Fiber Radius Research Articles

Nanocellular foams confined within PS microfibers obtained by CO2 batch foaming process

Nanocellular foams localized in PS (polystyrene) microfibers with 10.0---20.0 µm-diameter were prepared by CO2 (carbon dioxide) one-step foaming method. The CO2-soaking conditions were operated at the slightly above Tg (glass transition temperature) of PS that maintains the fiber structures was investigated. The control of bubble nucleation in the foaming mechanism was driven by the depressurization rate adjustment. The higher depressurization rate and the use of the lower diameter of PS microfibers have resulted in the reduction of the obtained cell diameter and larger of cell density. The average of the smallest cell size diameter of 60---70 nm with the number of cell density of 1.1 × 1014 cmź3 was obtained by the depressurization rate of 3.0 MPa/min using PS microfibers of 10 µm-diameter. The ratio of foamed region (core region) and the fiber radius was slightly increased by the reduction of the depressurization rate and using the larger diameter of PS microfibers. The formation of cellular structures could be clarified by P---T (pressure---temperature) profiles combined phase diagram of CO2---PS system. The changed in thermal properties and mechanical properties of the PS fibers caused by the formation of the nanocellular foams were reported in this study.

Free vibration of thick laminated circular and annular plates using three-dimensional finite element analysis

This paper presents a numerical analysis of free vibration of thick laminated circular plates, having free, clamped as well as simply-supported boundary conditions at outer edges. The finite element method that works on the basis of three-dimensional theory of elasticity is employed to evaluate the first ten natural frequencies of uniform circular plate. The effect of thickness ratios, fiber orientation angle (angle-ply and cross-ply), stacking sequences (symmetric and antisymmetric), radius ratios and boundary conditions on frequency parameter are discussed in detail. The first five natural modes of flexural vibrations for different boundary conditions are presented in pictorial forms. Verification of the accuracy of these results is verified by appropriate convergence study and checked with the results available in the literature.

Modeling and computation of nonlinear rotating polymeric jets during forcespinning process

In the usual forcespinning (FS) process, a meso-scale fluid jet is forced through an orifice of a rotating spinneret, where the ambient fluid is air. This leads to the formation of a jet with a curved centerline. In this study we make use of a phenomenological viscosity model for polymeric fluid to investigate the properties of nonlinear polymeric fiber jets during FS process. We apply multi-scale and perturbation techniques to determine the governing modeling systems for such nonlinear rotating jets and their stabilities. First, we calculate numerically the expressions for the leading order nonlinear steady solutions for the jet quantities such as radius, speed, stretching rate, strain rate and trajectory versus arc length, and we determine, in particular, these quantities for different values of the parameters that represent effects due to rotation, viscosity and relaxation time. Next, we calculate the stability of the nonlinear jet versus different types of perturbations. We find that the nonlinear fiber jet flow can be stable in most cases and uncover conditions for which fiber radius reduces and the jet speed or stretching rate increases.

Fiber optic macro-bend based sensor for detection of metal loss

Metal loss in metallic structures, often as a result of corrosion, is a severe problem across multiple industries. Catastrophic consequence of structural failure due to such loss of structural metal requires an accurate determination and assessment of corrosion. Widely used electrochemical methods can only suggest the likelihood of the metal loss due to corrosion while failing to provide a quantitative measure of the accumulated amount of corrosion. Due to its unique advantages such as small size, light weight, resistance to electromagnetic interference and corrosion, fiber optic sensing technique has been emerging as a promising alternative for most sensing applications. In this paper, a novel type of ferromagnetic distance-based metal loss sensor is proposed based on the principle of fiber optic macro-bend loss. The proposed sensor is composed of the bended optical fiber, the magnet and a spring. The magnet is connected to the spring and the fiber bend is attached to the spring in such a way that the movement of the magnet will induce a change in bending radius of the optical fiber. Metal loss in the monitored sample increases the distance between the magnet and the metal surface and thereby reducing the magnetic force. A change in magnetic force will lead to the variation in light intensity loss of the fiber optic macro-bend, thus metal loss, such as in the form of corrosion pits, can be detected by the proposed metal loss sensor. The practicality of the proposed distance sensor for metal loss measurement is validated through scanning the fabricated corrosion samples.

Optical bottle versus acoustic bottle and antibottle resonators.

The theory of slow acoustic modes propagating along the optical fiber and being controlled by the nanoscale variation of the effective fiber radius (analogous to the theory of slow optical whispering gallery modes) is developed. Surprisingly, it is shown that, in addition to acoustic bottle resonators (which are similar to optical bottle resonators), there exist antibottle resonators, the neck-shaped deformations of the fiber that can fully confine acoustic modes. It is also shown that an eigenfrequency of the mechanical vibrations of a silica parabolic bottle resonator can match the separation between the eigenfrequencies of a series of its optical modes, thereby enabling the resonant mechanical excitation of these series. The developed theory paves the groundwork for slow-mode optomechanics in an optical fiber.

On the CFD Analysis of a Stratified Taylor-Couette System Dedicated to the Fabrication of Nanosensors

Since the pioneering work of Taylor, the analysis of flow regimes of incompressible, viscous fluids contained in circular Couette systems with independently rotating cylinders have charmed many researchers. The characteristics of such kind of flows have been considered for some industrial applications. Recently, Taylor-Couette flows found an innovative application in the production of optical fiber nanotips, to be used in molecular biology and medical diagnostic fields. Starting from the activity of Barucci et al., the present work concerns the numerical analysis of a Taylor-Couette system composed by two coaxial counter-rotating cylinders with low aspect ratio and radius ratio, filled with three stratified fluids. An accurate analysis of the flow regimes is performed, considering both the variation of inner and outer rotational speed and the reduction of fiber radius due to etching process. The large variety of individuated flow configurations provides useful information about the possible use of the Taylor-Couette system in a wide range of engineering applications. For the present case, the final objective is to provide accurate information to manufacturers of fiber nanotips about the expected flow regimes, thus helping them in the setup of the control process that will be used to generate high-quality products.

An improved shear lag model for predicting stress distribution in hybrid fiber reinforced rubber composites

An improved micromechanical shear lag model, which considers the interphase and bonded fiber end, is developed to investigate the load-carrying characteristics and stress profiles in hybrid aramid/sepiolite fiber reinforced rubber composites. The properties of the equivalent matrix, which is combination of sepiolite fiber and rubber matrix, are determined by Mori-Tanaka method. The axial and shear stresses at the fiber end are resolved by the imaginary fiber technique. The results obtained from the improved model show the tensile stress has a maximal at the real fiber center and the interfacial shear stress has a maximal at the end of the real fiber. Comparing with the results from Tsai’s model, the improved model has a better agreement with the numerical simualtion results. The effects of the imaginary fiber length on the stress transfer are analyzed and the results show that the effects can be ignored when the imaginary fiber length is greater than twice of the fiber radius. The effects of interphase modulus and thickness on the maximal axial and shear stresses are discussed. The results show that the interphase modulus and thickness of about 106.3 MPa and 0.2 μm are optimal to prevent interfacial debonding and improve the strength of hybrid fiber reinforced rubber composites.

The glass transition and interfacial dynamics of single strand fibers of polymers.

We investigate the glass transition and interfacial dynamics of single strand fibers of flexible polymers by employing molecular dynamics (MD) simulations along with a coarse grained model. While the polymer fiber has drawn significant attention due to its applicability in tissue engineering and stretchable electronics, its dynamic properties, especially the glass transition temperature (Tg), are yet to be understood at the molecular level. For example, there has been a controversy on the effect of the polymer fiber radius (R) on Tg: Tg decreased with a decrease in R for some polymer fibers, whereas Tg of other polymer fibers was not sensitive to R. In this article, we estimate the bond relaxation time of polymers and evaluate both Tg and fragility (m) as a function of R. We illustrate that Tg of the polymer fiber decreased with a decrease in R monotonically and also that the values of Tg follow faithfully the empirical equation proposed by Keddie et al. as a function of R, which was successfully employed to fit the values of Tg of both polyvinyl alcohol (PVA) fibers and polyethylene (PE) fibers. We also find that the dynamics of polymers at the interface between a polymer fiber and air is faster than that of polymers at the center. By employing Adam-Gibbs theory, we show that the fast interface dynamics of polymer fibers should influence the cooperative motion of monomers, which should be responsible for the decrease in Tg for smaller values of R. Near the interface there are more mobile monomers that participate in the cooperative motions of polymers. Interesting is that due to the curved surface (unlike flat polymer films) the cooperative motion of monomers is anisotropic in polymer fibers.

Modeling of dopant segregation in sapphire single crystal fibre growth by Micro-Pulling-Down method

Experiments and numerical simulations are conducted in order to study the causes and solutions for the Ti inhomogeneity problem in Ti doped sapphire Micro-Pulling-Down (μ-PD) growth. The measurement and modeling of the thermal and flow fields, electromagnetic field, Ti concentration in the molten zone and along the fibre axis are compared. For the mean Ti concentration along the fibre and temperature along the iridium crucible, the modeling results are consistent with experiments. Results showed that for high pulling rate, the mass transfer in the capillary is dominated by convection. Marangoni convection is strong in the meniscus due to the large temperature gradient, which has great impact on the Ti distribution for different fibre radii. For high pulling rate, Ti concentration increases quickly from the seed along the fibre axis, and reaches a constant value after about 0.5–2mm. Radial segregation is high for large diameter fibres. The constant Ti concentration along the fibre axis is increasing when increasing the fibre radius from 0.2 to 0.6mm. For 0.8mm, it decreases due to the change of the vortex. At low growth rate, the transport in the capillary is diffusive, back to the crucible, which leads to a Scheil-like Ti distribution, in full agreement with the experimental results.

Study of structure–mechanical heterogeneity of polyacrylonitrile-based carbon fiber monofilament by plasma etching-assisted radius profiling

We present a feasible method, named as radius profiling, to study the structure–mechanical heterogeneity of polyacrylonitrile (PAN)-based carbon fibers (CFs). CF monofilaments were peeled layer by layer by plasma etching, and their electrical resistances and mechanical properties were measured by a multimeter and a single-filament tensile tester, respectively. The method allows the structure and mechanical properties of CF monofilament to be explored with high spatial resolution. The measurement results show that the modulus distribution along the fiber radius direction is highly dependent on the local crystallinity, and the maximums of both modulus and crystallinity appear at approximately 1.1–1.6 μm beneath the surface. The observation is used to explain the high modulus of PAN-based CFs.

Cellulose dissolution: insights on the contributions of solvent-induced decrystallization and chain disentanglement

The dissolution of cellulose is a critical step for the efficient utilization of this renewable resource as a starting material for the synthesis of high value-added functional polymers and chemicals and also for biofuel production. The recalcitrance of semicrystalline cellulose microfibrils presents a major barrier to cellulose dissolution. Despite research efforts, important aspects of cellulose dissolution such as solvent-induced decrystallization and chain disentanglement are not well-understood. Here we address these fundamental issues with the practical goal of gaining insights into the swelling and dissolution of cellulose that cannot be obtained from macroscopic experimental data. To this end, we have used a newly-developed phenomenological model that captures the phenomena governing the dissolution of semicrystalline polymers as well as the thermodynamics and kinetics of dissolution. This model fits well experimental data for swelling and dissolution of cotton fibers in the ionic liquid [bmim]Cl, and allows the quantification of two important aspects, i.e., solvent effectiveness in cellulose (1) decrystallization and (2) chain disentanglement, the balance of which controls the mechanism and kinetics of cellulose dissolution. The activation parameters of cellulose decrystallization, estimated using the obtained decrystallization constant values, reveal that the decrystallization of cellulose in [bmim]Cl is associated with positive enthalpy and entropy and it is also very sensitive to temperature. When the solvent effectiveness in the disruption of cellulose crystals is relatively lower than its ability to disentangle the chains, the kinetics of dissolution are controlled by decrystallization. Furthermore, conditions that facilitate cellulose chain disentanglement, in addition to increasing the rate of dissolution, can result in faster decrystallization. The solvent effectiveness in chain disentanglement is the only factor that determines the decrease of the cellulose fiber radius. In cases where the fiber dissolution rate is lower than the decrystallization rate, the dissolution of cellulose is mostly controlled by the solvent ability to disentangle the chains. The insights obtained from this study improve the understanding of cellulose–solvent interactions underlying decrystallization and disentanglement and their contributions in controlling the kinetics of cellulose swelling and dissolution.

Assessing the Increase in Specific Surface Area for Electrospun Fibrous Network due to Pore Induction.

The effect of pore induction on increasing electrospun fibrous network specific surface area was investigated in this study. Theoretical models based on the available surface area of the fibrous network and exclusion of the surface area lost due to fiber-to-fiber contacts were developed. The models for calculation of the excluded area are based on Hertzian, Derjaguin-Muller-Toporov (DMT), and Johnson-Kendall-Roberts (JKR) contact models. Overall, the theoretical models correlated the network specific surface area to the material properties including density, surface tension, Young's modulus, Poisson's ratio, as well as network physical properties, such as density and geometrical characteristics including fiber radius, fiber aspect ratio and network thickness. Pore induction proved to increase the network specific surface area up to 52%, compared to the maximum surface area that could be achieved by nonporous fiber network with the same physical properties and geometrical characteristics. The model based on Johnson-Kendall-Roberts contact model describes accurately the fiber-to-fiber contact area under the experimental conditions used for pore generation. The experimental results and the theoretical model based on Johnson-Kendall-Roberts contact model show that the increase in network surface area due to pore induction can reach to up to 58%.

Modeling blood filtration in hollow fibers dialyzers coupled with patient’s body dynamics

We develop a mathematical model for cross filtration in a hollow fibers dialyzer, taking into account not only the phenomena occurring within the machine, but also the redistribution of chemicals between intra- and extracellular compartments in the patient’s body. The scheme for the cross flow is derived with reference to a single fiber, starting from the basic laws of fluid dynamics and exploiting the smallness of the ratio between the fiber radius and the fiber length to obtain significant simplifications. We end up with a system of integral and partial di¤erential equations in which the input data are in principle unknown. Indeed, the blood composition is considerably altered while going through the fiber and the body reacts redistributing urea, sodium ions, etc. between intra- and extracellular compartments with its own dynamics, thus updating the various concentrations at the dialyzer inlet. Such a coupling is an essential feature of the model. We present numerical simulations, showing a reasonable agreement with the data for a specific patient taken from the literature.

Influence of selected process parameters on changes of the fiber orientation in unidirectional reinforced thermoplastics during a hot pressing process

The research described in this paper is part of SFB 639 “Textile‐reinforced composite components for function integrating multi‐material design in complex lightweight applications.” In this study, changes in fiber orientation of unidirectional (UD) aligned glass polypropylene hybrid yarn were investigated. A new model for simulating changes in fiber orientation of unidirectional reinforced thermoplastics during a hot pressing process was developed. The material observed during this study is a hybrid yarn made out of homogenous aligned glass and polypropylene filaments. The mechanistic model, which was used to simulate the fiber behavior, allows the user to define different input data, such as velocity, vorticity, and viscosity of the molten polymer as well as fiber radius, initial fiber positions and fiber stiffness to characterize material properties. To determine fiber orientations, a program was written in MATLAB. Hence, it is possible to calculate the angle of any desired segment at any point of time during the hot pressing process. Thereby, it was feasible to compare the numerical obtained changes in fiber orientation with those measured during the experiments conducted at the end of the research. The post‐processing algorithm was used to determine the effects of selected processing parameters and model parameters on the fiber behavior respectively. POLYM. COMPOS., 39:2241–2249, 2018. © 2016 Society of Plastics Engineers

Tube-side mass transfer for hollow fibre membrane contactors operated in the low Graetz range

Transformation of the tube-side mass transfer coefficient derived in hollow fibre membrane contactors (HFMC) of different characteristic length scales (equivalent diameter and fibre length) has been studied when operated in the low Graetz range (Gz<10). Within the low Gz range, mass transfer is generally described by the Graetz problem (Sh=3.67) which assumes that the concentration profile comprises a constant shape over the fibre radius. In this study, it is experimentally evidenced that this assumption over predicts mass transfer within the low Graetz range. Furthermore, within the low Gz range (below 2), a proportional relationship between the experimentally determined mass transfer coefficient (Kov) and the Graetz number has been identified. For Gz numbers below 2, the experimental Sh number approached unity, which suggests that mass transfer is strongly dependent upon diffusion. However, within this diffusion controlled region of mass transfer, tube-side fluid velocity remained important. For Gz numbers above 2, Sh could be satisfactorily described by extension to the Lévêque solution, which can be ascribed to the constrained growth of the concentration boundary layer adjacent to the fibre wall. Importantly this study demonstrates that whilst mass transfer in the low Graetz range does not explicitly conform to either the Graetz problem or classical Lévêque solution, it is possible to transform the experimentally derived overall mass transfer coefficient (Kov) between characteristic length scales (dh and L). This was corroborated by comparison of the empirical relationship determined in this study (Sh=0.36Gz) with previously published studies operated in the low Gz range. This analysis provides important insight for process design when slow tube-side flows, or low Schmidt numbers (coincident with gases) constrain operation of hollow fibre membrane contactors to the low Gz range.

Diffusion in hierarchical systems: A simulation study in models of healthy and diseased muscle tissue

PurposeTo investigate the sensitivity of diffusion‐MR signal to microstructural change in muscle tissue associated with pathology, and recommend optimal acquisition parameters.MethodsWe employ Monte‐Carlo simulation of diffusing spins in hierarchical tissue to generate synthetic diffusion‐weighted signal curves over a wide range of scan parameters. Curves are analyzed using entropy—a measure of curve complexity. Entropy change between a baseline and various microstructural scenarios is investigated. We find acquisitions that optimize entropy difference in each scenario.ResultsPermeability changes have a large effect on the diffusion‐weighted signal curve. Muscle fiber atrophy is also important, although differentiating between mechanisms is challenging. Several acquisitions over a range of diffusion times is optimal for imaging microstructural change in muscle tissue. Sensitivity to permeability is optimized for high gradient strengths, but sensitivity to other scenarios is optimal at other values.ConclusionsThe diffusion‐attenuated signal is sensitive to the microstructural changes, but the changes are subtle. Taking full advantage of the changes to the overall curve requires a set of acquisitions over a range of diffusion times. Permeability causes the largest changes, but even the very subtle changes associated with fiber radius distribution change the curves more than noise alone. Magn Reson Med 78:1187–1198, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.

Removal of thiophenes from FCC gasoline by using a hollow fiber pervaporation module: Modeling, validation, and influence of module dimensions and flow directions

In the recent years, pervaporation is widely investigated for removal of thiophene compounds from fluidized catalytic cracker (FCC) gasoline. Commercialization of this process depends on the development of suitable membrane modules. The earlier studies were performed on flat sheet membranes. Hollow fiber modules seem to be the most suitable module for this process on account of high packing density for handling the large amount gasoline-stream in the refineries. In this study, a mathematical model is derived for determining the performance of axial flow type hollow fiber module. The experiment could not be performed on hollow fiber module due to unavailability of commercial modules. However, tubular modules are commercially available. Tubular and hollow fiber modules have similar geometry, and their mathematical models are also the same. Therefore, for the model validation, experiments are carried out on a commercially available tubular module for removal of thiophene from n-heptane/thiophene model gasoline. Furthermore, the experimental results from the literature for the ethanol/water separation are also used for model validation. The mathematical model is successfully validated in both cases.Since the performance of membrane modules also depends on different dimensional parameters related to module geometry, the performances of the hollow fiber module are determined by varying these parameters. Simulations show that lower membrane area, lower length/diameter ratio of module, higher module porosity and lower fiber radius leads to improvement in the module performances.Furthermore, the performance of module is compared for both co-current and counter-current flow arrangements, which shows that the values of average fluxes are higher for counter-current flow arrangement. These results suggest that for given conditions, counter-current hollow fiber module is a better option than the co-current hollow fiber module.

Permanent matching of coupled optical bottle resonators with better than 0.16 GHz precision.

The fabrication precision is one of the most critical challenges to the creation of practical photonic circuits composed of coupled high Q-factor microresonators. While very accurate transient tuning of microresonators based on local heating has been reported, the record precision of permanent resonance positioning achieved by post-processing is still within 1 and 5 GHz. Here we demonstrate two coupled bottle microresonators fabricated at the fiber surface with resonances that are matched with a better than 0.16 GHz precision. This corresponds to a better than 0.17 Å precision in the effective fiber radius variation. The achieved fabrication precision is only limited by the resolution of our optical spectrum analyzer and can be potentially improved by an order of magnitude.

Tunable photonic elements at the surface of an optical fiber with piezoelectric core.

Tunable photonic elements at the surface of an optical fiber with piezoelectric core are proposed and analyzed theoretically. These elements are based on whispering gallery modes whose propagation along the fiber is fully controlled by nanoscale variation of the effective fiber radius, which can be tuned by means of a piezoelectric actuator embedded into the core. The developed theory allows one to express the introduced effective radius variation through the shape of the actuator and the voltage applied to it. In particular, the designs of a miniature tunable optical delay line and a miniature tunable dispersion compensator are presented. The potential application of the suggested model to the design of a miniature optical buffer is also discussed.

Simulation of convective-diffusional processes in hollow fiber membrane contactors

Abstract A model for the simulation of the shell-side convective diffusion of a solute in the hollow-fiber contactor with a regular arrangement of fibers is advanced. The model takes into account the impact of the fiber packing density on the shell-side constrained flow and solute transport. A system of periodic rows of parallel monodisperse fibers arranged normally to the laminar flow direction is considered as a model contactor. Flow and solute concentration fields are calculated from the numerical solution of the Navier-Stokes and convective diffusion equations over a broad range of the inter-fiber distances, Reynolds (Re) and diffusion Peclet (Pe) numbers. For the calculated fiber drag force and for the fiber Sherwood number (Sh) for high and intermediate Peclet numbers Pe ≫ 1 and at Re ⩽ 50, the best-fit regression formulas are found. The fiber Sherwood number governs the contactor retention efficiency via E = 1 - exp ( - 4 α H Sh / a ) , where a is the fiber radius, H is the thickness of the layer of fibers, α is the packing density of fibers. For a dense fibrous medium, a retarded increase in Sh with Re at constant Pe and packing density is demonstrated. Fiber retention efficiency is shown to follow the fiber drag force, which makes it possible to estimate the solute retention efficiency of a contactor from its hydrodynamic resistance to the shell-side flow. A convective-diffusional transport of a solute in the hollow fiber contactor with a radial Stokes flow is also considered.