Structure and mechanics of aegagropilae fiber network
Résumé
Fiber networks encompass a wide range of natural and man-made materials. The threads or filaments from which they are formed span a wide range of length scales: from nanometers, as in biological tissues and bundles of carbon nanotubes, to millimeters , as in paper and insulation materials. The mechanical and thermal behavior of these complex structures depends on both the individual response of the constituent fibers and the density and degree of entanglement of the network. A question of paramount importance is how to control the formation of a given fiber network to optimize a desired function. The study of fiber clustering of natural flocs could be useful for improving fabrication processes, such as in the paper and textile industries. Here, we use the example of aegagropilae that are the remains of a seagrass (Posidonia oceanica) found on Mediterranean beaches. First, we characterize different aspects of their structure and mechanical response, and second, we draw conclusions on their formation process. We show that these natural aggregates are formed in open sea by random aggregation and compaction of fibers held together by friction forces. Although formed in a natural environment, thus under relatively unconstrained conditions, the geometrical and mechanical properties of the resulting fiber aggregates are quite robust. This study opens perspectives for manufacturing complex fiber network materials. fiber network | fiber aggregation | Posidonia oceanica | fibrous material A egagropilae are a natural aggregate of fibers produced by the decomposition of leaves and roots of Posidonia ocean-ica. The fibers are entangled by sea motion until the clusters reach the shore (1) (Fig. 1). P. oceanica is an endemic plant of the Mediterranean Sea with very long and thin leaves (about 1-m long, 1-cm wide, and less than 1-mm thick). The name aega-gropilae originates from the Greek [α´ιγαγρoςα´ιγαγρoς (wild goat) and π˜ιλoςπ˜ιλoς (fur)] and refers to the resemblance between the shape of these balls and those regurgitated by goats. Natural fiber clustering occurs for different species of aquatic plants, such as the so-called seaballs that can be found on the Atlantic Ocean and lake shores (2, 3). P. oceanica meadows play an important ecological role in the preservation of Mediterranean coasts. They constitute plant barriers that promote sediment trapping and oxygen production in seabed, and the accumulated remains protect the beaches from erosion. Moreover, aegagropilae fibers are suitable materials for insulation in construction and automotive industries. After submitting them to various tests, they recently landed in the marketplace under the name of Neptutherm. Beyond the curiosity that these natural objects can provoke, aegagropilae samples found on beaches raise several fundamental questions. The first set of questions is on their formation process. How can fibers be entangled and packed by a flow without any confinement? How long does it take to form a cluster? Can one explain the size distribution of these aggregates? The second set of questions is on the cohesion of these structures. How can we relate the apparent stiffness of these balls to the interaction of the fibers and the topological properties of the network? Natural and manmade fiber networks are abundant structures and arise on a wide range of length scales. Examples may be found in both biological systems, such as the cytoskeleton of a cell (4), blood clots (5), and biological tissues (6), and technology , such as nanotube bundles (7), paper (8), textiles, and felts (9–11). The mechanical response of fiber assemblies depends on both the properties of the elementary thread and the density , connectivity, and ordering of the network. Therefore, the functional properties of these materials can be tuned by controlling their formation processes. The manufacturing of some athermal networks, such as paper, involves transport of fibers in a fluid flow (8). During this process, fibers are advected and deformed elastically, and they interact through interfiber friction (12, 13). From this perspective, aegagropilae are an archetype of these fiber networks, and understanding the clustering mechanism can shed light on the fundamental aspects of fiber aggre-gation dynamics. Here, we perform various measurements on aegagropilae to characterize their structural and mechanical properties. Our observations provide a qualitative understanding of the formation process of this fiber network. Structural Properties of Aegagropilae Aegagropilae were collected at two different locations of the Mediterranean shore: at Six Fours, France (43 • 06 03 N, 5 • 49 20 E) and on Porquerolles Island, France (43 • 00 02 N, 6 • 13 38 E) with the authorization of Port-Cros National Park. On the first beach, nearly 2,000 samples were collected to determine their size and mass distributions. To avoid any bias in the sampling caused by damage of aegagropilae by human activity, all samples were collected on a morning in winter, the day after a storm. Moreover, only balls found at a distance less than 10 m from the sea were collected, assuming that the ones located farther away from the sea might have been released by a precedent storm. At the second spot, few samples were picked up in seabed to investigate the geometrical and mechanical properties of fiber Significance Aegagropilae are centimeter-sized, nearly spherical fiber aggregate found on Mediterranean beaches. They result from aggregation and compaction of the remains of the seagrass Posidonia oceanica. We find that they possess remarkable mechanical properties, which are unexpected for a structure formed by random agitation caused by sea motion and held together solely by friction between the fibers. The study of such material offers perspectives for complex materials, such as felts and insulation materials.