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Fluidization (or fluidisation) is a process similar to liquefaction whereby a granular material is converted from a solid-like state to a liquid-like state. This process occurs when a fluid (liquid or gas) is passed up through the granular material.
At a critical velocity (Minimum Fluidization Velocity (Umf)) fluid drag lifts the grains into suspension and they lose contact with their neighbours. As the granular material's strength comes from friction between grain contacts, the material loses its strength as a result of going into suspension. Without strength between the grain contacts, the material is free to flow as a liquid. Reducing the fluid velocity below Umf, returns the material to a solid-like state.
The process is key in the formation of a sand volcano and fluid escape structures in sediments and sedimentary rocks.
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