The discovery of cold-drawing at DuPont in the 1930s was fundamental to the development of synthetic fibers, such as polyester and nylon. In such industrial applications, tensile stress reduces the fiber diameter and orients the polymer chains.
MIT MRSEC researchers have demonstrated for the first time a selective cold drawing process in multi-material fibers in which one material undergoes cold-drawing while the others do not. By exploiting a mechanical-geometric instability associated with neck propagation upon cold-drawing, they are able to controllably and sequentially fragment embedded cores into uniformly sized rods along meters of fiber or films.
Top: Photograph of a cylindrical polymer fiber undergoing cold-drawing under axial stress at a speed of approximately 5 mm/s. Multiple shots taken over 1 min are overlaid to highlight the extent of fiber elongation. Bottom: Transmission optical micrograph of a multimaterial cylindrical fiber after undergoing cold-drawing, as shown in the top panel. The fiber consists of a brittle glass core (diameter ~ 10 – 20 microns) embedded in a larger diameter ductile polymer cladding ( diameter ~ 1 mm).
This new fragmentation process enables the scalable fabrication of nano- and micro-particles of complex shape and from a broad range of materials including crystalline semiconductors, bio-degradable polymers, gold, silk, and even ice. Micro-rods are obtained after cold-drawing and dissolving the cladding (top images).
When coupled with a thermal restoration process, self-healing (middle images) of the fragmented structure can be achieved.
Films subject to the same process can be used to produce large-area nano-photonic structures with mechanically induced diffraction gratings (bottom images).