Synopsis: Sliding Sand

Adding a small amount of water to sand can significantly reduce the sliding friction.
Synopsis figure
A. Fall et al., Phys. Rev. Lett. (2014)

Everyone who has been to the beach will know that dry sand doesn’t make good sandcastles—the grains slump into a puddle when the bucket is lifted. Adding water can solve this problem: the grains stick and the castle holds its shape. This is due to liquid bridges that start to form between the grains when water is added. Once there is enough water, these bridges act like glue, keeping the grains in place. This is great for sand castle building, and also, it turns out, for sand transportation. Writing in Physical Review Letters, Daniel Bonn from the University of Amsterdam, Netherlands, and colleagues show that adding water to sand significantly reduces the sliding friction of an object moving over the sand, but only for small amounts of water.

Bonn et al. tested the sliding friction of dry and wet sand when a weighted sled was pulled across the surface. As water was added, both the force needed to pull the sled and the friction coefficient were found to decrease below that of the dry sand, before increasing at higher water contents. When the sand was dry, a heap of sand formed in front of the sled, hindering its movement; a relatively high force was needed for the sled to reach a steady state. Adding water made the sand more rigid, and the heaps decreased in size until no heap formed in front of the moving sled and therefore a lower applied force was needed to reach a steady state. Why then did the sliding friction increase at higher water contents? The authors suggest that this was due to a decrease in the stiffness accompanying water saturation, similar to that seen in sandcastles—add too much water and the capillary bridges, which previously acted like a glue between individual grains, start to merge and disappear. – Katherine Wright


More Features »


More Announcements »

Subject Areas

Materials Science

Previous Synopsis

Related Articles

Synopsis: A Crystal Ball for 2D Materials
Materials Science

Synopsis: A Crystal Ball for 2D Materials

Researchers predict new two-dimensional materials whose structures differ from their three-dimensional counterparts. Read More »

Viewpoint: Electron Pulses Made Faster Than Atomic Motions
Atomic and Molecular Physics

Viewpoint: Electron Pulses Made Faster Than Atomic Motions

Electron pulses have shattered the 10-femtosecond barrier at which essentially all atomic motion is frozen in materials. Read More »

Focus: Ultrafast Switch with Organic Crystal
Condensed Matter Physics

Focus: Ultrafast Switch with Organic Crystal

An organic crystal was switched between paraelectric and ferroelectric states in a picosecond. Similar materials could eventually serve as extremely fast digital switches. Read More »

More Articles