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We are familiar with the fact that some adhesives are thin and runny while others are thick and resistant to flow. A thick adhesive can be said to be “viscous” and the measure of the ease of flow is “viscosity”. If we define water as having a viscosity of 1 (the units are cP, which means centipoise) then some typical numbers for familiar materials are found in Table 2.1.

Table 2.1 Typical viscosity measurements of familiar materials.

Liquid	Viscosity, cP
Water	1
Olive oil	100
Glycerine	1000
Honey	5000
Ketchup	50 000
Lard	100 000
Peanut butter	250 000

Adhesives manufacturers have to worry incessantly about viscosity. Users can be equally as frustrated by an adhesive that is too viscous to flow nicely into a joint, as by one with such a low viscosity that it runs everywhere. Fortunately, there are some tricks they can play. For many viscous adhesives, if you apply a shearing or sliding force, their viscosity decreases rapidly (this is called “shear thinning”), allowing them to flow in the gap between the substrates. Others are “thixotropic” which means that they become thin when stirred or sheared (so they are also shear thinning), making them easy to apply, then, after some time, become thick again, stopping them from “slumping” out of a joint.

The study of how things flow is called rheology and interested readers can find a friendly (but technical) eBook and set of apps on my Practical Rheology web pages, https://www.stevenabbott.co.uk/practical-rheology/.

Even if the manufacturer has created the perfect viscosity, there is still the problem that squeezing a joint together is not as simple as it sounds.