Deformation Of Filaments Research Articles

Effect of Heat Diffusion During State Transitions in Resistive Switching Memory Device Based on Nickel-Rich Nickel Oxide Film

The switching behaviors of the resistive switching device based on Ni-rich nickel oxide thin film during the set and reset processes in the pulse voltage experiment have been examined. In the switching from a high-resistance state (HRS) to a low-resistance state (LRS) during the set process, the formation of filament is the dominant process. The switching is easier to occur with a shorter off time between pulses, indicating that heat diffusion plays an important role. On the other hand, in the switching from the LRS to the HRS during the reset process, there is a competition between the formation and deformation of filament, which is much stronger than that in the set process. The heat diffusion during the off time between pulses affects both the formation and deformation of filaments. Thus, there is no definite relationship in statistics between the off time and the occurrence of the reset switching.

Targeting Lysophosphatidic Acid Signaling Retards Culture-Associated Senescence of Human Marrow Stromal Cells

Marrow stromal cells (MSCs) isolated from mesenchymal tissues can propagate in vitro to some extent and differentiate into various tissue lineages to be used for cell-based therapies. Cellular senescence, which occurs readily in continual MSC culture, leads to loss of these characteristic properties, representing one of the major limitations to achieving the potential of MSCs. In this study, we investigated the effect of lysophosphatidic acid (LPA), a ubiquitous metabolite in membrane phospholipid synthesis, on the senescence program of human MSCs. We show that MSCs preferentially express the LPA receptor subtype 1, and an abrogation of the receptor engagement with the antagonistic compound Ki16425 attenuates senescence induction in continually propagated human MSCs. This anti-aging effect of Ki16425 results in extended rounds of cellular proliferation, increased clonogenic potential, and retained plasticity for osteogenic and adipogenic differentiation. Expressions of p16Ink4a, Rb, p53, and p21Cip1, which have been associated with cellular senescence, were all reduced in human MSCs by the pharmacological inhibition of LPA signaling. Disruption of this signaling pathway was accompanied by morphological changes such as cell thinning and elongation as well as actin filament deformation through decreased phosphorylation of focal adhesion kinase. Prevention of LPA receptor engagement also promoted ubiquitination-mediated c-Myc elimination in MSCs, and consequently the entry into a quiescent state, G0 phase, of the cell cycle. Collectively, these results highlight the potential of pharmacological intervention against LPA signaling for blunting senescence-associated loss of function characteristic of human MSCs.

The matching of a “one-dimensional” numerical simulation and experiment results for low viscosity Newtonian and non-Newtonian fluids during fast filament stretching and subsequent break-up

This paper develops a model for fast filament stretching, thinning, and break-up for Newtonian and non-Newtonian fluids, and the results are compared against experimental data where fast filament relaxation occurs. A 1D approximation was coupled with the arbitrary Lagrangian Eulerian (ALE) formulation to perform simulations that captured both filament thinning and break-up. The modeling accounts for both the initial polymer stretching processes from the precise movement of the two moving pistons and also the subsequent thinning when the pistons are at rest. The simulations were first validated for a low viscosity Newtonian fluid matched to experimental data obtained from a recently developed apparatus, the Cambridge Trimaster. A non-Newtonian polymer fluid, with high frequency linear viscoelastic behavior characterized using a piezoaxial vibrator rheometer, was modeled using both an Oldroyd-B and FENE-CR single-mode constitutive models. The simulations of the filament deformation were compared with experiment. The simulations showed a generally reasonable agreement with both the stretch and subsequent relaxation experimental responses, although the mono mode models used in this paper were unable to capture all of the details for the experimental time evolution relaxation profile of the central filament diameter.

The shape of an elastic filament in a two-dimensional corner flow

The deformation of a flexible filament held fixed at one end in a nonuniform viscous flow with curved streamlines is considered, with a focus on the filament dynamics and steady-state shape. Our motivation arises from recent microfluidic experiments on biofilm formation in a channel with bends, where thread-like structures, or streamers, were observed, attached to the side walls downstream of each corner and connecting consecutive corners while floating in the middle plane of the channel [Rusconi et al., J. R. Soc. Interface 7, 1293 (2010)]. We discuss the time evolution and final shape of the filament in different corner geometries as a function of a non-dimensional elasticity parameter that compares viscous and elastic effects. Since the filament develops tension, when the flow has curved streamlines the filament does not align with the flow, as occurs in a rectilinear flow, but rather it crosses the streamlines.

Influences of Different ECAE Routes on Filament Deformation in Cu Clad Nb Composite Wires

A strategy to obtain higher critical current density (Jc) in Nb3Sn conductors is to increase the Nb fraction. This can be achieved by improving the deformation behavior of the Nb filaments so less Cu is needed between filaments enabling smaller diameter filaments and more filaments in the subelement region. The objective of this project is to better understand the influence of different pre-processing strain path (texture) on the deformation of a Nb filament in a Cu-matrix. Initial severe plastic deformation by equal channel angular extrusion (ECAE) was used to obtain a uniform fine grain size with slightly different textures in starting Nb rods. Cu-Nb composite bars were warm extruded and wire drawn to evaluate the performance of different starting Nb fine grained microstructures on Nb filament deformation behavior. The initial textures in the various Nb samples were different but quite weak; little difference is seen in the final Nb filament circularities. However, this study indicates the Cu-Nb interface roughness is influenced somewhat by non-uniformity in the Nb microstructure. The evidence suggests that to obtain better deformation characteristics in Nb filaments, one should start with fine-grain weak or untextured Nb with a globally uniform microstructure. This should enable finer as-drawn Nb filaments with better concentricity, composite wires with a higher Nb loading fraction, and conductors with higher Jc.

Modeling the Mechanical Property of Single Actin Filament

Actin filaments play many important roles in the cellular processes including motility, morphogenesis, and mechanosensing of the environment. One of the keys to better understanding of how actin filaments perform those roles lies in understanding the mechanical properties of actin filament, such as persistence length. There have been intensive studies on the mechanical properties of actin filament and its network. The measurements so far show the diversity of persistence length, ranging from several to a few tens of microns, also dependent upon the chemical states of actin molecules. Another interesting issue is the description of actin filament breaking. In order to understand these, we built up a simple model where each actin monomer is treated as a spherical particle connected by a set of springs. These spring stiffness parameters are determined from the known information on the chemical bonds in the actin filament and stretching deformation of the actin filament as an elastic rod. Our results show the length dependency of the persistence length, especially in a shorter length range which is relevant to the physiological conditions. They also show that the diversity of persistence length measurements is closely related to the breaking of the bonds in the actin filament, as well as the chemical states of actin monomers in the filament. Finally the mechanism of actin filament breaking is discussed.

Dynamics of a semiflexible polar filament in Stokes flow

In this work, the dynamics and transport of a polarly driven filament is examined using a continuum slender-body model. Immersed in a viscous fluid, the filament gains polar propulsion from the motor proteins (anchored on the motility assay) while experiencing a viscous drag from the bottom wall. Results from the linear analysis on a straight polar filament illustrate the necessity of spatial inhomogeneity in the polar forcing for the buckling instability. The ensuing buckling leads to filament deformation, undulation, and change of its direction of motion in the numerical simulations. Repeated filament buckling in two types of motor protein concentration landscape results in diffusive transport of a polar filament on scales much larger than the mean-free path and the average duration between filament buckling events.

Self-winding of helices in plant tendrils and cellulose liquid crystal fibers

Passiflora edulis, like other climbing plants, possesses long, tender, soft, curly and flexible organs called tendrils whose circumnutation allows the plant to find support. Tendrils curl into spirals or twist into a helix, often of one handedness over half of its length and of the opposite handedness over the other half, the two halves being connected by a short straight section – a perversion, depending on whether they are supported at just one end or supported at both ends, respectively. This is a consequence of an intrinsic curvature of the tendrils. We report on liquid crystalline cellulosic fibers and jets, which mimic the shapes of helical tendril structures. Liquid crystalline and isotropic cellulosic precursor solutions of curved and straight fibers are examined using nuclear magnetic resonance imaging (MRI) and polarizing optical microscopy (POM) techniques to determine morphological and structural features contributing to fiber curvature. We study the subtle physical mechanisms responsible for self-winding behavior as a result of the intrinsic curvature due to the non-uniform deformation of filaments. In the case of liquid-crystalline cellulosic fibers, this is due to a core of disclination forming off-axis along the filament. We also highlight the critical dependence of the helical structures on temperature, which offers a potential for direct fabrication of biocompatible tunable high-surface area membranes with mechanical adaptability.

Accurate simulation dynamics of microscopic filaments using “caterpillar” Oseen hydrodynamics

Microscopic semiflexible filaments suspended in a viscous fluid are widely encountered in biophysical problems. The classic example is the flagella used by microorganisms to generate propulsion. Simulating the dynamics of these filaments numerically is complicated because of the coupling between the motion of the filament and that of the surrounding fluid. An attractive idea is to simplify this coupling by modeling the fluid motion by using Stokeslets distributed at equal intervals along the model filament. We show that, with an appropriate choice of the hydrodynamic radii, one can recover accurate hydrodynamic behavior of a filament with a finite cross section without requiring an explicit surface. This is true, however, only if the hydrodynamic radii take specific values and that they differ in the parallel and perpendicular directions leading to a caterpillarlike hydrodynamic shape. Having demonstrated this, we use the model to compare with analytic theory of filament deformation and rotation in the small deformation limit. Generalization of the methodology, including application to simulations using the Rotne-Prager tensor, is discussed.

Coupling between axial stretch and bending/twisting deformation of actin filaments caused by a mismatched centroid from the center axis

A cytoskeletal actin filament with a spirally arranged centroid (a biopolymer with a double helical structure) exhibits mechanically interesting phenomena, such as tensile-twisting coupling behavior involved in the binding of actin-related proteins. To describe this mechanical behavior of the actin filament, we propose a Cosserat continuum model by focusing on the mismatch between the centroid and the center axis. We derive the equations of motion based on the variational principle and discuss the coupling behavior caused by the mismatch, which is responsible for interesting couplings between axial stretch and bending/twisting deformation. The proposed model can express the mechanical behavior of actin filaments with a spirally arranged centroid mismatched from its center axis, through which we can explore the fundamental filament-level mechanism of an actin cytoskeleton in relation to the mechanical regulation of biological functions.

A semiflexible polymer ring acting as a nano-propeller

The dynamics of a rotating elastic nano-ring driven in a viscous fluid by an externally applied torque about a specific axis is studied using elasto-hydrodynamic simulations. We show that a helical deformation of the ring filament is excited, and that this leads to directed propulsion which is independent of the direction of rotation. It is found that the propulsive force and efficiency initially increase as the torque is increased, and then decrease discontinuously at a buckling transition at a critical torque. This unique propulsive behavior at the shape transition arises due to its specific geometry, i.e., circularity of an elastic filament. The implications of the behavior for artificial microscopic devices are discussed.

Optimization of Cross-sectional Shapes of the Bi-2223/Ag Wires before Flat Rolling

Rolling process plays an important role in the manufacture of Bi-based high temperature superconductor tapes, and the plastic flow regularities of the superconducting wires during deformation will directly affect the ultimate quality of the tapes. In order to investigate the effect of cross-sectional shapes before flat rolling on the performance and homogeneity of the tapes, some numerical models of Bi-2223/Ag wires with different cross-sectional shapes including circular, square, elliptical and racetrack cross-sections are constructed during the rolling process. By comparing the relative density, logarithmic strain ratio and length-width ratio on the filaments, it is revealed that Bi-2223/Ag wire with special-shaped cross-section can achieve better conductivity than the round wire, in particular, the racetrack cross-sectional wire has the second best performance among four wires. Based on material processability and experimental condition, tri-pass racetrack drawing technique is employed to optimize the process and obtain racetrack cross-sectional wire. The rolling process of Bi-2223/Ag wire with racetrack cross-section causes more intensive deformation of filaments in the center of the tape and achieves the filaments with larger length-width ratio. Also, the deformation distribution of filaments verifies the numerical results. Consequently, the racetrack drawing technique can be utilized for a reference during the mechanical processing and to increase the current transmission capacities of Bi-2223/Ag tapes.

Shear and elongational flow behavior of acrylic thickener solutions. Part II: effect of gel content

We have investigated the effect of crosslink density on shear and elongational flow properties of alkali-swellable acrylic thickener solutions using a mixing series of the two commercial thickeners Sterocoll FD and Sterocoll D as model system. Linear viscoelastic moduli show a smooth transition from weakly elastic to gel-like behavior. Steady shear data are very well described by a single mode Giesekus model at all mixing ratios. Extensional flow behavior has been characterized using the CaBER technique. Corresponding decay of filament diameter is also well fitted by the Giesekus model, except for the highest crosslink densities, when filament deformation is highly non-uniform, but the non-linearity parameter α, which is independent of the mixing ratio, is two orders of magnitude higher in shear compared to elongational flow. Shear relaxation times increase by orders of magnitude, but the characteristic elongational relaxation time decreases weakly, as gel content increases. Accordingly, variation of gel content is a valuable tool to adjust the low shear viscosity in a wide range while keeping extensional flow resistance essentially constant.

Transport Properties of a PIT-${\hbox{Nb}}_{3}{\hbox{Sn}}$ Strand Under Transverse Compressive and Axial Tensile Stress

A study of the critical current of a PIT strand under transverse compressive and axial tensile loads is presented. The conductor is not reinforced and has a proof strength, , of 142 MPa. Although the tensile strain at the maximum of the critical current, , is only 0.14% (low thermal precompression) the strand behaves as expected. However it is particularly sensitive to transverse compressive forces. Even relatively small forces yield to a reduction of the critical current. Qualitatively this degradation may be explained by a non-uniform deformation of filaments, yielding to irreversible micro cracks, and by a reduced of intact filaments. The recovery of upon unloading of the transverse compressive force has also been investigated.

Nonlinear dynamics of semiflexible magnetic filaments in an ac magnetic field

Flexible spontaneously magnetized filaments exist in the living world (magnetotactic bacteria) and arise in magnetic colloids with large magnetodipolar interaction parameter. We demonstrate that these filaments possess variety of novel nonlinear phenomena in an ac magnetic field: orientation of the filament in the direction perpendicular to the field and the development of the oscillating U-like shapes, which presumably can lead to the formation of rings of magnetic filaments. It is found that these phenomena are determined by the development of the localized boundary modes of the filament deformation. We have illustrated by qualitative estimates that the phenomena found may be useful for insight into the complex pattern formation phenomena in ensembles of magnetic particles under the action of an ac magnetic field.

Deformation of a helical filament by flow and electric or magnetic fields

Motivated by recent advances in the real-time imaging of fluorescent flagellar filaments in living bacteria [Turner, Ryu, and Berg, J. Bacteriol. 82, 2793 (2000)], we compute the deformation of a helical elastic filament due to flow and external magnetic or high-frequency electric fields. Two cases of deformation due to hydrodynamic drag are considered: the compression of a filament rotated by a stationary motor and the extension of a stationary filament due to flow along the helical axis. We use Kirchhoff rod theory for the filament, and work to linear order in the deflection. Hydrodynamic forces are described first by resistive-force theory, and then for comparison by the more accurate slender-body theory. For helices with a short pitch, the deflection in axial flow predicted by slender-body theory is significantly smaller than that computed with resistive-force theory. Therefore, our estimate of the bending stiffness of a flagellar filament is smaller than that of previous workers. In our calculation of the deformation of a polarizable helix in an external field, we show that the problem is equivalent to the classical case of a helix deformed by forces applied only at the ends.

Analyses on Deformation of Helical Flagella of Salmonella

In the present investigation, we constructed formulation to analyze the deformation of flagellar filament combining the evolution equations for space curves with the Kirchhoff rod model as well as the detailed structure of the filament of Salmonella. In the analytical results of the present study, experimental results of the large elongation of close-coiled filament (Hoshikawa and Kamiya, 1985) and the small deformation of normal filament rotating in water (Kudo et al.) are reproduced. Comparing the results of deformation of flagellar filament between the analyses and the experiments, the torsional and the flexural rigidity of the flagellar filament are estimated to be GJ=4.6pNµm2 and EI=6.1pNµm2.

Numerical simulation of bubble growth in Newtonian and viscoelastic filaments undergoing stretching

Numerical results are presented concerning bubble growth in Newtonian and viscoelastic filaments undergoing stretching. In practice, such bubbles or cavities develop in materials (either in their bulk or at their interface with a substrate) such as the pressure sensitive adhesives. The problem we address here is how such an initially spherical bubble deforms inside a filament (which at the beginning has the shape of a cylinder with uniform radius) undergoing stretching with a constant pulling velocity. The lower end of the liquid filament is fixed at the substrate; stretching along the filament axis is achieved by means of an imposed force on the upper end. The governing equations consist of the momentum, continuity and constitutive equations and the free surface boundary conditions at the two interfaces (the bubble–liquid and the liquid–air interfaces). These are solved by a finite element/Galerkin method coupled with a first-order implicit Euler scheme for time integration. In the case of a viscoelastic medium (here the Phan-Thien/Tanner constitutive model is chosen), the elastic viscous stress splitting-G (EVSS-G) technique is used to separate the elastic and viscous contributions to the stress tensor together with a streamline upwind Petrov-Galerkin (SUPG) discretization of the constitutive equation. Our numerical calculations provide information on the effect of a number of important parameters on bubble and filament growth rate and deformation. These include mainly the capillary and Deborah numbers. The effect of other variables such as the relative size of the bubble, the proximity of the bubble to and liquid slippage on the substrate are also studied and discussed in detail.

An analytical model for material line kinematics in steady and unsteady buoyancy driven flows in 2-d channels

In this note, we use both developing and fully developed buoyant flow in a 2-d channel as a simple flow model useful for approximating buoyancy driven interpenetrating mixing phenomenon, such as the Rayleigh–Taylor instability. Our approach towards understanding and quantifying mixing within this flow follows the development by Ottino (The kinematics of mixing: stretching, chaos and transport, Cambridge University Press, Cambridge, 1989) that mixing is the kinematically efficient stretching and folding of material lines. Like Ottino, we use simple flows as a prototype to understand more complex problems. Here we derive the stretch length i.e. the deformation of a unit filament subjected to both the fully developed and developing buoyant channel flow. We use the simpler fully developed flow to compute the specific rate of stretching for the fully developed flow. An early time contraction–expansion effect is noted for stretching. Asymptotically, the stretching rate decays by the expected t −1 behavior. This note indicates that early time buoyancy induced inter-penetrative mixing is a typical 2-d, shear driven phenomenon, exhibiting neither strong re-orientation (with attendant enhance mixing) nor negligible filament deformation (indicating minimal disruption of material surfaces).

Experiments on instability of columnar vortex pairs in rotating fluid

We present results from a new series of experiments on the geophysically important issue of the instability of anticyclonic columnar vortices in a rotating fluid in circumstances such that the Rossby number exceeds unity. The vortex pair consisting of a cyclonic and an anticyclonic vortex is induced by a rotating flap in a fluid which is itself initially in a state of solid-body rotation. The anticyclonic vortex is then subject to either centrifugal or elliptical instability, depending on whether its initial ellipticity is small or large, while the cyclone always remains stable. The experimental results demonstrate that the perturbations due to centrifugal instability have a typical form of toroidal vortices of alternating sign (rib vortices). The perturbations due to elliptical instability are of the form of sinuous deformation of the vortex filament in the plane of maximal stretching which corresponds to the plane of symmetry for the vortex pair. The initial perturbations in both cases are characterized by a definite wave number in the vertical direction. The characteristics of the unstable anticyclone are determined by the main nondimensional parameter of the flow - the Rossby number. The appearance of both centrifugal and elliptical instabilities are in accord with the predictions of theoretical criteria for these cases.