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We have to agree on a few measurement units and technical terms. Most readers will be familiar with them as they aren't too exotic.

For length we will use metres, m, millimetres, mm, micrometres, µm and nanometres, nm, each 1000× smaller than the previous. The unit of centimetres, cm, doesn't fit into that nice scheme but is so common that it has to be included.

For time we will go down to µs, ns and ps, micro, nano and picoseconds, one millionth, billionth and thousand billionth of a second.

For weights and loads we will use kilograms, kg, grams, g and Newtons, N, which, if a weight is involved is just weight times gravity. For our purposes, gravity is 10 m s⁻¹ s⁻¹ so 1 kg is 10 N.

Force per unit area has the units of N m⁻² which is also expressed as Pascals, Pa (Figure 2.4). One Pa is rather small, so we often have kPa, MPa and GPa for kilo, mega and giga, 1000, 1 million, 1 billion Pascals.

Figure 2.4 Force, in N, is applied over an area in m². 1 N m⁻² is called a Pascal, Pa.

If you pull, say, a piece of plastic which has a cross-sectional area of A with a force F, then you will get a fractional increase of length (the change in length divided by the original length), ε (Figure 2.5). The force per unit area is the stress in Pa. The fractional increase in length, strain, has no units. If you divide stress by strain you get modulus which gives an idea of the strength of the material. Modulus is also measured in Pa, as strain is dimensionless. Dividing by a small number gives a larger number, so the smaller ε for a given stress, the larger the modulus, which makes sense because a stronger material will stretch less. The modulus of typical polymers lies in the 1–4 GPa range, steel is 200 GPa. We will find later in the book that adhesion doesn't necessarily rely on brute strength; if the adhesive on a strong household tape has a modulus greater than 0.3 MPa, it does not work (it has no stickiness) – this really is strength through (a special type of) weakness.

It is unfortunate that the words stress and strain start with the same three letters and in common language mean the same thing, but we are stuck with the terms and you just have to get used to remembering which is which.

Figure 2.5 When we apply a Stress, Force/Area (F/A) in Pa we get an elongation, ΔL of the original length L. Their ratio is ε, which is the Strain, which is unitless. The ability to resist stresses is the Modulus, Stress/Strain, also in Pa.

We often have to discuss work of adhesion and surface energy, each of which is measured as Joules per square metre, J m⁻². It happens that work is the same as energy, which is why the two measures have the same units and why “work of adhesion” is sometimes called “energy of adhesion”.

The strength of an adhesive joint is often measured in force per unit length, i.e. N m⁻¹. If you sit down and do the sums (or if you go to my app page https://www.stevenabbott.co.uk/practical-adhesion/basics.php) you will find that a peel strength of 1 N m⁻¹ is the same as a work of adhesion of 1 J m⁻² (Figure 2.6).

Figure 2.6 Classically, adhesion is measured as the force, N, per width, m in N m⁻¹, or as work, J, to create 1 m² of separated adhesive, in J m⁻². The two measures are exactly the same!

It is tricky to know whether to add a space between a number and a unit. Some prefer 1N m⁻¹, others prefer 1 N m⁻¹. For clarity and readability I have standardized on using the space.