Cooperation of two intruders moving side by side in a granular medium

In bird colonies, schools of fish, and cycling protons, important interactions occur between individuals via the surrounding fluid. These interactions are well understood in fluids such as air and water, but what happens when an object moves through something like sand? Similar interactions occur in granular materials such as soil and sand. , it turns out that it plays an important role in daily life. Think of plows cutting through farmland, animals digging holes in the ground, roots pushing through the soil, and even robots exploring the surface of Mars.

Recently, we came across an interesting discovery. That is, when two objects (what we call “intruders”) move side by side through a granular material, they can actually help each other by reducing the resistance they face. is. This collaborative effect was discovered by a team of researchers from the FAST laboratory at the University of Campinas School of Mechanical Engineering (UNICAMP) in Brazil and the CNRS at the University of Paris-Saclay in France.

To investigate this, we set up an experiment to recreate a granular medium using spherical objects immersed in glass beads. The goal was to pull these objects at a constant speed and measure the drag the particles experienced as they moved through the grain. Previous research had looked at lateral forces between objects, but our team wondered if moving together might also reduce drag.

Interesting numerical simulations by two UNICAMP researchers, DD Carvalho and EM Franklin, published in Physics of Fluids in 2022 suggested that this was possible, but we I wanted to test it in the real world.

What we discovered was interesting. When two intruders were brought closer together, the resistance exerted on each was significantly reduced, by 30% compared to when they were separated. And the deeper they are buried in the material, the more pronounced this effect will be. What’s the explanation? When two objects move side by side, the movement of one breaks the chain of forces between the surrounding particles of the other. The break in particle contact reduces the overall resistance experienced by each object.

In addition to observing this effect, we also developed a semi-empirical model to explain it. This model is based on the idea that interactions between closely packed objects break these granular force chains, making them easier to move. The study, currently published in Physical Review Fluids, focuses on a previously understudied aspect of granular dynamics: the coordinated motion of multiple objects.

As research into these dynamics continues, new technologies and methods for moving particulate matter on Earth and beyond will be developed, potentially enabling more efficient solutions for a variety of industries and scientific endeavors. there is.

This story is part of the Science X Dialog, where researchers can report findings from published research papers. To learn more about Science X Dialog and how to participate, visit this page.

Douglas De Carvalho is a PhD student at the Mechanical Engineering Department of the University of Campinas (UNICAMP), Brazil, where he performs numerical simulations and experiments on granular systems. He realized the experiment presented in this article. The study was supervised by Baptiste Darbois Texier, a researcher at the Fluids, Automated Systems and Termikes Laboratory (FAST) at the Université Paris-Saclay. His research focuses on the mechanics of granular media and the rheology of these materials and related transport issues.