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1. Packaging Electronics on Textiles: Identifying Fiber Junctions for Automated Placement

   https://doi.org/10.1115/MSEC2020-8456  

   Challa, Sushmita ; Harnett, Cindy ( September 2020 , 15th International Manufacturing Science and Engineering Conference)

   Electronic textile (E-textile) research requires an understanding of the mechanical properties of fabric substrates used to build and support electronics. Because fibers are often non-uniform and fabrics are easily deformed, locating fiber junctions on the irregular surface is challenging, yet is essential for packaging electronics on textiles at the resolution of single fibers that deliver power and signals. In this paper, we demonstrate the need to identify fiber junctions in a task where microelectromechanical structures (MEMS) are integrated on fabrics. We discuss the benefits of fiber-aligned placement compared with random placement. Thereafter we compare three image processing algorithms to extract fiber junction locations from sample fabric images. The Hough line transform algorithm implemented in MATLAB derives line segments from the image to model the fibers, identifying crossings by the intersections of the line segments. The binary image analysis algorithm implemented in MATLAB searches the image for unique patterns of 1s and 0s that represent the fiber intersection. The pattern matching algorithm implemented in Vision Assistant - LabVIEW, uses a pyramid value correlation function to match a reference template to the remainder of the fabric image to identify the crossings. Of the three algorithms, the binary image analysis method had the highest accuracy, while the pattern matching algorithm was fastest. 

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2. Nylon Fabric Enabled Tough and Flaw Insensitive Stretchable Electronics

   https://doi.org/10.1002/admt.201800466  

   Sim, Kyoseung ; Gao, Yang ; Chen, Zhou ; Song, Jizhou ; Yu, Cunjiang ( December 2018 , Advanced Materials Technologies)

   The architecture of stretchable electronics, typically in the fashion of very thin functional electronics on a stretchable rubber substrate, defines their mechanical robustness which is dominantly attributed to the stretchable rubber substrate. Most of the existing and reported stretchable electronics are vulnerable to flaws or cracks in the substrate and subject to fracture upon mechanical deformation, which limits their practical usages. Here, a class of tough and flaw insensitive stretchable electronics enabled by a Nylon/rubber composite substrate is reported. The woven and stretchable fibers in the Nylon fabric are responsible for its high toughness and flaw insensitivity, as they prevent crack propagation by dissipating the energy into the nearby fiber network and also the rubber matrix to yield enhanced toughness and flaw insensitivity. Stretchable electrodes, supercapacitors, and photodetectors with high toughness and flaw insensitivity are developed as examples to illustrate the validity of such a type of stretchable electronics. Systematic studies of the associated materials, fabrication, mechanical and electrical properties, and reliability illustrate the key aspects of such a type of stretchable tough and flaw insensitive electronics and also suggest routes toward stretchable electronics with other functions.

    

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3. Hysteresis in spreading and retraction of liquid droplets on parallel fiber rails

   https://doi.org/10.1039/D1SM00126D  

   Wang, Fang ; Schiller, Ulf D. ( January 2021 , Soft Matter)

   null (Ed.)

   Wetting and spreading of liquids on fibers occurs in many natural and artificial processes. Unlike on a planar substrate, a droplet attached to one or more fibers can assume several different shapes depending on geometrical parameters such as liquid volume and fiber size and distance. This paper presents lattice Boltzmann simulations of the morphology of liquid droplets on two parallel cylindrical fibers. We investigate the final shapes resulting from spreading of an initially spherical droplet deposited on the fibers and from retraction of an initial liquid column deposited between the fibers. We observe three possible equilibrium configurations: barrel-shaped droplet, droplet bridges, and liquid columns. We determine the complete morphology diagram for varying inter-fiber spacing and liquid volume and find a region of bistability that spans both the column regime and the droplet regime. We further present a simulation protocol that allows to probe the hysteresis of transitions between different shapes. The results provide insights into energies and forces associated with shape transformations of droplets on fibers that can be used to develop fiber-based materials and microfluidic systems for manipulation of liquids at small scale. 

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4. Electrospinning to Surpass White Natural Silk in Sunlight Rejection for Radiative Cooling

   https://doi.org/10.1002/adpr.202100008  

   Park, Bo Kyung ; Um, In Chul ; Han, Sang M. ; Han, Sang Eon ( May 2021 , Advanced Photonics Research)

   Natural white silk cocoons exhibit strong broadband optical scattering by Anderson localization within the silk fibers, providing a low‐light environment to the pupae inside. This scattering effect is due to thousands of densely packed parallel fibrillar nanovoids running within each fiber along the fiber axis. Herein, to enhance sunlight rejection from white silk, conventional diffusive optical transport without Anderson localization is used. For optical diffusion, natural white silk is restructured by electrospinning to destroy the fibrillar nanovoids. Sunlight rejection power of the electrospun structures is controlled by the fiber diameters. Relative to a nonwoven raw silk fabric, a restructured silk film with a mean fiber diameter of a quarter micron substantially increases optical scattering strength in the visible spectrum and emissivity in the mid‐infrared atmospheric transparency window. The restructured silk fibrous film can reduce the average temperature of a substrate, on which the film is coated, by 7.5 °C relative to a nonwoven raw silk fabric during daytime under solar radiation. The results suggest that artificially processed polymeric fiber mats can achieve substantially stronger sunlight rejection than natural silk, by using optical diffusion without Anderson localization. These polymeric mats are useful as sunshades in various applications.

    

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5. Quantification of Errors in Applying DIC to Fiber Networks Imaged by Confocal Microscopy

   https://doi.org/10.1007/s11340-022-00870-6  

   Sarkar, M. ; Notbohm, J. ( January 2022 , Experimental Mechanics)

   Background. An assumption of Digital Image Correlation (DIC) is that the displacement field within each subset is relatively smooth, captured with reasonable accuracy by, for example, linear or quadratic shape functions. Although this assumption works well for many materials, it becomes problematic for heterogeneous materials, such as fiber networks, wherein the length scale of heterogeneity matches the size of a subset. Objective. Here we applied DIC to fibrous networks made of collagen, for which displacements at the scale of a subset are highly heterogeneous, but errors caused by the heterogeneity are difficult to quantify. We developed a method to quantify such errors. Methods. We began by generating a synthetic three-dimensional fiber network with structure matching that of gels made of fibrous collagen. We then formulated an algorithm to mimic the way in which a confocal microscope images the fibers at its focal plane, thereby generating synthetic images similar to those obtained in experiments. Displacement boundary conditions were applied to the synthetic fiber networks, and the resulting displacement fields were computed using a finite element solver. DIC was applied to the synthetic images, and displacements were compared to the data from the finite element method, enabling rigorous quantification of error. Results. Point-wise errors in the DIC-measured displacements were substantial, often exceeding 40%, but over regions far larger than the length scales of heterogeneity or the DIC subset size, errors were modest, e.g., ≤15%. Conclusions. Although DIC can accurately measure displacements of fiber networks at length scales larger than the subset window, quantification of mechanical behavior at the scale of material heterogeneity will require new methods to complement or replace the use of DIC. 

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