‘Magic sand’ an ideal tool for exploring granular materials

Sand is a granular material; it can be poured to fill volumes, like a liquid, while retaining solid properties. Granular physics is the field of study of materials like these, such as rice, powders and the sand used in the construction industry.

A key challenge for researchers in this field is the sheer magnitude of the number of grains involved. While grains can be modelled using straightforward classical mechanics, the enormous number of interacting particles leads to a complexity of flow behaviour that cannot be adequately explained with simple equations. Scientists are not only searching for better models to account for this behaviour, but also convenient “model systems” that can be used to gain insights into how the microscopic structure of these materials give rise to their macroscopic properties.

Researchers at Tokyo Metropolitan University have been studying the behaviour of wet sand; it is well known that wet sand behaves very differently to dry sand, making it possible to make sand castles and other structures. This behaviour is mostly due to the formation of bridges of liquid between particles (capillary bridges) which bind grains of sand such that they can be used to create load-bearing structures. However, it is a challenge to mix water into sand evenly and sand dries very quickly, making it tricky to explore wet sand in the laboratory.

The researchers turned to magic sand to resolve this problem. Magic sand, which was originally developed to clean up oil spills and has since become popular as a toy, has a hydrophobic (trimethylhydroxysilane) coating; ordinary sand is hydrophilic.

They found that grains of magic sand strongly attract each other via thin strands of oil, but they do not interact with ordinary grains of sand, simply bumping into them like dry grains. By mixing magic sand and ordinary sand in various ratios, they could study how wet sand behaves, down to the smallest fractions of liquid which form few capillary bridges between grains.

Using three different methods - involving sieving, measuring density and forming stable mounds of sand - they found that the mechanical properties (clustering and rigidity) of the mixture of sands changes drastically when the fraction of magic sand exceeds 20 per cent. This could offer a very simple way to tune the properties of granular materials in industry, also presenting a possible model for granular physics.

Their conclusions reinforce findings from percolation theory: the study of how connections between particles fill space without breakages, such as in polymer gels. This behaviour helps unify theoretical approaches which apply to completely different materials.