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Polymer cold-drawing (1-4) is a process in which tensile stress reduces the diameter of a drawn fibre (or thickness of a drawn film) and orients the polymeric chains. Cold-drawing has long been used in industrial applications (5-7), including the production of flexible fibres with high tensile strength such as polyester and nylon (8,9). However, cold-drawing of a composite structure has been less studied. Here we show that in a multimaterial fibre (10,11) composed of a brittle core embedded in a ductile polymer cladding, cold-drawing results in a surprising phenomenon: controllable and sequential fragmentation of the core to produce uniformly sized rods along metres of fibre, rather than the expected random or chaotic fragmentation. These embedded structures arise from mechanical-geometric instabilities associated with 'neck' propagation (2,3). Embedded, structured multimaterial threads with complex transverse geometry are thus fragmented into a periodic train of rods held stationary in the polymer cladding. These rods can then be easily extracted via selective dissolution of the cladding, or can self-heal by thermal restoration to re-form the brittle thread. Our method is also applicable to composites with flat rather than cylindrical geometries, in which case cold-drawing leads to the break-up of an embedded or coated brittle film into narrow parallel strips that are aligned normally to the drawing axis. A range of materials was explored to establish the universality of this effect, including silicon, germanium, gold, glasses, silk, polystyrene, biodegradable polymers and ice. We observe, and verify through nonlinear finite-element simulations, a linear relationship between the smallest transverse scale and the longitudinal break-up period. These results may lead to the development of dynamical and thermoreversible camouflaging via a nanoscale Venetian-blind effect, and the fabrication of large-area structured surfaces that facilitate high-sensitivity bio-detection., When a longitudinal tensile stress is applied to a ductile polymer fibre or sheet, the mechanical instability known as 'necking' reduces the transverse dimensions of the sample and longitudinally orients [...]

From drug delivery (1,2) to chemical and biological catalysis (3) and cosmetics (4), the need for efficient fabrication pathways for particles over a wide range of sizes, from a variety [...]

Author(s): Mehmet Bayindir (corresponding author) [1]; Fabien Sorin [2, 3, 5]; Ayman F. Abouraddy [1, 5]; Jeff Viens [1, 3]; Shandon D. Hart [1, 3]; John D. Joannopoulos [1, 2, [...]