it and if you can find a liquid glue that rapidly changes to a solid, then surface energy adhesion works OK.
If we recall that surface energies change, across most relevant materials, only by a factor of 2.5 across most relevant materials, then we cannot get a 10× increase by increasing the surface energy. This means that we need some very different science to help us. We will find that there are two very different approaches to strong adhesion, one of which relies on having weak adhesives!
Before we start exploring strong adhesion, we need to understand how to determine whether adhesion is strong or not. We already have a hint that this is not an easy matter. The two strong men found that the adhesion was strong, then the little girl showed that it wasn't.
In most adhesion science books, the question of measuring adhesion is kept until later, because it is assumed that measurement is something rather straightforward. My view is the other way around. We cannot understand what makes strong adhesion without understanding that there is no objective way to measure it! Very quickly we will find that the force needed to separate the same samples of pure rubber, held together by surface energy, can vary by more than 1000× depending on how the force is applied.
CHAPTER 4
How Stuck Is Stuck?
It seems very easy to know how well two things are stuck together. You measure the area of overlap then measure the force needed to pull them apart. Force divided by area in standard units is N m−2, also known as a Pascal, Pa. Because a Newton is rather small and a square metre is rather large, a Pa is small and adhesion values tend to be quoted in MPa or GPa.
Let us start with the system we introduced in the chapter above, with gecko-style, glueless, surface energy adhesion. Take two pieces of smooth rubber (rubber strips can be cast onto glass to produce super-smooth surfaces) and place them together so they spontaneously stick to each other.
We can pull them apart in three ways (Figure 4.1):
Pull up one end and peel the samples apart: Peel test
Pull along the join and try to shear them apart: Lap shear test
Attach something strong to the back of one piece and pull it up vertically while holding the lower piece in place: Butt test
Figure 4.1 The three types of adhesion tests: Peel, Shear and Butt. Even for the same material the force need to separate them differs by factors of 100s or 1000s.
Rubber typically has a surface energy