I am reading an article in the current issue of The American Scientist. The subject is acoustic interaction with mud (on the ocean floor). Early on comes the following sentence:
“Marine sediments can generally be divided into two broad categories: granular or coarse-grained sediments (such as sand or gravel) in which the individual grains are held together by gravitational forces, and fine-grained sediments (muds) in which the grains are held together primarily by electrochemical forces.”
Is this possible? Is there any significant gravitational force at the scale of a grain of sand. Surely it is chemical bonds that hold that grain together, no? It seems to me that even at the size of an asteroid, the gravitational forces holding the body together are rather weak and easily disrupted.
I don’t think they mean that the pebble itself is held together by gravity. You’re right that is clearly nonsense. What they’re saying is that the pebbles are held in place in the pile by gravity. But smaller things (grains of sand) are held together by weak attractions between them. Id guess this is something like static electricity or hydrogen bonding in water molecules.
If you get a handful of sand or gravel and open your hand, it will all fall to the ground separately. The main thing holding the grains together is gravity. Mud on the other hand is sticky and will hold together despite gravity. That’s all they’re saying. They’re not, as far as I can tell, suggesting that grains of sand will stick together from their own minuscule gravity.
I suspect they are talking about the dominant forces that stop the unit from behaving like a fluid. Gravity, and thus the pressure from above as well as the individual grain’s weight along with the surface roughness makes large grains a solid. Very fine grains may not hold structure without more inter-particle forces.
ETA: Aimed at @Richard_Pearse before the wise and greatly esteemed @Francis_Vaughan snuck in.
Tl;dr: Ninja’ed.
'Zactly.
Is, or is not, the grain behavior of the bulk material dominated by inter-grain Van der Waals and static electric forces? If not, gravity (and in aqueous environments the surrounding water motion, surface tension, etc.) acting on each macro-lump (read “sand grain or larger pebble”) is all that’s left to explain how the bulk material behavior arises from the particulate constituents
Perhaps thinking “sinking” in place of “gravity”.
Really small things are more dominated by being sticky than by their weight. Flies can land on the ceiling. Smoke particles hardly care at all about gravity. We, on the other hand, hardly care at all about sticking to things.
I’m sure you’re right, but the way they worded it suggested my interpretation. The article was poorly written in other ways.
My initial reading of that was that the individual grains are held to each other by gravity or electrochemical forces, and I was really surprised by your question.
I spent the whole of Wednesday looking at gravel; this thread is my opportunity to mention the term ‘angle of repose’.
Loose materials like gravel form piles where the flanks cannot exceed a certain angle before they start to collapse or slide - the angle is different depending on the material properties such as shape and smoothness of the particles and the coefficient of friction of the surfaces of the grains.
Muds and clays and other fine-grained materials can be formed with steeper flanks, even if the grains are not permanently cemented together - for reasons others have mentioned above.
So if I dumped a couple buckets of sand out of my spaceship while in deep space, what would happen? My guess is that it would eventually clump together but I’m not sure if it could form sandstone or if it would remain a loose ball of sand forever? My instinct says the latter, but I’d never have expected something like vacuum welding for metal, so who knows…
For something like sand, the points of contact between the grains are fairly small - on earth, sandstone formation requires pressure to compact it, then cementation by new minerals, usually in solution in water.
A bucketful of sand in space would clump together and form a tiny ‘rubble heap’ type asteroid. It might become more coherent as it attracted additional particles of dust and maybe ices -depending on how warm it was; its quite a small mass so it might not get cold enough.
A bucketful of sand will eventually coalesce, but it’s never going to have more than an extremely tiny pressure in the center, astronomically far from the amount needed to adhere anything together.
And, for that matter, on astronomical scales, sandstone is just loose sand, too. The larger a body gets, the more relevant gravity becomes, and the less relevant material strength is. For practical purposes, all astronomical objects are basically liquid.
Liquid? Dont drop a 15km rock on my dinosaurs and tell me it’s raining!
Mostly, what killed the dinosaurs wasn’t the material that rained down - it’s the material that did not, and stayed in the atmosphere blocking the sun.