The physics of mud (it's a load of *&%$!!)

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:

  1. Why is mud slippery
  2. Why do we slowly sink in mud but not water
  3. Why does water make loose soil stick together?
  4. Why does the removal of water then turn mud into solid?
  5. How can a solid flow
  6. 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).

  • Solid
  • Liquid
  • Gas
  • Plasma

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:

  • Softness
  • Adhesiveness
  • Slipperyness
  • Churability
  • Suction
  • … 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.

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5 Responses to The physics of mud (it's a load of *&%$!!)

  1. golf charlie says:

    Look up Atterberg Limits!
    Clay is made up of such small particles that each one can have a film of water around it, and it can be squeezed and shaped, ie it is “plastic” or mouldable.
    The stickiness of mud has more to do with particle size.
    It is also why clay expands and contracts, depending on water content.
    As it dries, it ceases to be mouldable, and becomes crumbly. That is its Plastic Limit.
    As it becomes wetter, it will move as a liquid. That is its Liquid Limit

  2. Phil Rae says:

    The comment about clays is accurate and this is a reasonable place to start if you want to differentiate “mud” from “dirty water”. Many soils and sedimentary rock formations contain significant amounts of clays. The most common swelling clay is bentonite or smectite (sodium montmorillonite) and this is the clay generally responsible for giving mud its cohesive but slippery texture. There are many natural and/or synthetic clays that form gels (liquid suspensions with cohesive properties) and they are widely used as suspending agents in many applications. Suspensions of bentonite exhibit thixotropy – they are thin and fluid when moved of shaken but will thicken if left quiescent and may ultimately gel to become a rigid material. However, generally, when re-sheared, the “rigid gel” will typically break and become fluid again.

    • Scottish-Sceptic says:

      Wow! I never expected that there would be so many long words that I don’t understand in a reply about mud! “montmorillonite”? – I can’t even pronounce it! I think I’m right that despite so many chemical names, you’re not implying any chemical reaction (except perhaps salts going into solution – and weird surface behaviour).
      What caused me to start researching the subject was that I just couldn’t understand how mud formed from soil – and that I presume was because I was just imagining it as much smaller sand particles.
      I guess from your response that I should be thinking of them as being a gel – with larger particle suspended in the gel. I can visualise this to some extent by thinking about the boulder flows I’ve seen on Youtube – but instead of having a massive amount of energy to keep it suspended – it’s slight agitation needed.
      I also guess the prime mechanism for “fluidisation” (I don’t know the correct term) is the application of large amounts of shear stress, which forces the particles to move relative to each other and allows water to come between them as they break apart transitioning from a “pile” of particles lying on each other to presumably “floating” particles largely cushioned from each other by a lubricating layer of water.

  3. golf charlie says:

    A “clay” is made up of very small particles, each of which attracts a thin film of water.
    If you put a mixture of gravel and soil in a bottle with water and shake it up, the larger particles will settle out first, and the smallest last. Over time, much the same happens with a muddy track or footpath.
    A bit of gravel, whether wet or dry, will not stick to your tyre or welly. Sand may stick briefly, but wet silts, clays, “muds” will.
    You can “squeeze” a handful of sand underwater, but out of water it will crumble. Damp clay soil can be squeezed. A small piece of clay can be rubbed between thumb and forefinger, without feeling gritty.
    You can add as much water as you like to sand, and it won’t flow like a liquid, and self-level. When clay is wet enough, it reaches its Liquid Limit, and will start to flow as a liquid.

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