laboratoire de physique statistique
laboratoire de physique statistique

The tangled yarn balls of the sea

Aegagropilae do not puzzle only the sea shore hikers, they rise also scientific questions. A team of researchers of the IRPHE, the LPS and the ENS Lyon have tried to understand the formation of these balls of vegetal fibers. This work has been published in PNAS.

You may find on the Mediterranean beaches few centimeters wide balls of vegetal fibers called aegagropilae. They are the result of the decay of an aquatic plant named posidonia. Reaserchers from the Institut de recherche sur les phénomènes hors équilibre (CNRS/Aix Marseille Université/Centrale Marseille), from Laboratoire de physique statistique (CNRS/UPMC/ENS Paris/Université Paris Diderot) and from the lab of physics of ENS Lyon (CNRS, UCBL, ENS Lyon) described the formation of those surprisingly stiff pack of fibers. Their work has involved field observations, statistical studies, mechanical testing and X-ray tomography.

They have shown that the formation undergoes several steps. First, fibers gather in loose and disordered aggregates. Which, in turn, get compacted by rolling and tumbling on the sea floor, pushed by the waves and the currents. This process is confirmed by the tomography that shows the internal structure : an increasing density from the core of the ball to its surface. This feature is shared with other situations of compacted elastic objects, like for example a crumpled sheet of paper. Finally, the deformations of the ball are partially irreversible. The combined effect of the fibers elasticity and their friction allows the cohesion of the aegagropilae despite the absence of wall to keep it compressed. Understanding the structure and the mechanical properties of these natural fiber aggregates may inspire the design of future materials both light and rigid.

Aegagropilae of various shapes and sizes placed on millimeter paper.

Equatorial section of an aegagropilae revealing its internal structure : a losse core surrounded by a denser crust.

Références :
G. Verhille, S. Moulinet, N. Vandenberghe, M. Adda-Bedia and P. Le Gal, « Structure and mechanics of aegagropilae fiber network ».
PNAS, vol. 114 no. 18 (4607-4612).
Doi : 10.1073/pnas.1620688114

Contact chercheur :

  • Sébastien MOULINET

Gautier Verhille - IRPHE