From my previous experience examining the physics of falling over, I’ve realised that the simplest things are often the hardest to explain.
This is an unfinished article – unfinished because I’m metaphorically “stuck in the mud”. I’ve been reading up what I can, but e.g. I’ve bought a book on “Soil Mechanics” which doesn’t even mention Mud in the index. Mud is a pretty ubiquitous thing – but it seems that just like physical mud, most scientists just try to avoid it altogether.
So, any comments or inspiration will be gratefully received in comments.
The physics of mud can be distinguished by many questions:
- Why is mud slippery
- Why do we slowly sink in mud but not water
- Why does water make loose soil stick together?
- Why does the removal of water then turn mud into solid?
- How can a solid flow
- Why is mud – mud coloured?
The specific question I have relates to why a patch of waterlogged ground will turn to mud. In science there are four states of matter (and two that do not matter).
Mud however does not fit any of these. It has some of the properties of a solid, in that small quantities can support their own weight. But it also behaves like a liquid. But ground is clearly “solid” – and waterlogged ground is still solid, at least until it is agitated, whereupon it can (but does not always) turn to mud.
The specific properties of mud are:
- … and capacity to dampen explosives (OK from this you might guess I got the list from a military book).
One of the big questions about mud – is why when soil is just small rocks – doesn’t something like sand turn to mud. A possible reply was found in this reply on Stackexchange by John Rennie:
There is a profound difference between sand and mud.
Sand is composed of grains of silicon dioxide, and these grains are relatively large and approximately round but with jagged facets. The grittiness of water/sand mixtures is due to physical interactions between these grains. At low volume fractions sand water dispersions have low viscosity, but at volume fractions of greater than around 0.5 interactions between the grains cause the dispersions to become dilatant.
On the other hand mud is composed largely of clays. The word clay is a generic term for a large class of aluminosilicates all of which have a layered crystal structure and many of which delaminate in water to form small and very thin platelike crystals. This has two consequences. Firstly a small amount of clay expands to take up a large amount of water, so clay dispersions are typically very viscous. Hence the gluely feel of clays. Secondly, under shear the plates tend to align and can slide over each other even at high volume fractions of clay. That is why clay is thick at low shear rates but can be deformed at high stresses – this property is known as shear thinning.
Both sand/water dispersions and clays are non-Newtonian fluids, but their behaviour under shear is essentially opposite.
This answer seems to explain the difference, but given the lack of any reference to evidence to support the answer I would wonder whether it was a knowledgeable person’s guess rather than the result of careful testing. My main concern, is that it would appear to me that mud moves because the water lubricates the particles – and that is as true of clay used by potters as it is of boot-sucking mud.
The sinking of person in mud is mostly due non-Newtonian behaviour of mud. i.e Mud can have very high viscosity initially when force is applied to it.