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Materials

Materials

Wet and dry spheres pack together in the same way

29 Apr 2019
Spheres
Liquid bridges: wet packing of disks at 3.1% liquid content. The tomography image on the left is the raw data after noise reduction. The processed image on the right shows the spheres in blue and the liquid in yellow. (Courtesy: S Weis, G Schröder-Turk and M Schröter/New Journal of Physics)

Adding water to a granular material does not necessarily result in significant structural changes, according to a team in Germany led by Matthias Schröter at Friedrich Alexander University Erlangen-Nuremberg. The group made the discovery by taking X-ray tomographic images of the packing arrangements of both wet and dry spheres. The result is surprising because it is well known that mechanical properties of granular materials such as sand can change significantly when water is added.

Anyone who has built a sandcastle knows that wet sand is much better at sticking together than dry sand. This stickiness arises because water droplets wet the surfaces of sand grains and form networks of capillary bridges between them. This creates tensile forces between grains that boost the mechanical stability of the sand. In contrast, the forces between dry sand grains are far weaker – which is why a dry sandcastle will quickly collapse in a heap.

It is tempting, therefore, to assume that inter-grain forces also affect how the grains are packed together – which could also affect the mechanical properties of a material. There is however, no universal quantitative theory that confirms or refutes this idea.

Filling a gap

To fill this gap in our knowledge, Schröter and colleagues set-out to compare the packing structures of wet and dry granular materials for the first time. To do this, they placed 5000 polymer spheres, each roughly 3.5 mm in diameter, inside a cylinder. Using X-ray tomography, they then obtained images of cross-sectional slices of the cylinder at regular intervals, in both wet and dry conditions. To quantify packing structures, the team used these images to compute the number of contacts between the spheres, and for the wet particles, the number of liquid bridges connected to each sphere.

The team was surprised to find that even when liquid bridges introduced tensile forces between spheres, the packing structures of the wet spheres was not significantly different to that of the dry spheres. Furthermore, they established a clear relationship between the number of contacts and the number of liquid bridges. This suggested that instead of actively changing packing structures by drawing spheres together, the bridges simply varied their shapes to accommodate for existing packing structures.

Schröter’s team now hopes to repeat their experiment using smaller particles, where liquid bridges can stabilize structures at far lower packing fractions than in dry materials. They will also aim to explore the behaviour of non-spherical particles, which introduce new bridge geometries and novel geometrical features. Ultimately, they will aim to guide the development of the first universal particle-based models and mechanical descriptions of granular materials.

The study is described in the New Journal of Physics.

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