Intro:

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Let's talk about sand. I'm sure many of you Houdini artists out there have seen or made a good sand simulation. But...I think it's wise to say, we take sand and it's overall physics for granite. :) :) So let's talk about sand, it's physics, and how we can better understand the substance to make our VFX better.

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Science Behind Sand

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Sand is defined as small loose grains of minerals, rocks, or other material. So pretty much any grain of anything under 1/16mm or 2mm. Sand can contain anything based on where its location is. Its quality is mostly judged by it's size and shape. The more round a sand grain is the greater amount of weathering the grain has been through. Sand weathering can be caused by the distance the sand has traveled, and it's exposure to the elements. For example, grain size can be related to the amount of water movement around it. The larger the grain, the more wave movement needed to shift its position, and keep it suspended in the liquid.

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Fun Facts About Sand

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- Sand found on land is called Terrigenous Sands.

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- Weathered granite sands contain clear quartz and dark and light minerals.

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- Weathered volcanic sand will appear glassy and black. They can even be green or red. 

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- Sand found underwater or in the sea is called Biogenous Sands.

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- Silica based sand will not dissolve on its own. But sands made from limestone will, and in the process create carbon dioxide.

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- If sands appear purple or an unnatural color, this could mean they are Metamorphic Sands. Metamorphic sands are made up of metamorphic rock. (More on this later.)

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- Biogenic sand is sand made from the bone fragments of dead sea creatures.

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Even though sand is a solid, it cannot be broken into smaller pieces. This is because it is already broken, or is already at its smallest size.  This means it is a Clastic Material. It can also be deformed quite easily, and behave like a fluid when needed. Therefore, sand is seen as an ideal tool for certain experimental scientific investigations. Such as, in Soil Mechanics, and other investigations into the movement of the Earth's crust. Even certain geomaterials have been created to mimic sands behavior.

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The study of sand is called Arenology. A person who studies sand is called an Arenologist. Arenologists are important, as sand not only fits into the  division of geology, but also the studies of chemistry and biology. So understanding where the tiniest grain fits into the environment can have a diverse impact on other environmental issues of that area.

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Sand vs Water

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I think it's easy to say the interaction between water and sand is a given. When we are at the beach, we don't really think about how the environment is working around us. We are just there to enjoy our vacation. So now that we have some time, let's talk about how sand and water behave together.

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As mentioned above, sand is an easily deformed material, and under the right amount of pressure can behave like a fluid. ( Mark Rober has an excellent example of this behavior HERE.) But there are also other important abilities of sand.  Marine sediment has a special level of something called Porosity. Porosity is the measurement of the space between materials. Depending on if the sediment is dry or wet, the Porosity of it can change. 

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If a sand sample is found underwater, it is going to be composed of different materials than sediment found on land. Often, the oceanic crust will introduce other foreign materials that are not found on land. The crust is made up of a volcanic material called basalt. Because of the pressure and weight of the water surrounding it, the basalt gets pushed back down into the surrounding dirt. Therefore, most ocean basins are entirely made up of this material. Basalt is also denser than granite and darker in color. This is because it contains minerals such as iron and manganese. 

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Amongst the minerals, you'll also be able to find the remains of creatures in underwater sediment. Animals exist in the oceans, and there is only one place for their bodies to go after they die. And that is down. However, sands can also be made up of living organisms as well. There are certain creatures on the ocean floor that create their own biological sands. These creatures can be corals, mollusks, worms, or even sea urchins. All of their shells are made up of deposits of Calcium Carbonate, and they have the freedom to remove excess deposits of Calcium that form around them. Sometimes these sands are called Calcium Sands, or Limey Sands.

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Back on the surface, there are some other types of sand formations called Sand Dunes. Sand dunes are huge collections, or hills of sand that you can usually find in random parts of the Canadian wilderness. (Shout out to Chalk River.) But more commonly along rivers, and other waterways. Dunes are special because they move with the flow of water. They start as two very close mounds of sand, and over time get pushed away from the water source and away from each other as they grow.  Any dune that forms in front of them will also have it's growth affected by the previous dune's sand movement.  For example, an older upstream dune can prevent a younger downhill  dune from forming by migrating it's sediment away.

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When dunes are exposed to winds or water, the dune shape starts to change. The sediment will start to move downstream with the flow, and often the dune itself will disappear over time. Therefore, only large groups of dunes are found in remote areas like the desert or in dune fields. 

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Often in the field of Arenology, sand will be experimented on with different forces of water to see how it behaves. Depending on the study, scientists will choose their preferred type of sand, and place it in a controlled environment. This can be useful to understand how to prevent sediment build up in certain water based industries. 

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Mining Sand and Protection of The World's Sand Resources

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As small as sand is, it is an incredibly important resource for many industries across the globe. So much so, is that it is the most sought out resource besides water on the planet. It is used in everything from manufacturing cement, glass, asphalt, paper, and toothpaste. About 40-50 billion metric tons of sand are mined each year. These sand deposits are mostly taken from tropical beaches as many of our sand needs require rougher grains of sand. Such as sediment that contains crushed corals. This of course is having a huge environmental impact for our coastlines.

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Big mining companies such as Glencore and BHP Billiton are currently mining sand deposits in Asia and Africa. As countries in these continents have fewer regulations on resource mining.

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In the near future, we are going to start running into the issue of disappearing beaches. If there is no sand left to collide with the water, then the oceans themselves will just keep coming inwards. This will more than likely be a disastrous outcome for any cities or towns near watershed plains, as progressive mining will remove any barriers between us and the ocean. And with global warming encouraging the seas to rise, this process will only happen faster. These mining efforts have also destroyed many marine ecosystems, as coral and other life suddenly have nowhere to grow or create burrows.

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There is hope however. A few recent studies have come forward and proved we have been measuring the coarseness of sand all wrong. In the majority of cases, companies don't take into consideration the size of the grain of sand they are using, and just take as much as they possibly can. Often the larger the grain, the more rough it will be. So instead of taking sand from our oceans, we could just be mining elsewhere for the same result.

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There are also other options to mining sand entirely. Some industrial companies are now starting to create "fake sand". This fake sand is called Artificial Sand, Crushed Sand, or Mechanical Sand. It is mainly made up of industrial waste that is finely mechanically crushed. In China and India it is currently being mixed into concrete for power plants and infrastructure construction.

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Micro-Visualizing Sand

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So now that we have talked about how sand moves, it's time to dive a bit deeper into its biological state. 

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We already know that sand is made up of small pieces of minerals, rocks, and other sediment. However, based on where those sediments are located around the world, the components are going to change and have a unique composition. You can usually analyze these components under a microscope.

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Usually when a scientist does this, it is called Sand Inspection.  These "inspections" happen more frequently than you think. When construction or agriculture companies need to build  a faculty in an unknown area, they will often hire a geologist to look at the surrounding soil that they are building on. This can be very useful to better understand what materials will be needed, how much erosion will happen in that area, and what type of rock they are building on. 

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Here is some more insight into the different sand types and characteristics. 

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White: White sand usually contains materials such as limestone, shells, coral, magnetite, and calcium-carbonate. It is usually found on tropical beaches such as Hawaii. 

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Black: Black sand is usually from volcanoes. Any volcanic minerals and lava fragments will be found in this sand type. It is usually found in areas like Hawaii, Greece, and other geothermal areas.

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Pink: Pink sand will be made up of shells, calcium carbonate, and marine organisms known as foraminifera. This is an incredibly rare sand type, and is usually found on islands. For example, beaches in the Bahamas or Bermuda.

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Red and Orange: These sands are made from volcanic events, and other iron-oxide rich rocks. They can be found in Greece, and Maui.

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Purple: Purple sands contain metamorphic rocks. Quartz is the main component to purple sand, but it gets it color from the garnet grains inside of it. It can be found in California.

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Grey: Grey sand is mostly made up of granite, feldspar, mica, and magnetite. Sometimes it is also made up of broken coral.

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Annnnd.....Just for fun, there are two other sand types worth mentioning.

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Gypsum Sand: This type of sand is made up of calcium sulfate dihydrate. This is a very rare sand type as gypsum tends to dissolve in water. However, it is a popular tool in monument building.

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Ooid Sand: These sands are kind of weird. They are made up of Oolite pellets. Ooids are mineralized rock that develop in shallow bodies of water. 

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Sand Simulations in Houdini

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Now it's time for the Houdini section of the article. 

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We know for a fact we can create sand simulations in Houdini. But what is the best way to start this task? Well depending on what type of sand, and what type of scene you are trying to achieve, this can be a mixed answer.

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Let's start with particles and grains.

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POP grains are probably going to be your first stop when attempting a sand simulation. They have the advantage over regular pops as they have the ability to stack.  So if you are destroying a sand castle, or making a loose beach simulation, this is the way to go. Pop grains also have a built in ability to move it's particles directly apart, which stops them from penetrating each other when they are stacked.

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You can quickly create a grain setup by going to the shelf tools and finding the Grain Tools Option.

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- In the default setup you might run into the issue of your sand appearing uniform in position. If this is the case, turn on the Jitter Scale on the grain source node. This will randomize the position of your particles.

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- Another thing worth mentioning is that you could also try using Vellum grains. vellum is known for creating soft bodies. However, Vellum grains can generate some pretty interesting results.

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However, if you are building something like a sand storm, you might want to use a regular POP network for your simulation. This is because you are trying to simulate sand in the air rather than the ground. It will also give you more freedom to manipulate the direction of your sandstorm.

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Before we trail off, it's probably worthwhile mentioning The difference between PBD and FEM. Why may you ask? Well, if you are building something diverse as sand, you might use a few different systems to get the job done.

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PBD stands for Position Based Dynamics. It is mostly used in the POPs Grain Solver and sets the constraints between points. These constraints contain the default rest position for pairs of points in the simulation. When using PBD you can have a fast solving speed, the ability to change point positions, and create bouncy objects easily.

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FEM is a bit similar. FEM stands for Finite Element Methods. This system is based around physical stain and volume compression models. It is great for simulating realistic and physically based simulations. So if you'd ever like to measure the compressibility of sand in Houdini, you could 100% do so with FEM. 

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References:

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Arenology- The Study of Sand: https://sites.google.com/site/dougfosterprogramsite/arenology--the-study-of-sand

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Sand under a Microscope: https://www.microscopemaster.com/sand-under-a-microscope.html

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Sand: https://www.newworldencyclopedia.org/entry/Sand

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Sand Lab: http://www.hawaii.edu/gk-12/evolution/pdfs/meiofauna.sand.lab.highschool.pdf

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8-20.8 Statistics: https://sites.google.com/site/646lieb/spreadsheets/8-20-8-statistics

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Arenology: http://impossiblegeology.net/geopathology/arenology

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Experimental study of sand grains behavior at their contacts with force- and displacement-controlled sliding tests: https://www.sciencedirect.com/science/article/pii/S2467967417300065

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A Study in Sand: https://360.lubrizol.com/2020/A-Study-in-Sand

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To Protect the World’s Sand, We Need to Know How to Measure It: https://eos.org/articles/to-protect-the-worlds-sand-we-need-to-know-how-to-measure-it

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A Study on Sand Movement Due to Wave Action: https://www.tandfonline.com/doi/abs/10.1080/05785634.1967.11924054?journalCode=tcej19

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The Importance of Sand in Earth Sciences: https://link.springer.com/chapter/10.1007/978-3-642-11445-8_27

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Sand and Gravel - A Study of Their Permeabilities: https://www.onepetro.org/conference-paper/SPE-4772-MS

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Sand: The study of quartz sand in sediments provides much information about ancient geological environments: https://www.jstor.org/stable/27843128?seq=1

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Comparative Study of Sand Porosity and a Technique for Determining Porosity of Undisturbed Marine Sediment: https://www.tandfonline.com/doi/abs/10.1080/10641190490900844

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Recipe for a dune: Sand, wind, water, plants: https://ceoas.oregonstate.edu/feature-story/recipe-dune-sand-wind-water-plants

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Experimental Study of Sand and Slurry Jets in Water: https://www.researchgate.net/publication/245297861_Experimental_Study_of_Sand_and_Slurry_Jets_in_Water

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Sand dunes can 'communicate' with each other: https://www.sciencedaily.com/releases/2020/02/200204091403.html

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About Grains; https://www.sidefx.com/docs/houdini/grains/about.html

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How to simulate a stable pile of sand without grid patterns: https://www.sidefx.com/docs/houdini/grains/stablepile.html

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List of largest mining companies by revenue: https://en.wikipedia.org/wiki/List_of_largest_mining_companies_by_revenue

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Artificial Sand - What Is It and How to Make It: https://www.ftmmachinery.com/blog/artificial-sand-what-is-it-and-how-is-the-development-prospect.html#:~:text=Artificial%20sand%2C%20also%20called%20crushed,the%20construction%20of%20hydropower%20systems.