Antlion larvae catch prey by digging conical pits in sand. The steep walls of the trap are near the angle of repose, the largest angle a granular material can maintain before grains slide down. When a hapless ant wanders into the trap, the antlion throws sand from the center of the pit, triggering a sandslide that carries the ant downward. The act of flinging sand also helps the antlion maintain the pit, correcting any disruptions to the pit’s steep sides caused by its flailing prey. (Image and research credit: S. Büsse et al.; via Science)
Tag: angle of repose
The Angle of Repose
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Granular materials like sand tend to form heaps when poured. The steepness of the heap at rest is described by the angle of repose, which is determined by a balance between gravity, normal force, and friction on the grains. When a heap of grains is disturbed, it can trigger an avalanche. As can be seen in the video above, avalanches are a surface phenomenon, only moving the top few layers of grain while most of the heap remains stationary. (Video credit: Peddie School Physics)
Sand Dunes
Sand dunes form with a gentle incline facing the wind and a steeper slip face pointing away from the wind. Most slip faces are angled at about 30 to 34 degrees–called the angle of repose. The shape is determined by the dune’s ability to support its own weight; add more sand and it will cascade down the slip face in a miniature avalanche. Similarly, if you disturb sand on the slip face by digging a hole at the base, you get the cascading collapse seen in this video. By removing sand, the dune’s equilibrium is broken and it can no longer support its weight. This makes sand flow down the slip face until enough is shifted that the dune can support itself. Being a granular material, the sand itself appears to flow much like a fluid, with waves, ripples and all. (Video credit: M. Meier; submitted by Boris M.)
