Understanding the physics of granular matter using Magic sand

Children's toys an ideal modifier for interparticle adhesion and mechanical properties.

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Sand sometimes shows fluid and solid-like behaviors. It can flow and be poured like a liquid but retains many of the solids’ properties, clogging pipes or forming dunes.

The behavior of collections of small particles like sand is known as granular physics. It is an enormously significant field for transporting the wide range of granular materials out there like grains, rice, powders, and the tremendous sand measures utilized in the construction industry.

A vital problem of this part of physics is the sheer numbers included. Grains interact through basic, Newtonian mechanics, but since such countless particles interact immediately, there is a developing intricacy of flow behavior that cannot yet be clarified by simple conditions.

Scientists are thus looking for better theoretical models to explain granular behavior but convenient “model systems” that can be handled and tuned in the lab to give insights into how granular materials’ microscopic structure gives rise to their macroscopic properties.

Scientists from Tokyo Metropolitan University have been studying what happens to sand when sand gets wet. It is well known that beach sand behaves entirely differently. This behavior is largely due to the formation of “bridges” of liquid between particles, known as capillary bridges.

The capillary bridges firmly tie the grains together to form load-bearing structures. In contrast, homogeneous wet sand is notoriously hard to mix evenly and dries very quickly. It is challenging to prepare it in the lab as well.

To overcome this issue, scientists used magic sand, hydrophobic sand particles coated in silicone oil. Scientists studied the macroscopic features of composite sand systems where the silicone-coated sand is mixed to dry. They found that the sand particles were found to interact with each other only and not with other sand particles.

The team found that not only does “magic sand” strongly attract each other via thin strands of oil, but it completely ignores normal sand, simply bumping into it like dry grains. By mixing “magic sand” and normal sand in different ratios, the team could freely study how wet sand behaves down to even the smallest liquid fractions. Only some grains are connected via capillary bridges.

Scientists used three different methods that involve sieving, measuring density, and forming stable mounds of sand. Using these methods, scientists found that the mixture’s mechanical properties change drastically when the fraction of magic sand to normal sand exceeds 20%.

This concurred with discoveries from perlocation theory, which shows how interactions between particles span space with no breakages, letting the sand mixture behave as solid and bear its weight. This behavior is known for polymer gels and binds together hypothetical approaches applied to various materials.

The mixture also offers a suitable material for an industry with mechanical properties that can be easily modified. Notably, the method provides a new, convenient, accurate, and informative way to explore granular physics and may become the new default for scientists in future investigations.

Journal Reference:
  1. Marie Tani et al. Transition Behavior in Silicone-coated Sand Mixtures. DOI: 10.7566/JPSJ.90.033801