Buckling initiation and displacement dependence in compression of vertically aligned carbon nanotube arrays

Carbon nanotube (CNT) arrays have shown the remarkable ability to react as foam-like structures and exhibit localized buckling coordinated within specific regions. Here, we report on the low-cycle compression of bulk vertically aligned CNT arrays to observe initiation and growth of the buckling as a function of compressive strain. A critical strain is found above which the buckling region length increased and below which it remained at or below the applied strain. As previously observed, the buckling region of the CNT array propagates from the surface where growth occurred, which, in the test specimen, is a free surface and later receives compressive contact by a polished silicon substrate. The results are corroborated with nanoindentation on the surfaces, which indicate a stiffening of the near surface with increasing applied strain. Observation and results of the buckling region nature are important for applications of nanotube arrays as energy absorbing cushions, tunable dampers, thermal contacts, or in sliding contact.     

NMR analysis of butyl acrylate/methylmethacrylate/α‐methyl styrene terpolymers

Composition analysis for butyl acrylate (BA)/ methyl methacrylate (MMA)/α‐methyl styrene terpolymers was carried out by NMR spectroscopy methods. ¹H‐NMR was used primarily for this analysis, but because the method did not provide independent measurements for the BA and MMA fractions, the terpolymer composition analysis was open to higher than normal levels of uncertainty. Supplementary analyses were made with quantitative ¹³C‐NMR methods to confirm the results from ¹H‐NMR (quantitative ¹³C‐NMR was used to provide corroboration of selected composition analyses). To confirm spectral assignments in the ¹H‐ and ¹³C‐NMR spectra, heteronuclear multiple quantum coherence and J Modulated Spin Echo (JMOD) pulse sequences were used. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2093–2098, 2007     

DNA Comet Giemsa Staining for Conventional Bright-Field Microscopy

This study was undertaken to evaluate the compatibility of Giemsa staining protocol with the comet assay. We showed, for the first time, that DNA comets can be visualized and analyzed using Giemsa staining. We generated DNA damage dose response curves for human peripheral blood lymphocytes exposed to X-ray radiation using the comet assay with either SybrGreen I or Giemsa stain. The dose response curves were fitted by linear regressions (R² > 0.977). The SybrGreen I results showed only ~1.2-fold higher slope coefficient (method sensitivity) compared to the Giemsa results. The unexpectedly high sensitivity of Giemsa staining for the comet assay is due to the Romanowsky-Giemsa effect, the stain photo-stability and the higher resolution of bright-field imaging compared to fluorescence imaging. Our results demonstrate that Giemsa staining can effectively be used for measuring DNA damage by the comet assay. The low cost and availability of Giemsa stain makes this method affordable for any low budget research and will facilitate new applications of the comet assay in biology and medicine.     

Domain Wall Motion Across Various Grain Boundaries in Ferroelectric Thin Films

Domain wall movement at and near engineered 10°, 15°, and 24° tilt and 10° and 30° twist grain boundaries was measured by band excitation piezoresponse force microscopy for Pb(Zr,Ti)O₃ films with Zr/Ti ratio of 45/55 and 52/48. A minimum in nonlinear response was observed at the grain boundary for the highest angle twist and tilt grain boundaries, while a maximum in nonlinear response was observed at the 10° tilt grain boundaries. The observed nonlinear response was correlated with the domain configurations imaged in cross section by transmission electron microscopy.     

Nanoindentation study of polydimethylsiloxane elastic modulus using Berkovich and flat punch tips

This article explores polydimethylsiloxane (PDMS) mechanical properties, and presents nanoindentation experiments with Berkovich and flat punch indenters. In the Berkovich tip quasi‐static nanoindentation test, there are pull‐in and pull‐off events observed during the initial tip contact, and when withdrawing from the surface, respectively. The pull‐in interaction needs to be accounted for to properly determine the initial contact point, and thus the accurate contact area. Once accounted for the pull‐in event, the Berkovich and flat punch tips quasi‐static nanoindentation tests give comparable results of about 1.5 MPa for the PDMS elastic modulus (5 : 1 elastomer base to the curing agent ratio). However, PDMS unloading stiffness is higher than the loading stiffness, and dynamic PDMS testing yields higher elastic modulus of about 3.6 MPa. While these results are comparable with the large strain macroscopic compression test results, the difference underscores the complexity of elastomer mechanical characterization and illustrates the discrepancies typical of the reported values. This article describes nanoindentation methods and critical aspects of interpreting results to assess PDMS mechanical properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41384.     

Detection and Identification of Probiotic Lactobacillus plantarum Strains by Multiplex PCR Using RAPD-Derived Primers

Lactobacillus plantarum 2035 and Lactobacillus plantarum ACA-DC 2640 are two lactic acid bacteria (LAB) strains that have been isolated from Feta cheese. Both display significant potential for the production of novel probiotic food products. The aim of the present study was the development of an accurate and efficient method for the molecular detection and identification of the above strains in a single reaction. A multiplex PCR assay was designed for each strain, based on specific primers derived from Random Amplified Polymorphic DNA (RAPD) Sequenced Characterized Amplified Region (SCAR) analysis. The specificity of the assay was tested with a total of 23 different LAB strains, for L. plantarum 2035 and L. plantarum ACA-DC 2640. The multiplex PCR assay was also successfully applied for the detection of the above cultures in yogurt samples prepared in our lab. The proposed methodology may be applied for monitoring the presence of these strains in food products, thus evaluating their probiotic character. Moreover, our strategy may be adapted for other novel LAB strains with probiotic potential, thus providing a powerful tool for molecular discrimination that could be invaluable to the food industry.     

Physical aging in the mechanical properties of miscible polymer blends

Changes in mechanical properties during isothermal physical aging were investigated for three miscible blends: polystyrene (PS)/poly(2,6-dimethyl 1,4-phenylene oxide) (PPO), PS/poly(vinylmethylether) (PVME), and poly(methylmethacrylate) (PMMA)/poly(ethyleneoxide) (PEO). The kinetics of stress relaxation was investigated for the blend, dilute in one component, and compared with that of the neat major component at equal temperature distances, Tg-T, from the midpoint glass transition temperature. It is demonstrated that for all three blends, the mean stress relaxation time (τ) does not scale with Tg-T. For PS/PPO and PS/PVME blends, the stress relaxation rates are faster compared to neat PS; for PMMA/PEO, they are slower than for neat PMMA. Two effects appear to be important in contributing to this discrepancy. First, addition of the second component produces a change in the packing density of the blend: less dense for PS/PPO and PS/PVME; more dense for PMMA/PEO. Comparison of average free volume hole sizes and fractional free volumes measured via orthopositronium annihilation lifetime measurements for all three blends versus the pure constituents are qualitatively consistent with this interpretation. Second, because of the presence of concentration fluctuations in the blend, it is expected that the initial stress decay is dominated by regions enriched in the more mobile component. From observations of the change in width of the stress relaxation time distribution, this effect appears to be particularly significant in the PS/PVME blend. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 483–496, 1997     

Composition-dependent xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 bulk ceramics for high energy storage applications

This work reports the composition dependent microstructure, dielectric, ferroelectric and energy storage properties, and the phase transitions sequence of lead free xBa(Zr_0.2Ti_0.8)O₃-(1-x)(Ba_0.7Ca_0.3)TiO₃ [xBZT-(1-x)BCT] ceramics, with x?=?0.4, 0.5 and 0.6, prepared by solid state reaction method. The XRD and Raman scattering results confirm the coexistence of rhombohedral and tetragonal phases at room temperature (RT). The temperature dependence of Raman scattering spectra, dielectric permittivity and polarization points a first phase transition from ferroelectric rhombohedral phase to ferroelectric tetragonal phase at a temperature (T_R-T) of 40?°C and a second phase transition from ferroelectric tetragonal phase - paraelectric pseudocubic phase at a temperature (T_T-C) of 110?°C. The dielectric analysis suggests that the phase transition at T_T-C is of diffusive type and the BZT-BCT ceramics are a relaxor type ferroelectric materials. The composition induced variation in the temperature dependence of dielectric losses was correlated with full width half maxima (FWHM) of A₁, E(LO) Raman mode. The saturation polarization (P_s) ≈8.3?μC/cm² and coercive fields ≈2.9?kV/cm were found to be optimum at composition x?=?0.6 and is attributed to grain size effect. It is also shown that BZT-BCT ceramics exhibit a fatigue free response up to 10⁵ cycles. The effect of a.c. electric field amplitude and temperature on energy storage density and storage efficiency is also discussed. The presence of high T_T-C (110?°C), a high dielectric constant (ε_r ≈?12,285) with low dielectric loss (0.03), good polarization (P_s ≈?8.3?μC/cm²) and large recoverable energy density (W?=?121?mJ/cm³) with an energy storage efficiency (η) of 70% at an electric field of 25?kV/cm in 0.6BZT-0.4BCT ceramics make them suitable candidates for energy storage capacitor applications.     

Coarse-grained modeling of model poly(urethane urea)s: Microstructure and interface aspects

Poly(urethane urea) elastomers are versatile and can be tailored to exhibit a broad range of mechanical response under high strain rate deformation. In this work, we utilize coarse-grained molecular dynamics simulations to elucidate the molecular mechanisms, particularly the effects of hard segment content, intermolecular interaction, and rigidity of the interface between the hard and soft segments on local morphology and rate-dependent stress-strain behavior in the ballistic regime. Simulation results qualitatively agree with available experimental data, where analysis of hard segment orientation during tensile and compression deformation and dynamic strain rate sensitivity was also performed. Further study of the intermolecular interaction on the stress-strain behavior reveals that it has a strong effect on strain hardening, particularly for a rigid interface, once the hard segment content reaches the percolation threshold. Simulation results also show that interface intermolecular interaction could become more dominant over interface rigidity in the initial stress-strain response, particularly below percolation.     

The collision efficiency in a shear flow

Fluid flows with cohesive particles are present in oil industry (e.g. natural gas/oil with hydrates, wax or asphaltenes), medicine (e.g. blood cells), nano- and ferro-fluidic applications (e.g. fluids with nanoparticles subject to the van der Waals and electrostatic interactions) and even in astrophysics (e.g. grains in planetary rings). Such flows may lead to formation of agglomerates that, for example in pipelines, may result in unwanted phenomena such as formation of deposits. The main process parameter governing this is the “collision efficiency”, which is the ratio of the number of collisions resulting in agglomeration to the total number of collisions. This is commonly considered to depend on the relativemagnitudes of attractive and repulsive interactions during a collision. The effect of the particles' mechanical properties on the agglomeration efficiency has, however, not yet been studied. In this paper the agglomeration efficiency is studied as a function of inter-particle friction, stiffness, density and volume fraction by numerical simulation. By running direct numerical simulations (DNS) with Lagrangian particle tracking of a shear flow laden with solid particles, the parameters influencing the agglomeration efficiency are demonstrated and their effects quantified. Finally, an expression that relates the collision efficiency to the salient dimensionless physical parameters is proposed.