The structure and stability of soil changes as it gets wet, dry and wet again – something architects and engineers know only too well.
New US research combining geophysics and fluid mechanics provides some clues to how and why this happens and, more generally, to how particles stick together and then pull apart.
The short answer is that it’s all about bridging, and that the size of the particles involved may be more important than their chemical properties.
The work by a team from the University of Pennsylvania – described in a paper in the journal PNAS – grew out of earlier research to determine how the needle-like fibres in asbestos stick to each other, and to other materials, to form aggregates.
Ali Seiphoori and colleagues began thinking more generally about what determines the strength and stability of an aggregate – so they created a simple model.
They suspended very small glass spheres of two sizes – three and 20 microns – in a droplet of water. As the water evaporated, the edges of the droplet retreated, dragging the particles inward.
Eventually the shrinking water droplet transformed into multiple smaller droplets connected by a thin water bridge, known as a capillary bridge, before that too evaporated.
The extreme suction pressures caused by evaporation pulled the small particles so tightly together, the researchers say, that they fused in the capillary bridges, leaving behind solid bridges between the larger particles, to which they also bound, once the water evaporated completely.
When they rewet the particles with a controlled flow, the aggregates composed solely of the 20-micron particles were much easier to disrupt and resuspend than those composed of either the smaller particles, or mixtures of small and larger particles.
“We found that if aggregates composed of only particles larger than five microns were rewet, they collapsed, but under five microns nothing happens, the aggregates were stable,” says geophysicist and study co-author Douglas Jerolmack.
Subsequent tests with particles of four sizes that better reflected natural soil composition found the same bridging affect occurring at different scales. The largest particles were bridged by the second largest, which were bridged by the third largest, and so on.
Even mixtures that contained only a small fraction of smaller particles became more stable thanks to solid bridging.
And really quite stable. After a rather painstaking process to quantify the “pull-off force” required to remove particles from an aggregate, Seiphoori’s team found they were 10 to 100 times harder to pull off when they had formed a solid bridge structure than in other configurations.
And the principles still held, they say, when they moved from experimental glass beads to two types of clay that are common components of natural soils. Smaller particles and the presence of solid bridges made aggregates stable, but when particles smaller than five microns were removed from the suspensions their resulting aggregates lost cohesion.
“Clay soils are thought to be fundamentally cohesive and that cohesiveness has usually been attributed to their charge or some other mineralogic property,” says Jerolmack.
“But we found this very surprising thing that it doesn’t seem to be the fundamental properties of clay that make it sticky, but rather the fact that clay particles tend to be very small. It’s a brand new explanation for cohesion.”
Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.