Tailoring Surface Properties of Fiber Materials : Novel Opportunities in the Fabrication of Multi-scale Fiber-based Architectures

Many surface properties of materials can be tailored by a proper physical patterning. Such patterned surfaces, commonly fabricated by wafer-based techniques, have been widely exploited for light trapping in advanced photovoltaic systems, for tailoring the hydrophobicity of surface, and for enabling the preferential positioning and growth of biological cells. Nevertheless, wafer-based techniques, despite being versatile and mature, are limited to small, flat and rigid Silicon substrates. There is however an increasing demand for micro and even nanoscale patterns to be deployed over large-area and flexible substrates, over fabrics and textiles, or within confined 3D hollow cavities. Other techniques such as roll-to-roll processes bring some solutions but cannot address the patterning of high curvature surfaces that has resisted scientist and engineers for decades. In this thesis, we propose to exploit the emerging field of multi-material thermal drawing to realize submicrometer scale structure on the surfaces of fibers and ribbons. We then go one step further in the assembly of multi-scale functional architectures by using textured fibers as building blocks in novel additive manufacturing processes. At the heart of the project is the fabrication of potentially kilometers-long polymer fibers with controlled hierarchical surface textures of unprecedented complexity and with feature sizes down to a few hundreds of nanometers. To achieve this result, we first establish a theoretical framework to understand the reflow behavior of the structure during the drawing process, which is identified as the reason behind structure collapsing. From this framework, a strategy is developed to reduce the surface/interfacial tension of textured polymers, thus drastically slowing down the reflow, enabling to create for the first time submicrometer textured fibers. These developments are shown in Chapter II, III and IV of this thesis. In addition, the understanding of reflow during the drawing process, in combination with previous work on capillary break-up, allows us to propose an empirical law, presented in Chapter V, to predict the cross-sectional preservation during a preform-to-fiber deformation, the most important feature for the drawing of functional fibers so far. The law is well verified by successful draws of preforms with freely movable domains, based on which we fabricate micro electro-mechanical fiber devices that can detect and localize multiple pressure points along their length with submillimeter resolution. Finally, in Chapter VI we demonstrate the use of multi-material textured fibers as building blocks for the assembly of advanced 2D and 3D functional constructs. We show two examples of microfluidic devices and of capacitive-touch sensing panels, highlighting the novel opportunities in the scalable nano-scale fabrication of complex devices enabled by the results of this thesis.