Comprehensive contribution of filament thickness and crosslinker failure to the rheological property of F-actin cytoskeleton

Rheological property of F-actin cytoskeleton is significant to the restructuring of cytoskeleton under a variety of cell activities. This study numerically validates the rheological property of F-actin cytoskeleton is not only a result of kinetic energy dissipation of F-actin, but also greatly depends on the configuration remodeling of networks structure. Both filament geometry and crosslinker properties can affect the remodeling of F-actin cytoskeleton. The crosslinker unbinding is found to dissipate energy and induce prominent stress relaxation in the F-actin adjacent to cross-linkages. Coupled with F-actin elasticity, the energy dissipation and stress relaxation are more significant in bundled F-actin networks than in single F-actin networks.

Self-Assembly of Bile Steroid Analogues: Molecules, Fibers, and Networks

Structural and rheological features of a series of molecular hydrogels formed by synthetic bile salt analogues have been scrutinized. Among seven gelators, two are neutral compounds, while the others are cationic systems among which one is a tripodal steroid derivative. Despite the fact that the chemical structures are closely related, the variety of physical characteristics is extremely large in the structures of the connected fibers (either plain cylinders or ribbons), in the dynamical modes for stress relaxation of the associated SAFINs, in the scaling laws of the shear elasticity (typical of either cellular solids or fractal floc-like assemblies), in the micron-scale texture and the distribution of ordered domains (spherulites, crystallites) embedded in a random mesh, in the type of nodal zones (either crystalline-like, fiber entanglements, or bundles), in the evolution of the distribution and morphology of fibers and nodes, and in the sensitivity to added salt. SANS appears to be a suitable technique to infer all geometrical parameters defining the fibers, their interaction modes, and the volume fraction of nodes in a SAFIN. The tripodal system is particularly singular in the series and exhibits viscosity overshoots at the startup of shear flows, an “umbrella-like” molecular packing mode involving three molecules per cross section of fiber, and scattering correlation peaks revealing the ordering and overlap of 1d self-assembled polyelectrolyte species.

Absence of reptation in highly confined polymers

We present results of mechanical stress relaxation measurements on polymers confined at the air-water interface in the form of a monolayer. Systematic measurements allow, to our knowledge, for the first time, observation of the scaling of the stress relaxation time of the highly confined polymers as a function of both surface concentration and molecular weight. The observed scaling is found to be very close to that expected for motion of unentangled polymer solutions with hydrodynamic interactions. Our experimental observations thus clearly rule out the possibility of entanglement and hence reptation as a mode of relaxation in such highly confined polymeric systems.

Equivalent constitutive model-based design of wave-absorbing material system and controller

A design methodology for wave-absorbing active material system is reported. The design enforces equivalence between an assumed material model having wave-absorbing behavior and a set of target feedback controllers for an array of microelectro-mechanical transducers which are integral part of the active material system. The proposed methodology is applicable to problems involving the control of acoustic waves in passive-active material system with complex constitutive behavior at different length-scales. A stress relaxation type one-dimensional constitutive model involving viscous damping mechanism is considered, which shows asymmetric wave dispersion characteristics about the half-line. The acoustic power flow and asymptotic stability of such material system are studied. A single sensor non-collocated linear feedback control system in a one-dimensional finite waveguide, which is a representative volume element in an active material system, is considered. Equivalence between the exact dynamic equilibrium of these two systems is imposed. It results in the solution space of the design variables, namely the equivalent damping coefficient, the wavelength(s) to be controlled and the location of the sensor. The characteristics of the controller transfer functions and their pole-placement problem are studied. (c) 2005 Elsevier Ltd. All rights reserved.

Method of Analysis if Uniaxial Deformation Data

A comprehensive scheme for analysing uniaxial deformation data, taking into account the finite stiffness of the testing machine is presented. Equations relevant to tension and stress relaxation tests carried out under cross head speed control, and to creep testing under constant load, are described. For the first two cases, the implications of not using gauge length extensometry but relying upon cross head displacement for inferring specimen extension, and the role of uncertainty in machine stiffness are also examined. The final section touches upon the extension of the present scheme to account for specimen anelasticity.

Influence of Tempered Microstructures on the Transformation Behaviour of Cold Deformed and Intercritically Annealed Medium Carbon Low Alloy Steel

This research is focused on understanding the role of microstructural variables and processing parameters in obtaining optimised dual phase structures in medium carbon low alloy steels. Tempered Martensite structures produced at 300, 500, and 650 degrees C, were cold rolled to varied degrees ranging from 20 to 80% deformation. Intercritical annealing was then performed at 740, 760, and 780 degrees C for various time duration ranging from 60 seconds to 60 minutes before quenching in water. The transformation behaviour was studied with the aid of optical microscopy and hardness curves. From the results, it is observed that microstructural condition, deformation, and intercritical temperatures influenced the chronological order of the competing stress relaxation and decomposition phase reactions which interfered with the rate of the expected alpha -> gamma transformation. The three unique transformation trends observed are systematically analyzed. It was also observed that the 300 and 500 degrees C tempered initial microstructures were unsuitable for the production of dual structures with optimized strength characteristics.

Excess dissipation in dense emulsions

A new model for the structure, elastic properties and dynamics of foams and concentrated emulsions is presented, based on the idea of local regions lacking shear-rigidity in one or more directions which vary randomly through the medium. It is shown to lead naturally to slow (t(-1/2)) stress-relaxation, implying a piece of the dynamic modulus scaling with frequency omega as omega(1/2). Striking experimental confirmation of this prediction using a novel experimental technique is reported, and challenges for the theoretician are offered. This work was done in collaboration with Andrea Liu, Tom Mason, Hu Gang, and David Weitz [1].

Nonlinear viscoelasticity of freestanding and polymer-anchored vertically aligned carbon nanotube foams

Vertical arrays of carbon nanotubes (VACNTs) show unique mechanical behavior in compression, with a highly nonlinear response similar to that of open cell foams and the ability to recover large deformations. Here, we study the viscoelastic response of both freestanding VACNT arrays and sandwich structures composed of a VACNT array partially embedded between two layers of poly(dimethylsiloxane) (PDMS) and bucky paper. The VACNTs tested are similar to 2 mm thick foams grown via an injection chemical vapor deposition method. Both freestanding and sandwich structures exhibit a time-dependent behavior under compression. A power-law function of time is used to describe the main features observed in creep and stress-relaxation tests. The power-law exponents show nonlinear viscoelastic behavior in which the rate of creep is dependent upon the stress level and the rate of stress relaxation is dependent upon the strain level. The results show a marginal effect of the thin PDMS/bucky paper layers on the viscoelastic responses. At high strain levels (epsilon - 0.8), the peak stress for the anchored CNTs reaches similar to 45 MPa, whereas it is only similar to 15MPa for freestanding CNTs, suggesting a large effect of PDMS on the structural response of the sandwich structures. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.3699184]

Enhancement in creep resistance of Ti-6Al-4V alloy due to boron addition

The addition of B, up to about 0.1 wt%, to Ti-6Al-4V (Ti64) reduces its as-cast grain and colony sizes by an order of magnitude. In this paper, the creep resistance of this alloy modified with 0.06 and 0.11 wt% B additions was investigated in the temperature range of 475-550 degrees C and compared with that of the base alloy. Conventional dead-weight creep tests as well as stress relaxation tests were employed for this purpose. Experimental results show that the B addition enhances both elevated temperature strength and creep properties of Ti64, especially at the lower end of the temperatures investigated. The steady state creep rate in the alloy with 0.11 wt% B was found to be an order of magnitude lower than that in the base alloy, and both the strain at failure as well as the time for rupture increases with the B content. These marked improvements in the creep resistance due to B addition to Ti64 were attributed primarily to the increased number of inter-phase interfaces - a direct consequence of the microstructural refinement that occurs with the B addition - that provide resistance to dislocation motion. (C) 2014 Elsevier B.V. All rights reserved.

Using heat treatment effects and EBSD analysis to tailor microstructure of hybrid Mg nanocomposite for enhanced overall mechanical response

In this study, a detailed investigation on the effect of heat treatment on the microstructural characteristics, texture evolution and mechanical properties of Mg-(5.6Ti+2.5B(4)C)(BM) hybrid nanocomposite is presented. Optimised heat treatment parameters, namely, heat treatment temperature and heat treatment time, were first identified through grain size and microhardness measurements. Initially, heat treatment of composites was conducted at temperature range between 100 and 300 degrees C for 1 h. Based on optical microscopic analysis and microhardness measurements, it was evident that significant grain growth and reduction in microhardness occurred for temperatures > 200 degrees C. The cutoff temperature that caused significant grain growth/matrix softening was thus identified. Second, at constant temperature (200 degrees C), the effect of variation of heat treatment time was carried out (ranging between 1 and 5 h) so as to identify the range wherein increase in average grain size and reduction in microhardness occurred. Furthering the study, the effect of optimised heat treatment parameters (200 degrees C, 5 h) on the microstructural texture evolution and hence, on the tensile and compressive properties of the Mg-(5.6Ti+2.5B(4)C)(BM) hybrid nanocomposite was carried out. From electron backscattered diffraction (EBSD) analysis, it was identified that the optimised heat treatment resulted in recrystallisation and residual stress relaxation, as evident from the presence of similar to 87% strain free grains, when compared to that observed in the non-heat treated/as extruded condition (i.e. 2.2 times greater than in the as extruded condition). For the heat treated composite, under both tensile and compressive loads, a significant improvement in fracture strain values (similar to 60% increase) was observed when compared to that of the non-heat treated counterpart, with similar to 20% reduction in yield strength. Based on structure-property correlation, the change in mechanical characteristics is identified to be due to: (1) the presence of less stressed matrix/reinforcement interface due to the relief of residual stresses and (2) texture weakening due to matrix recrystallisation effects, both arising due to heat treatment.

Tailoring viscoelastic response of carbon nanotubes cellular structure using electric field

Cellular structures of carbon nanotubes (CNT) are novel engineering materials, which are finding applications due to their remarkable structural and functional properties. Here, we report the effects of electric field, one of the most frequently used stimulants for harnessing the functional properties of CNT, on the viscoelastic response, an important design consideration for the structural applications of a cellular CNT sample. The application of an electric field results in electrostriction induced large actuation in freestanding CNT samples; however, if the CNT are prohibited to expand, an electric field dependent force is exerted by the sample on the constraining platens. In addition, the above force monotonically decreases with the pre-compressive strain imposed onto the sample. The viscoelastic recovery reveals a decrease in the stress relaxation with an increase in the pre-compressive strain in both the presence and absence of the electric field; however, the stress relaxation was significantly higher in the presence of the electric field. A model, based on a simple linear viscoelastic solid incorporating electric field, is developed to understand the experimental observations.

Strain Rate-Dependent Compressive Deformation Behavior of Nd-Based Bulk Metallic Glass

Compressive deformation behavior of the Nd60Fe20Co10Al10 bulk metallic glass was characterized over a wide strain rate range (6.0 x 10(-4) to 1.0x10(3) s(-1)) at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate. Serrated flow and a large number of shear bands were observed at the quasi-static strain rate (6.0 x 10(-4)s(-1)). The results suggest that the appearance of a large number of shear bands is probably associated with flow serration observed during compression; and both shear banding and flow serration are a strain accommodation and stress relaxation process. At dynamic strain rates (1.0 x 10(3) s(-1)), the rate of shear band nucleation is not sufficient to accommodate the applied strain rate and thus causes an early fracture of the test sample. The fracture behavior of the Nd60Fe20Co10Al10 bulk metallic glass is sensitive to strain rate.

Molecular dynamics simulation of interaction of a dislocation array from a crack tip with grain boundaries

The interaction of a dislocation array emitted from a crack tip under mode II loading with asymmetric tilt grain boundaries (GBs) is analysed by the molecular dynamics method. The GBs can generally be described by planar and linear matching zones and unmatching zones. All GBs are observed to emit dislocations. The GBs migrated easily due to their planar and linear matching structure and asymmetrical type. The diffusion induced by stress concentration is found to promote the GB migration. The transmissions of dislocations are either along the matched plane or along another plane depending on tilt angle theta. Alternate processes of stress concentration and stress relaxation take place ahead of the pileup. The stress concentration can be released either by transmission of dislocations, by atom diffusion along GBs, or by migration of GBs by formation of twinning bands. The simulated results also unequivocally demonstrate two processes, i.e. asymmetrical GBs evolving into symmetrical ones and unmatching zones evolving into matching ones during the loading process.

Crack growth by grain boundary cavitation in the transient and extensive creep regimes

Crack growth due to cavity growth and coalescence along grain boundaries is analyzed under transient and extensive creep conditions in a compact tension specimen. Account is taken of the finite geometry changes accompanying crack tip blunting. The material is characterized as an elastic-power law creeping solid with an additional contribution to the creep rate arising from a given density of cavitating grain boundary facets. All voids are assumed present from the outset and distributed on a given density of cavitating grain boundary facets. The evolution of the stress fields with crack growth under three load histories is described in some detail for a relatively ductile material. The full-field plane strain finite element calculations show the competing effects of stress relaxation due to constrained creep, diffusion and crack tip blunting. and of stress increase due to the instantaneous elastic response to crack growth. At very high crack growth rates the Hui-Riedel fields dominate the crack tip region. However. the high growth rates are not sustained for any length of time in the compact tension geometry analyzed. The region of dominance of the Hui-Riedel field shrinks rapidly so that the near-tip fields are controlled by the HRR-type field shortly after the onset of crack growth. Crack growth rates under various conditions of loading and spanning the range of times from small scale creep to extensive creep are obtained. We show that there is a strong similarity between crack growth history and the behaviour of the C(t) and C(t) parameters. so that crack growth rates correlate rather well with C(t) and C(t). A relatively brittle material is also considered that has a very different near-tip stress field and crack growth history.

Influences of CO2 laser irradiation on the structure and photoluminescence of zinc oxide thin films

CO2 laser irradiation experiments on ZnO thin films are reported. The structural, optical, luminescent and vibrational properties of the samples were investigated by X-ray diffraction (XRD), transmittance, photoluminescence (PL) and Raman measurements. XRD results show that the crystalline of the irradiated films was improved. The (002) peaks of irradiated ZnO films shift to. higher 20 angles due to the stress relaxation in the case of laser beam irradiation. From optical transmittance spectra, all films exhibit high transmittance in the visible range, the optical band edge of irradiated films showed a redshift compared with that of as-grown films. Compared with the as-grown films, the photoluminescence emission (in particular the relative intensities of visible emissions) intensities of irradiated samples enhanced. In the Raman scattering spectral both the A I. and E modes exhibited slight Raman blueshift. (c) 2005 Elsevier B.V. All rights reserved.