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What is Porosity?
Mary McMahon
Mary McMahon
Porosity is a term which is used to describe an important physical property of most materials. The porosity of a material is determined by measuring the amount of void space inside, and determining what percentage of the total volume of the material is made up of void space. Porosity measurements can vary considerably, depending on the material, and high or low porosity will impact the way in which the material performs.
The property of porosity is actually slightly more complex than the simple percentage of void space inside a material. Another important consideration is the shape and size of the void spaces in the material. Swiss cheese, for example, is famous for having fairly large and distinct bubbles, while Alpine lace cheese has much smaller bubbles, and these two cheeses behave differently because their void spaces are differently shaped.
Another issue is the level of interconnection between void spaces. If all of the spaces are discrete and isolated from each other, a material will not behave in the same way as a material with the same percentage of void space in the form of connected void spaces. When people study the rate of flow through materials, this property becomes especially important. Obviously, if the spaces are connected, the flow is going to be higher, because liquids can move freely.
This property can be measured in a wide range of materials including wood, stone, paper, cement, and ceramic. There are a number of ways to measure porosity, including soaking materials to see how much water they absorb. In the case of stone, people may also be interested in whether the porosity is primary or secondary. Primary porosity is the result of processes which occurred during deposition, while secondary porosity happens due to processes which occur after rock is formed. For example, deposits of a more soluble mineral inside a hard mineral may be gradually dissolved over time, making the rock more porous.
Porosity plays an important role in a wide range of processes. Porous membranes are routinely used for filtration of all sorts of materials, with the effectiveness of the filter being limited by the size of the pores, and porous deposits of rock are commonly exploited to access deposits of oil which may be embedded in the rock. People even take advantage of this property in the kitchen; sponge cakes, for example, are designed to be highly porous so that they can soak up syrups used for flavoring.
Mary McMahon
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllTheScience researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.
Learn more...
Mary McMahon
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllTheScience researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.
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![Certain soils and materials are more porous than others.]()
By: godunk13Certain soils and materials are more porous than others.
Discussion Comments
Some people have problems with measuring the porosity, especially for nanomaterials researchers.
Now, researchers usually adopt the famous static volumetric principle. It uses nitrogen, etc. as an adsorbate and under certain pressure, lets the nitrogen be adsorbed by nanomaterials, then calculates the nitrogen amount and will know the porosity data like pore size, pore volume, pore size distributions and so on.
I find it fascinating that a rock can float because of its porosity. Pumice, a type of volcanic rock, is so porous that it floats.
I saw this demonstrated when my friend brought back some pumice from Hawaii. He learned from a volcanic rock collector that pumice is full of hot gases when it forms. The gas bubbles remain as the lava cools, so pumice is full of air pockets.
These multiple pockets of air make pumice less dense and lighter than an equal volume of water. This high porosity makes it able to float.
When planning my garden, I had an analyst examine my soil. I wanted to make sure that the soil porosity would suit the type of things I wanted to plant.
He told me that even though I had lots of clay in my soil, it had high porosity. I asked him how this could be, because clay is so sticky and it seems like the particles would be jammed up against each other.
The analyst said that the porosity of soil is a ratio of pore space volume to total volume. Since clay has a lot of small pore spaces, it has high porosity.
@kylee07drg - Yes, bread's porosity makes for some yummy food combinations. However, it can cause absolute mush.
I learned this when I started packing my husband's lunch. I personally never eat mayonnaise on my sandwiches, but he likes it.
I made his sandwich the night before with plenty of mayo. When he tried to eat it the next day, he said it was disgustingly squishy, and everything squirted out of the bread when he took a bite.
I have since learned to either make his sandwiches the morning of the day he will eat them or slip a packet of mayo in his lunch bag. The separate packet is best, because even a few hours of soaking in mayo can make the bread unappetizing.
I am thankful for the porosity of bread. I love bread soaked with gravy, syrup, butter, or jelly.
You can see the porosity of bread in action when you place a few pats of butter on a slice and stick it in the oven. The butter melts and covers the whole surface of the bread, but it also seeps into its pores, creating a wonderful flavor throughout the slice.
I love the way that jelly softens a piece of toast. I don't leave it on there too long, because it will get too soft to handle, but just a couple of minutes on a warm piece makes the perfect texture.
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