https://www.reddit.com/r/askscience/comments/1986qiq/is_sand_a_liquid/
created by OldContribution4677 on 16/01/2024 at 16:19 UTC
0 upvotes, 15 top-level comments (showing 15)
It takes the shape of its container?
Comment by agate_ at 16/01/2024 at 17:39 UTC
278 upvotes, 5 direct replies
No. The most important way that sand isn't a liquid is that you can make a pile of it. It *doesn't* always take the shape of its container, a small amount will form a self-supporting hill. As you add more that hill gets bigger but keeps the same steepness ( "angle of repose"[1] ). You can't make a pile of a liquid: given enough time, even the thickest and most viscous[2] liquid will have a flat surface on top.
1: https://en.wikipedia.org/wiki/Angle_of_repose
2: https://www.merriam-webster.com/dictionary/viscous
Technically, we say that granular materials like sand have "static shear strength[3]" while liquids do not: when subjected to forces that try to slide part of the material past another part, granular materials can stay still, but liquids always move.
3: https://en.wikipedia.org/wiki/Shear_stress
Comment by DAS_FUN_POLICE at 16/01/2024 at 17:29 UTC
44 upvotes, 1 direct replies
If I remember correctly from Engineering School the technical definition of a fluid vs solid is a solid will resist a shear force and a fluid will not, it will only resist the rate of shear which is also know as viscosity. Sand will resist a shear force (not as well as other materials) so there for it would meet that definition of a solid.
Comment by Chemomechanics at 16/01/2024 at 18:24 UTC
39 upvotes, 0 direct replies
It is a granular solid[1] that could be reasonably **modeled** as a complex fluid[2] or even as an ideal liquid **under certain scenarios and circumstances**, but distinctions exist. Sand is not a liquid.
1: https://en.wikipedia.org/wiki/Granular_material
2: https://en.wikipedia.org/wiki/Complex_fluid
Comment by CrustalTrudger at 16/01/2024 at 17:20 UTC*
52 upvotes, 1 direct replies
So I'll definitely defer to any material scientists etc. who want to weigh in, but broadly, sand would not meet the normal or complete definitions of a liquid[1], but when thinking about moving sand (or other granular material), it's common to treat it as a fluid[2], e.g., a random assortment of papers discussing the modelling of granular materials as complex fluids - 1[3], 2[4], 3[5], 4[6], etc. While in common, non-technical language we tend to think of "fluid" being synonymous with "liquid", in terms of material science / physics these terms are not equivalent.
1: https://en.wikipedia.org/wiki/Liquid
2: https://en.wikipedia.org/wiki/Fluid
3: https://www.sciencedirect.com/science/article/abs/pii/S0020740321002435
4: https://www.nature.com/articles/s41467-020-15263-3
5: https://www.nature.com/articles/nature24062
6: https://www.sciencedirect.com/science/article/abs/pii/S0301932222002452
Comment by regular_modern_girl at 16/01/2024 at 21:44 UTC*
11 upvotes, 0 direct replies
No, sand and similar substances like powders have a specific classification in physics; granular solids. They *do* have some properties of liquids (like conforming to the bottom of confined volumes in gravity, and flowing), as well as gases (they can also fly around the entirety of a confined volume) and conventional solids (they can become packed together into a rigid, non-flowing mass, reminiscent of an amorphous solid). They’re kind of their own thing, and there’s a whole area of physics that deals with them specifically, called “granular mechanics”.
Granular solids have actually historically been a headache to a lot of physicists and engineers, to the point where it is often joked that granular mechanics is a “harder” area of physics than even general relativity or quantum mechanics. As far as Newtonian classical systems go, it is true that, in spite of their mundanity, granular solids remain relatively difficult to model mathematically. They obviously can’t be treated exactly the same as a conventional bulk solid, even an amorphous one like glass, and they can’t be treated like conventional fluids either because they are composed of small but macroscopic, heterogeneous particles thoroughly limited to classical behavior, rather than as quantum-scale homogenous particles. This requires in most cases that granular materials be modeled as gigantic many-body systems, which is really taxing for computer simulations and really just mathematical models in general, from my understanding.
There are also a number of examples of physical behavior in granular systems that is more or less unique to them, and is only recently being better understood, like the phenomenon of granular convection, better known as the “Brazil nut effect”, as it can be readily observed in jars of mixed nuts. In a container of mixed nuts (or any equivalent granular solid with a wide range of grain sizes) you’ll notice that the bigger (and thus we’d assume, more massive) nuts seem to end up near the top of the container when shaken, whereas the smaller (and thus, less massive) nuts end up on the bottom, which goes against intuitions about how gravity would affect these systems, but somewhat recent computer simulations have found that this occurs because the larger grains (or nuts) tend to end up getting stuck together when the granular solid is perturbed, forming a rigid “sieve” through which the smaller particles can slip through the gaps of and fall to the bottom of the container, creating that distinctive “mixed nut” distribution of particles.
Another really interesting and beautiful granular phenomenon at the intersection with acoustics is Chladni figures[1].
1: https://publicdomainreview.org/collection/chladni-figures-1787/
It’s a surprisingly fascinating area of study, I’d encourage others to look more into it.
Comment by IWasSayingBoourner at 16/01/2024 at 20:35 UTC
8 upvotes, 0 direct replies
You seem to be confusing or conflating "liquid" and "fluid". While sand does exhibit certain fluid-like behaviors under certain conditions, it is actually neither. Sand forms hills and dunes in its fully settled state vs. gravity, and has static shearing properties not exhibited by true fluids or liquids.
Comment by Cheetahs_never_win at 16/01/2024 at 18:46 UTC
7 upvotes, 1 direct replies
What defines a liquid is its tendency to deform continuously under a continuous shear force.
If you take a bucket of (dry) sand and drop it, it'll form a mound. Fun fact: The angle of the mound relative to level ground is called the angle of repose.
Gravity is acting on it continously, but it no longer deforms.
We will "flow" sand by
While in a slurry forum, it can act like a liquid, but it's really just a bunch of solids hanging around inside a gel or paste.
It's worth noting that with a little water added, sand acts more solid than sand is.
By that token, toothpaste, paint, etc aren't strictly liquid, either, but a mishmash of liquid and solids, too, taking properties of both.
Comment by [deleted] at 16/01/2024 at 17:53 UTC
11 upvotes, 0 direct replies
No. Sand is just a whole bunch of small grains of some broken down rock /mineral (quartz for example) which are solids. It has interesting properties such as flow but sand is as much of a liquid as a pile of gravel is.
Comment by Not_an_okama at 16/01/2024 at 23:29 UTC
2 upvotes, 0 direct replies
In Brandon Sanderson’s “tress and the emerald sea” oceans are made of granulated solids that are suspended by air vents below the surface. The seas periodically settle when the air vents pause leaving a surface that can be walked on. Thought it would be neat to bring up with all the comments on granular solids being treated as liquids under certain circumstances.
Comment by VeryHungryDogarpilar at 17/01/2024 at 01:40 UTC
3 upvotes, 0 direct replies
All physical objects will take the shape of its container if you have enough of them and a big enough container. Are bricks a liquid? No, but if you fill up a shipping container full of bricks, the bricks take the shape of the container.
Comment by defyKnowing at 16/01/2024 at 17:34 UTC
2 upvotes, 0 direct replies
Sand behaves like a fluid because it's made up of fine particles, but the individual grains are solid. Technically, both liquids and gasses can be considered fluids due to the way they flow.
Technically speaking, everything has a rate of flow, from water all the way to mountains, and even people. There's a whole field of study about it, but idr the name
Comment by [deleted] at 16/01/2024 at 17:36 UTC
-4 upvotes, 1 direct replies
[removed]
Comment by 6strings10holes at 17/01/2024 at 01:36 UTC
1 upvotes, 0 direct replies
The grains are solid. A bunch of dry grains together are a mixture of sand and air. Mixtures don't necessarily fit classic definitions of states of matter.
Corn starch and water make a shear thickening fluid. Sand and air seem similar.
Comment by rededelk at 17/01/2024 at 05:32 UTC
1 upvotes, 1 direct replies
Random comment but I've read about debates whether or not glass is liquid or solid. I am in the camp that it's a very slow moving liquid, this can be evidenced in some very old wavey like glass windows. But that's just my 2¢. - I really don't know. I would say plain ole sand is a solid, but liquefaction is kind of unique scenario seen in and brought about by certain types of coastal earthquakes
Comment by Mockingjay40 at 18/01/2024 at 22:39 UTC*
1 upvotes, 0 direct replies
I think there are other good answers here but to weigh in as well in case another perspective would help. What you're referring to is actually an example of fluidization[1]. This series of articles can help you if you'd like to read more on the subject. However, in short, fluidization is a natural phenomenon that can occur when a solid particulate is exposed to a force from a fluid. You can often see this at science museums, where air will blow through a column and make a powdery substance form unique shapes. This is also what is happening if you were to have flour in your hand and blow on it. The particulate is released into the air, and looks like a "cloud" in that it appears to behave like a gas but is actually behaving more like a suspension.
1: https://www.sciencedirect.com/topics/chemistry/fluidization
However, in a rheological sense, a fluid is characterized as a material that flows when a certain amount of stress is imposed upon it[2]. In terms of phase, a liquid is a type of fluid that is denser than a gaseous fluid and retains some level of bulk molecular structure, in that the individual molecules in a liquid "stick" together but can also slide around if force is applied, which often also factors in some level of surface tension. Sand, in a non-molten state, doesn't do this. There are no significant intermolecular attractive forces that occur between two sand particles. This results in a material that has fluidlike properties, but is not a fluid in the classical sense. Examples of fluids that may surprise you are most gels. For example, poloxamer gels[3] have solid-like properties, and are about the same consistency as hair gel, but will flow when enough shear is applied. Blood[4] is similar to this, in that these fluids have a yield stress.
2: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5811736/
3: https://doi.org/10.1016/j.colsurfa.2021.128246
4: https://www.ahajournals.org/doi/pdf/10.1161/01.res.13.1.48