Research identifies key connection between gravitational instability in physical gels and granular media

What do jelly and sand have in common?

When sand beds (a) and gels (b) are destabilized, similar “finger ring” instabilities (left to right) form over time. Photo credit: Tokyo Metropolitan University

Tokyo Metropolitan University researchers have identified key similarities between the behavior of granular materials and melting gels. They found that falling sand beds exhibit the same destabilization mechanism as melting gelatin when heated from below, specifically how key parameters scale with the thickness of the churned area. Their results, published in Scientific Reportsprovide important insights into our understanding of destabilization under gravity as seen in avalanches, landslides, and industrial transport processes.

Sand and jelly may not look very similar, but they share similar physical properties. Sand is made up of billions of solid grains that flow like a liquid and can clog pipes like a solid. Materials such as gelatin solutions flow like a liquid at high temperature but suddenly assume solid-like properties upon cooling. Looking at the microscopic details, it is evident that the strength of gels is underpinned by polymer or protein networks that run through a material; This is similar to how “chains of force,” networks of grains crowding together, create the apparent solidity of sand. This fascinating combination of solid and liquid-like behavior forms the backbone of many natural phenomena, such as avalanches and landslides, but is still poorly understood.

These similarities inspired Dr. Kazuya Kobayashi and Professor Rei Kurita of Tokyo Metropolitan University to directly compare physical gels and sand beds as they liquify. Using high-speed cameras, they observed the fluidization of thin sand beds and gelatine solutions. For sand, preformed grain beds were inverted in air or water and observed as the base began to fall out. For gelatin, two layers with different gelatin concentrations were made on top of each other. The concentrations were chosen so that the lower layer completely fluidized first. If the material is heated from below, the top layer would destabilize and begin to fall.

What do jelly and sand have in common?

The typical distance between the fingers (a) and the velocity of the receding front (b) scale similarly in both gels and sand with the thickness of the fluidized region. This is evidence of common features in their physical mechanisms. Photo credit: Tokyo Metropolitan University

In both systems, the team found fingering instabilities, where thin fingers of material fall into the material (or air/water) below, resembling raindrops falling down a window. Over time, new fingers would appear between existing ones and the interface between the liquid and solid-like parts would recede. Using a special imaging technique, the team was also able to identify a “liquefied” interface region over the actual beginning of the fingers. The thickness of this region was found to correlate strongly with key parameters such as the speed at which the front is retreating and the distance between the fingers. This type of relationship is called a “scale” relationship and is important in physics to connect phenomena that may appear different at first, but may be related at a deeper level through their mechanisms. In this case, this is strong evidence of how the similarities between the materials, i.e. the connectivity of a force-bearing network, underlies their macroscopic physical behavior.

Through their extensive experiments, the team’s work offers valuable insights into how granular materials and gels destabilize under gravity, which has implications both for fluidization phenomena in nature and for the design of transport systems for granular materials on an industrial scale.

“Magic sand” could help us understand the physics of granular matter

More information:
Kazuya U. Kobayashi et al, Key connection between gravitational instability in physical gels and granular media, Scientific Reports (2022). DOI: 10.1038/s41598-022-10045-x

Provided by Tokyo Metropolitan University

Citation: Research Identifies Key Link Between Gravitational Instability in Physical Gels and Granular Media (2022 May 2) Retrieved May 2, 2022 from .html

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