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1. Effect of Network Architecture on the Mechanical Behavior of Random Fiber Networks

   Islam, M.R.; Picu, R.C. (June 2018, Journal of applied mechanics)

   Fiber-based materials are prevalent around us. While microscopically these systems resemble a discrete assembly of randomly interconnected fibers, the network architecture varies from one system to another. To identify the role of the network architecture, we study here cellular and fibrous random networks in tension and compression, and in the context of large strain elasticity. We observe that, compared to cellular networks of same global parameter set, fibrous networks exhibit in tension reduced strain stiffening, reduced fiber alignment, and reduced Poisson’s contraction in uniaxial tension. These effects are due to the larger number of kinematic constraints in the form of cross-links per fiber in the fibrous case. The dependence of the small strain modulus on network density is cubic in the fibrous case and quadratic in the cellular case. This difference persists when the number of cross-links per fiber in the fibrous case is rendered equal to that of the cellular case, which indicates that the different scaling is due to the higher structural disorder of the fibrous networks. The behavior of the two network types in compression is similar, although softening induced by fiber buckling and strain localization is less pronounced in the fibrous case. The contribution of transient interfiber contacts is weak in tension and important in compression 

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2. Mechanical behavior of nonwoven non-crosslinked fibrous mats with adhesion and friction

   https://doi.org/10.1039/c9sm00658c  

   Negi, V.; Picu, R. C. (July 2019, Soft Matter)

   We present a study of the mechanical behavior of planar fibrous mats stabilized by inter-fiber adhesion. Fibers of various degrees of tortuosity and of infinite and finite length are considered in separate models. Fibers are randomly distributed, are not cross-linked, and interact through adhesion and friction. The variation of structural parameters such as the mat thickness and the mean segment length between contacts along given fibers with the strength of adhesion is determined. These systems are largely dissipative in that most of the work performed during deformation is dissipated frictionally and only a small fraction is stored as strain energy. The response of the mats to tensile loading has three regimes: a short elastic regime in which no sliding at contacts is observed, a well-defined sliding regime characterized by strain hardening, and a rapid stiffening regime at larger strains. The third regime is due to the formation of stress paths after the fiber tortuosity is pulled out and is absent in mats of finite length fibers. Networks of finite length fibers lose stability during the second regime of deformation. The scaling of the yield stress, which characterizes the transition between the first and the second regimes, and of the second regime's strain hardening modulus, with system parameters such as the strength of adhesion and friction and the degree of fiber tortuosity are determined. The strength of mats of finite length fibers is also determined as a function of network parameters. These results are expected to become useful in the design of electrospun mats and other planar fibrous non-cross-linked networks. 

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3. Effects of sample shape and adhesive type on rheology of unidirectional carbon fiber prepregs

   Das, A; Chan, KJ; Dillard, DA; De_Focatiis, DSA; Bortner, MJ (June 2022, Annual Technical Conference - ANTEC, Conference Proceedings)

   The viscoelastic properties of carbon fiber reinforced thermoset composites are of utmost importance during processing such materials using composite forming. The quality of the manufactured parts is largely dependent on intelligent process parameter selection based on the viscoelastic and flow properties of the polymer resin. Viscoelastic properties such as the complex viscosity (η*), storage modulus (G'), loss modulus (G''), and loss tangent (tanδ) are used to determine the critical transition events (such as gelation) during curing. An understanding of the changes in viscoelastic properties as a function of processing temperature and degree of cure provides insight to establish a suitable processing range for compression forming of prepreg systems. However, tracking viscoelastic properties as a function of cure during the forming process is a challenging task. In this current work, we have investigated the effect of sample size and adhesive type on the rheological properties of a commercially available carbon fiber prepreg material. Specifically, determining the linear viscoelastic region (LVE) as a function of sample configuration and different adhesive chemistries were explored. The results suggest that the square-shaped sample geometries coupled with cyanoacrylate based adhesive are optimum for conducting rheological characterization on the carbon fiber prepreg system. 

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4. Random Fiber Networks With Superior Properties Through Network Topology Control

   https://doi.org/DOI: 10.1115/1.4043828  

   S. Deogekar, Z. Yan (June 2019, Journal of applied mechanics)

   In this work, we study the effect of network architecture on the nonlinear elastic behavior and strength of athermal random fiber networks of cellular type. We introduce a topology modification of Poisson–Voronoi (PV) networks with convex cells, leading to networks with stochastic nonconvex cells. Geometric measures are developed to characterize this new class of nonconvex Voronoi (NCV) networks. These are softer than the reference PV networks at the same nominal network parameters such as density, cross-link density, fiber diameter, and connectivity number. Their response is linear elastic over a broad range of strains, unlike PV networks that exhibit a gradual increase of the tangent stiffness starting from small strains. NCV networks exhibit much smaller Poisson contraction than any network of same nominal parameters. Interestingly, the strength of NCV networks increases continuously with an increasing degree of nonconvexity of the cells. These exceptional properties render this class of networks of interest in a variety of applications, such as tissue scaffolds, nonwovens, and protective clothing. 

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5. Push-pull mechanics of E-cadherin ectodomains

   K. Nagendra, A. Izzet (April 2023, Biophysical journal)

   E-cadherin plays a central role in cell-cell adhesion. The ectodomains of wild type cadherins form a crystalline- like two dimensional lattice in cell-cell interfaces mediated by both trans (apposed cell) and cis (same cell) interactions. In addition to these extracellular forces, adhesive strength is further regulated by cytosolic phenomena involving 𝛼 and 𝛽- catenin–mediated interactions between cadherin and the actin cytoskeleton. Cell-cell adhesion can be further strengthened under tension through mechanisms that have not been definitively characterized in molecular detail. Here we quantitatively determine the role of the cadherin ectodomain in mechanosensing. To this end, we devise an E-cadherin-coated emulsion system, in which droplet surface tension is balanced by protein binding strength to give rise to stable areas of adhesion. To reach the honeycomb/cohesive limit, an initial emulsion compression by centrifugation facilitates E-cadherin trans-binding, while a high protein surface concentration enables the cis-enhanced stabilization of the interface. We observe an abrupt concentration dependence on recruitment into adhesions of constant crystalline density, reminiscent of a first-order phase transition. Removing the lateral cis-interaction with a "cis mutant" shifts this transition to higher surface densities leading to denser, yet weaker adhesions. In both proteins, the stabilization of progressively larger areas of deformation can be rationalized by a stiffening catch-bond, whose strength increases with tension. This catch bond may well correspond to one that has been identified in the cadherin “X-dimer". 

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