Is flowing sand a 'fluid'?

Is flowing sand a 'fluid'?

Is flowing sand a fluid? This question, seemingly simple on the surface, can actually lead us on a fascinating journey into the realms of physics, materials science, and fluid dynamics.

TLDR; flowing sand is not a fluid.

Before we dive deeper into the answer, we must first delve into the definitions and characteristics of solids, fluids, and more specifically, granular materials like sand.

What is a Fluid?

In the most basic terms, a fluid is a substance that continuously deforms (flows) under an applied shear stress, or external force. Fluids include both gases and liquids. They are characterized by their ability to flow and take the shape of their containers, as well as other properties like viscosity and density.

What is a Solid?

In contrast, a solid is a state of matter characterized by particles arranged such that their shape and volume are relatively stable. The constituents of a solid, be it atoms, molecules, or ions, are packed close together and are not compressible. Therefore, a solid material has a fixed shape that is not easily changed.

What is Sand?

Sand, on the other hand, is a granular material composed of finely divided rock and mineral particles. It is classified as a solid due to the properties of its individual grains. However, when large amounts of sand are in motion—as in a sand dune shifted by wind, or sand flowing down an hourglass—the behavior of sand as a collective appears to be more fluid-like.

Granular Flow: The Intersection of Solids and Fluids

This seeming paradox in the behavior of sand brings us to the concept of granular flow. Granular materials like sand, when given enough energy, can behave like fluids. This type of movement is known as granular flow and is a sub-field of study within fluid dynamics.

Granular flow can be seen in various phenomena, like avalanches, landslides, and the aforementioned sand dunes and flowing hourglasses. But despite the fluid-like movement, granular flows possess unique characteristics that distinguish them from true fluids.

In a granular flow, individual grains move independently but are influenced by collisions with their neighbors. These collisions are dissipative, meaning they lose energy with each interaction, unlike the molecular interactions in fluids. This energy loss results in a phenomenon called "jamming," where the granular material suddenly transitions from a flowing state to a solid-like state, something we do not see in true fluids.

The Complexity of Flowing Sand

Understanding whether flowing sand is a fluid or not requires recognizing its complexity. On the one hand, as a collection of solid particles, sand exhibits properties of a solid. Each sand grain has a defined shape and volume, and the collection as a whole does not readily deform unless subjected to an external force.

On the other hand, when that external force (like gravity, wind, or even a child's hand in a sandbox) is applied, sand can flow much like a fluid. However, it's essential to understand that this behavior is more an emergent property of many particles interacting rather than a property of the individual sand grains themselves.

This fluid-like behavior, combined with the ability to suddenly stop flowing (jamming), places flowing sand in a unique category. It is a granular material demonstrating a phenomenon known as granular flow, which incorporates aspects of both solid and fluid dynamics.

Conclusion: Is Flowing Sand a Fluid?

So, is flowing sand a fluid? In strict scientific terms, no, flowing sand is not a fluid. Its behavior doesn't wholly align with the definition of fluids, especially considering its tendency towards jamming. However, it's also not a traditional solid while flowing, as it displays characteristics commonly associated with fluids.

Therefore, flowing sand is best described as a granular material exhibiting granular flow, occupying a fascinating middle ground in the world of physics. This conclusion highlights the complexity of natural phenomena and the importance of interdisciplinary studies in understanding our world. We must not rush to categorize but instead appreciate the nuanced behavior of materials and the science that allows us to explore such wonders.


  • raj87verma88
    Re: question

    Flowing sand is not a fluid. Think of a room filled to the top with balls. When you open the door all the balls roll out. Would you call it flowing and say that the balls are fluid. Its the same with sand. The particles are small tiny balls that roll over each other.
  • arpanmechengg
    yes flowing send is flud u can say solid flude like a (Hg) mercury u knw i kn ur smart k bye
  • Kaustubh Katdare
    Kaustubh Katdare
    Arpan - Please pay attention to following link - #-Link-Snipped-#
  • sriramchandrk
    You can justify that flowing sand is not fluid with this sentence:
    If you can stir it up with a spoon or blow it through a straw, it's a fluid.

    More info on: #-Link-Snipped-#

    This was the simple explaination i could get through search.

    Thanks & Regards
  • raj87verma88
    yes flowing send is flud u can say solid flude like a (Hg) mercury u knw i kn ur smart k bye
    What in the world is a solid fluid? Mercury is a liquid at room temperature and so are caesium, francium, gallium and rubidium. There are 4 sates of matter gas, liquid, solid and plasma(at very high temperatures).
    Gas and liquid are fluids.
    I have already explained the concept of flowing sand not being a fluid using an example. I will give one more. Sand granules are irregular in shape. If you enlarge them a 1000 times, they may look like boulders or huge rocks. Now imagine a land slide, number of rocks of all shapes and sizes rolling down the mountain slope, rolling on top of each other. Will you call that flowing and say that the boulders/rocks are fluid?
    By solid fluid, you may be meaning a gel like substance. If so then I think you are confusing Quicksand with Sand. Quicksand is a Colloidal Mixture of sand/silt, clay and salt water. It is a non-newtonian fluid. Looks like a solid if undisturbed. The other concepts are Dry Quicksand, Fetch Fetch etc. I have provided somelinks below.
    Dry Quicksand
    Fech Fech
  • mshearer
    I doubt arpens post was a serious answer, instead a repsonse to the initial butchery of the english language provided by krazzykalpana#-Link-Snipped-#
  • jackjp01
    Simple definition: A fluid is something that flows. I would definetly call flowing sand a fluid. The absolute viscosity would probably be very high. Here are a few more non-traditional examples of fluids:

    - a rock slide (as mentioned before is like sand)
    - a room full of balls when the door is opened
    - car traffic (a classic example of a pipe flow problem)

    If ya dont believe me then take your knowledge of fluid mechanics and apply it to the concept of the hour glass (using sand as the FLUID)
  • sgrshukla
    I think flowing sand is not fluid....because the basic property of the fluid is viscosity, though the sand layers flow over each other, it can't be said that it is having viscosity because viscosity is a function of temperature, if temperature increase viscosity decrease in real fluid, here it doesn't ultimately flowing sand is not fluid...
  • 6nv6
    no, because fluid includes gases and liquids only... sand is not either of them.. and sgrshukla is right about viscosity.. that called non-Newtonian fluid which can cause the viscosity to decrease, so the fluid appears "thinner"...
  • Abhijeet__
    Sand is a fluid,it act's just as water and only its molecules are bigger and its viscocity is very heigh as compaired to water.
  • raj87verma88
    Thread Closed

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