SciCombinator

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Concept: Continuum mechanics

137

Asymmetrical shear rolling with velocity asymmetry and geometry asymmetry is beneficial to enlarge deformation and refine grain size at the center of the thick plate compared to conventional symmetrical rolling. Dynamic recrystallization (DRX) plays a vital role in grain refinement during hot deformation. Finite element models (FEM) coupled with microstructure evolution models and cellular automata models (CA) are established to study the microstructure evolution of plate during asymmetrical shear rolling. The results show that a larger DRX fraction and a smaller average grain size can be obtained at the lower layer of the plate. The DRX fraction at the lower part increases with the ascending speed ratio, while that at upper part decreases. With the increase of the offset distance, the DRX fraction slightly decreases for the whole thickness of the plate. The differences in the DRX fraction and average grain size between the upper and lower surfaces increase with the ascending speed ratio; however, it varies little with the change of the speed ratio. Experiments are conducted and the CA models have a higher accuracy than FEM models as the grain morphology, DRX nuclei, and grain growth are taken into consideration in CA models, which are more similar to the actual DRX process during hot deformation.

Concepts: Finite element method, Cellular automaton, Continuum mechanics, Euclid's Elements, The Ambidextrous Universe, Asymmetry, Metallurgy, Symmetry

53

Understanding the compatibility between spider silk and conducting materials is essential to advance the use of spider silk in electronic applications. Spider silk is tough, but becomes soft when exposed to water. Here we report a strong affinity of amine-functionalised multi-walled carbon nanotubes for spider silk, with coating assisted by a water and mechanical shear method. The nanotubes adhere uniformly and bond to the silk fibre surface to produce tough, custom-shaped, flexible and electrically conducting fibres after drying and contraction. The conductivity of coated silk fibres is reversibly sensitive to strain and humidity, leading to proof-of-concept sensor and actuator demonstrations.

Concepts: Spider, Carbon nanotube, Fiber, Materials science, Continuum mechanics, Tensile strength, Spider silk, Silk

44

Soft actuators made from elastomeric active materials can find widespread potential implementation in a variety of applications ranging from assistive wearable technologies targeted at biomedical rehabilitation or assistance with activities of daily living, bioinspired and biomimetic systems, to gripping and manipulating fragile objects, and adaptable locomotion. In this manuscript, we propose a novel two-component soft actuator design and design tool that produces actuators targeted towards these applications with enhanced mechanical performance and manufacturability. Our numerical models developed using the finite element method can predict the actuator behavior at large mechanical strains to allow efficient design iterations for system optimization. Based on two distinctive actuator prototypes' (linear and bending actuators) experimental results that include free displacement and blocked-forces, we have validated the efficacy of the numerical models. The presented extensive investigation of mechanical performance for soft actuators with varying geometric parameters demonstrates the practical application of the design tool, and the robustness of the actuator hardware design, towards diverse soft robotic systems for a wide set of assistive wearable technologies, including replicating the motion of several parts of the human body.

Concepts: Failure assessment, Linear actuator, Continuum mechanics, Human body, Numerical analysis, Design, Finite element method, Engineering

37

Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell’s scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric ‘micro-scallop’, a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids.

Concepts: Fluid mechanics, Newtonian fluid, Shear stress, Continuum mechanics, Fluid, Non-Newtonian fluid, Fluid dynamics, Viscosity

30

Prolonged sitting impairs endothelial function in the leg vasculature and this impairment is thought to be largely mediated by a sustained reduction in blood flow-induced shear stress. Indeed, preventing the marked reduction of shear stress during sitting with local heating abolishes the impairment in popliteal artery endothelial function. Herein, we tested the hypothesis that sitting-induced reductions in shear stress and ensuing endothelial dysfunction would be prevented by periodic leg movement, or “fidgeting”. In 11 young healthy subjects, bilateral measurements of popliteal artery flow-mediated dilation (FMD) were performed before and after a 3-hour sitting period during which one leg was subjected to intermittent fidgeting (1 min on/4 min off) while the contralateral leg remained still throughout and served as an internal control. Fidgeting produced a pronounced increase in popliteal artery blood flow and shear rate (pre-fidgeting: 33.7±2.6sec(-1) to immediately post-fidgeting: 222.7±28.3sec(-1); mean±SE; p<0.001) that tapered off during the following 60 sec. Fidgeting did not alter popliteal artery blood flow and shear rate of the contralateral leg, which was subjected to a reduction in blood flow and shear rate throughout the sitting period (pre-sit: 71.7±8.0sec(-1) to 3-hr sit: 20.2±2.9sec(-1); p<0.001). Popliteal artery FMD was impaired after 3 hours of sitting in the control leg (pre-sit: 4.5±0.3% to post-sit: 1.6±1.1%; p=0.039) but improved the fidgeting leg (pre-sit: 3.7±0.6% to post-sit: 6.6±1.2%; p=0.014). Collectively, the present study provides evidence that prolonged sitting-induced leg endothelial dysfunction is preventable with small amounts of leg movement while sitting, likely through the intermittent increases in vascular shear stress.

Concepts: Blood vessel, Sitting, Continuum mechanics, Viscosity, Force, Heart

29

The rheological properties of wet powder masses used in the preparation of pharmaceutical pellets by extrusion/spheronization were evaluated utilizing capillary and rotational rheometers. A ram extruder was used as a capillary rheometer to construct flow and viscosity curves for each wet mass under different extrusion rates and die geometry. As a result, shear thinning behavior was observed for all wet masses. Among the considered rheological models Power Law and Herschel-Bulkley models fitted well with the experimental results. For the majority of the wet masses, water separation and migration occurred during extrusion which led to uneven water content in the extrudate. The effect of extrusion condition including extrusion speed, die geometry and water content on the occurrence of water separation was investigated and the surface quality of the extrudates was compared. In addition, dynamic rheometry tests were done by a parallel plate rheometer to investigate the viscoelastic properties of the wet masses. The frequency sweep tests showed that as water content of the wet masses decreases storage (G') and loss modulus (G″) increase. The storage modulus values were much higher than those of the loss modulus showing dominated elastic rather than viscous behavior for the wet masses at low deformation rates.

Concepts: Rheometry, Fluid mechanics, Viscoelasticity, Liquid, Rheology, Viscosity, Continuum mechanics, Rheometer

28

How cells sense their mechanical environment and transduce forces into biochemical signals is a crucial yet unresolved question in mechanobiology. Platelets use receptor glycoprotein Ib (GPIb), specifically its α subunit (GPIbα), to signal as they tether and translocate on von Willebrand factor (VWF) of injured arterial surfaces against blood flow. Force slows VWF-GPIbα dissociation (catch bond) and unfolds the GPIbα leucine-rich repeat domain (LRRD) and juxtamembrane mechanosensitive domain (MSD). How these mechanical processes trigger biochemical signals remains unknown. Here we analyze these extracellular events and the resulting intracellular Ca(2+) on a single platelet in real time, revealing that LRRD unfolding intensifies the Ca(2+) signal analogously whereas MSD unfolding determines the Ca(2+) type digitally. The >30nm macroglycopeptide separating the two domains transmits VWF-GPIbα bond lifetime prolonged by LRRD unfolding to enhance MSD unfolding cooperatively at an optimal force, which may serve as a design principle for a generic mechanosensory machine.

Concepts: Continuum mechanics, Cell biology, Coagulation, Bernard-Soulier syndrome, Blood, Von Willebrand disease, Platelet, Von Willebrand factor

28

Arapaima gigas, a fresh water fish found in the Amazon Basin, resist predation by piranhas through the strength and toughness of their scales, which act as natural dermal armour. Arapaima scales consist of a hard, mineralized outer shell surrounding a more ductile core. This core region is composed of aligned mineralized collagen fibrils arranged in distinct lamellae. Here we show how the Bouligand-type (twisted plywood) arrangement of collagen fibril lamellae has a key role in developing their unique protective properties, by using in situ synchrotron small-angle X-ray scattering during mechanical tensile tests to observe deformation mechanisms in the fibrils. Specifically, the Bouligand-type structure allows the lamellae to reorient in response to the loading environment; remarkably, most lamellae reorient towards the tensile axis and deform in tension through stretching/sliding mechanisms, whereas other lamellae sympathetically rotate away from the tensile axis and compress, thereby enhancing the scale’s ductility and toughness to prevent fracture.

Concepts: Plastic deformation in solids, Freshwater fish, Tensile strength, Work hardening, Amazon Basin, Continuum mechanics, Deformation, Amazon River

28

Bone fragility depends on its post-yield behavior since most energy dissipation in bone occurs during the post-yield deformation. Previous studies have investigated the progressive changes in the post-yield behavior of human cortical bone in tension and compression using a novel progressive loading scheme. However, little is known regarding the progressive changes in the post-yield behavior of bone in shear. The objective of this short study was to address this issue by testing bone specimens in an inclined double notch shear configuration using the progressive loading protocol. The results of this study indicated that the shear modulus of bone decreased with respect to the applied strain, and the rate of degradation was about 50% less than those previously observed in compression and tension tests. In addition, a quasi-linear relationship between the plastic and applied strains was observed in shear mode, which is similar to those previously reported in tension and compression tests. However, the viscous responses of bone (i.e. relaxation time constants and stress magnitude) demonstrated slight differences in shear compared with those observed in tension and compression tests. Nonetheless, the results of this study suggest that the intrinsic mechanism of plastic deformation of human cortical bone may be independent of loading modes.

Concepts: Elasticity, Continuum mechanics, Shear strain, Shear stress, Cortical bone, Young's modulus, Fluid, Solid mechanics

28

We demonstrate an innovative technique for the direct measurement on the shear modulus of an individual nanorod. This measurement is based on atomic force microscopy (AFM) and micro-fabrication techniques. A nanorod is first aligned along the edge of a small trench in a silicon substrate, and then one end of the nanorod is fixed on the substrate. When an AFM tip scans over the nanorod in contact mode, the nanorod will be twisted by the comprehensive action from the force of the AFM tip, confinement from the trench edge and the fixing end. The shear deformation and the corresponding force that caused the deformation can be retrieved from topography and lateral force image respectively. By small angle approximation, the shear modulus of the ZnO NR, which has a radius of 166nm and a length of 4 µm, is measured to be 8.1 ± 1.9 GPa. This method can be applied directly to characterize the shear modulus of any nanowire/nanorod that possesses a polygon cross section.

Concepts: Continuum mechanics, Units of measurement, Shear stress, Geometry, Young's modulus