Out-of-equilibrium microrheology inside living cells

Both forced and spontaneous motions of magnetic microbeads engulfed by Dictyostelium cells have served as experimental probes of intracellular dynamics. The complex shear modulus G*(omega), determined from active oscillatory measurements, has a power-law dynamics and increases with the probe size, reflecting intracellular structural complexity. The combined use of passive microrheology allows one to derive the power spectrum of active forces acting on intracellular phagosomes and to test the validity of the fluctuation-dissipation theorem inside living cells.     

On interference effects in the elastodynamic Green's functions G33(x, omega) of Si and GaAs

In this paper, we analyze interference effects present in the elastodynamic Green's functions G33(x,omega) of the cubic crystals Si and GaAs, which are associated with folded portions of the wave surface of the slow transverse (ST) acoustic mode. G33(x,omega) represents the three dimensional extension of the amplitude distribution imaged in the transmission acoustic microscopy of these crystals. The intensity contrast for oscillations of a particular wave vector k in the interference pattern is determined essentially by the 3D Fourier transform of G33(x,omega)G33*(x,omega). According to the Fourier autocorrelation theorem, that transform is equivalent to the autocorrelation function of the corresponding distribution G(33)(k,omega) in k-space. We show that due to the linear mapping between k-space and the slowness vector s-space, the interference phenomena discussed here are related to geometrical features of the slowness surface of the ST mode. We present calculations of these effects based on the angular spectrum technique.     

Enhanced elasticity and soft glassy rheology of a smectic in a random porous environment

We report studies of the frequency-dependent shear modulus, G(*)(omega) = G(')(omega) + iG(')(omega), of the liquid crystal octylcyanobiphenyl (8CB) confined in a colloidal aerosil gel. With the onset of smectic order, G' grows approximately linearly with decreasing temperature, reaching values that exceed by more than 3 orders of magnitude the values for pure 8CB. The modulus at low temperatures possesses a power-law component, G(*)(omega) approximately omega(alpha), with exponent alpha that approaches zero with increasing gel density. The amplitude of G' and its variation with temperature and gel density indicate that the low temperature response is dominated by a dense population of defects in the smectic. In contrast, when the 8CB is isotropic or nematic, the modulus is controlled by the elastic behavior of the colloidal gel.     

Influence of parity violating weak nuclear potentials on vibrational and rotational frequencies in chiral molecules

We study the effect of parity violation on the vibrational and rotational frequencies of CHBrClF. We report the parity violating potentials as a function of reduced normal coordinates for all nine internal vibrational modes omega(1) to omega(9), using our new, accurate multiconfigurational-linear response (RPA and complete-active-space self-consistent field) approach. All modes omega(i) show a strongly mode dependent relative shift Delta(pv)omega(i)/omega(i) (between 0.08x10(-16) and 13.3x10(-16), much smaller than all previous experimental tests could detect, including the most recent ones). The results are discussed in relation to other tests of parity violation.     

利用单分子光学探针测量幂律分布的聚合物动力学

研究聚合物薄膜纳米尺度的动力学特性对于高性能材料的制备具有重要的意义.本文利用尼罗红单分子作为光学探针吸附在聚丙烯酸甲酯(PMA)聚合物链上,研究该聚合物薄膜的动力学特性.通过单分子散焦宽场荧光成像显微镜技术测量了单分子随PMA聚合物链转动弛豫的三维再取向特性,当环境温度高于PMA的玻璃点温度19 K时,发现处于PMA聚合物薄膜中的单分子光学探针的转动态和非转动态的持续时间概率密度服从指数截止的幂律分布.研究结果表明该温度下PMA聚合物薄膜的纳米环境动力学仍存在空间和时间异构性.     

Noise thermal impedance of a diffusive wire

The current noise density S2 of a conductor in equilibrium, the Johnson noise, is determined by its temperature T: S2 = 4k(B)TG, with G the conductance. The sample's noise temperature T(N) = S2/(4k(B)G) generalizes T for a system out of equilibrium. We introduce the "noise thermal impedance" of a sample as the ratio deltaT(N)omega/deltaP(J)omega of the amplitude deltaT(N)omega of the oscillation of T(N) when heated by an oscillating power deltaP(J)omega at frequency omega. For a macroscopic sample, it is the usual thermal impedance. We show for a diffusive wire how this (complex) frequency-dependent quantity gives access to the electron-phonon interaction time in a long wire and to the diffusion time in a shorter one, and how its real part may also give access to the electron-electron inelastic time. These times are not simply accessible from the frequency dependence of S2 itself.     

The effects of chain segment motion on ionic diffusion in solid polymer electrolytes

The “vehicular effects” of chain segment motion on ionic diffusion in solid polymer electrolytes have been investigated via numerical simulation on a two-dimensional square lattice where the dynamical variation of chain configuration is presented by translational or rotational bond movement. It is found that (a) both types of bond motion promote continuous diffusion when the fraction (p) of available bonds is below the static percolation threshold of p=0.5 in two dimensions; (b) translational motion of bonds parallel to the direction of diffusion produces larger diffusion coefficients (D) than that by random renewal of the dynamic bond percolation model (DBPM), while the perpendicular motion or rotational motion gives smaller values of D; (c) Smooth lines instead of “stair-case like” curves generated by DBPM are obtained in the mean-squared displacement versus time plot, when bonds are shifting along the diffusion route. The dependence of diffusion coefficients on the variation of motion patterns of bonds is expected to be related to the temperature change under which these patterns are excited accordingly, such that VTF behavior of certain polymer electrolytes may be deduced.     

Mechanical and dielectric properties of SEBS modified by graphite inclusion and composite interface

Tough and flexible dielectrics were prepared using graphite (G), a natural and low-cost resource, as filler in polystyrene-b-(ethylene-co-butylene)-b-polystyrene (SEBS) and maleinized SEBS (SEBS-MA) matrices. The disintegration of graphite in submicron particles was accomplished by the shear forces during the melt processing step and it was highlighted by atomic force microscopy. Simultaneous increase of tensile strain, strength and Young's modulus was noticed for SEBS/G and SEBS-MA/G composites compared to unfilled matrices, this remarkable feature being previously reported only for some nanocomposites. Moreover, an exponential variation of the dielectric permittivity with the volume fraction of G was obtained. Higher reinforcing efficiency and better dielectric properties were observed in SEBS-MA/G composites, compared to the corresponding SEBS/G composites, due to the stronger polymer–filler interface and better dispersion of graphite. This study brings new insights into nanolevel properties of SEBS composites and it opens new perspectives on high performance composites by using graphite instead of expensive graphene and efficient melt mixing process.     

A pulse EPR study of longitudinal relaxation of the stable radical in gamma-irradiated L-alanine

The longitudinal relaxation rate of the first stable alanine radical, SAR1, was studied by employing pulse EPR technique over a wide temperature interval (5-290 K). The complex nonexponential recovery of the longitudinal magnetization in this temperature interval has been described with two characteristic relaxation times, 1/T*(1a) as the faster component and 1/T*(1b) as the slower component, respectively. It was shown that 1/T*(1a) is strongly affected by the CH(3) group dynamics of the SAR1 center. The complete temperature dependence of 1/T*(1a) was described by invoking several relaxation mechanisms that involve hindered motion of the CH(3) group from classical rotational motion to coherent rotational tunneling. It was shown that all relevant relaxation mechanisms are determined by a single correlation time with the potential barrier (Delta E/k=1570 K). On the other hand the temperature dependence of 1/T*(1b) is related to the motional dynamics of the neighborly NH(3) and CH(3) groups. We found a larger average potential barrier for this motion (Delta E/k=2150 K) corresponding to smaller tunneling frequencies of the neighbor groups.     

Analysis of Quantum Rod Diffusion by Polarized Fluorescence Correlation Spectroscopy

To measure the polarization dependence of fluorescent probes, a confocal-microscope-based polarized fluorescence correlation spectroscopy system was developed, and the polarization dependence on the rotational diffusion of well-defined quantum rods (Qrods) was investigated and characterized. The rotational diffusion region of the Qrods was observed over a time range of less than 10^?5 s in a water solution, and the rotational diffusion parameters were extracted using a rotational diffusion model in which the viscosity of the solution media was varied. Our work demonstrated that polarized fluorescence correlation spectroscopy (FCS) is useful for investigating both the rotational and translational diffusion of fluorescent probes.     

Floquet-van Vleck analysis of heteronuclear spin decoupling in solids: the effect of spinning and decoupling sidebands on the spectrum

We investigate the effect of magic angle spinning on heteronuclear spin decoupling in solids. We use an analytical Floquet-van Vleck formalism to derive expressions for the powder-averaged signal as a function of time. These expressions show that the spectrum consists of a centerband at the isotropic frequency of the observed spin, omega(0), and rotational decoupling sidebands at omega(0)+/-omega(1)+/-momega(r), where omega(1) is the decoupling field strength and omega(r) is the rotation frequency. Rotary resonance occurs when the rotational decoupling sidebands overlap with the centerband. Away from the rotary resonance conditions, the intensity of the centerband as a function of omega(r)/omega(1) is simply related to the total intensity of the rotational decoupling sidebands. Notably, in the absence of offset terms it is shown that as omega(1) decreases, the centerband intensity can decrease without any associated broadening. Furthermore, the centerband width is shown to be independent of spinning speed, to second order for the effects we consider. The effects of I spin chemical shift anisotropy and homonuclear dipolar couplings are also investigated. The analytical results are compared to simulations and experiments.     

Dynamics of paramagnetic agents by off-resonance rotating frame technique

Off-resonance rotating frame technique offers a novel tool to explore the dynamics of paramagnetic agents at high magnetic fields (B0 > 3T). Based on the effect of paramagnetic relaxation enhancement in the off-resonance rotating frame, a new method is described here for determining the dynamics of paramagnetic ion chelates from the residual z-magnetizations of water protons. In this method, the dynamics of the chelates are identified by the difference magnetization profiles, which are the subtraction of the residual z-magnetization as a function of frequency offset obtained at two sets of RF amplitude omega(1) and pulse duration tau. The choices of omega(1) and tau are guided by a 2-D magnetization map that is created numerically by plotting the residual z-magnetization as a function of effective field angle theta and off-resonance pulse duration tau. From the region of magnetization map that is the most sensitive to the alteration of the paramagnetic relaxation enhancement efficiency R(1rho)/R1, the ratio of the off-resonance rotating frame relaxation rate constant R(1rho) verse the laboratory frame relaxation rate constant R(1), three types of difference magnetization profiles can be generated. The magnetization map and the difference magnetization profiles are correlated with the rotational correlation time tauR of Gd-DTPA through numerical simulations, and further validated by the experimental data for a series of macromolecule conjugated Gd-DTPA in aqueous solutions. Effects of hydration water number q, diffusion coefficient D, magnetic field strength B0 and multiple rotational correlation times are explored with the simulations of the magnetization map. This method not only provides a simple and reliable approach to determine the dynamics of paramagnetic labeling of molecular/cellular events at high magnetic fields, but also a new strategy for spectral editing in NMR/MRI based on the dynamics of paramagnetic labeling in vivo.     

Conductivity or rheology? Tradeoff for competing properties in the fabrication of a gel polymer electrolyte based on chitosan-barbiturate derivative

Chitosan-barbiturate (Ch-Ba) derivative was synthesized to afford organosolubility. Attachment of Ba onto the Ch backbone was confirmed by ¹H NMR with peaks at 8.2 and 11.1 ppm, and FTIR with bands at 1679 and 1739 cm^?1 belonging to the Ba ring. This derivative was used as a host polymer in the preparation of gel polymer electrolytes. The components of the gel consist of tetrapropylammonium iodide (TPAI) as the salt, SiO₂ nanofiller (NF) as the mechanical stabilizer, and dimethyl sulfoxide (DMSO) as the solvent. The necessary formulation required to produce the gel was studied using response surface models by means of artificial neural networks. Electrochemical and rheological behaviors were studied and the simulated model predicted conductivities were as high as 8.51 mS cm^?1 while still maintaining a solid-like gel structure in the region where storage modulus dominated loss modulus, G″/G′?<?1.     

用NMR方法测定聚合物长侧基转动扩散系数

提出了一种测定聚合物溶液及有机固体中长侧基转动扩散系数的NMR方法,即先导出聚合物溶液和固体中长侧基运动的相关函数和谱密度函数,然后利用¹³C自旋弛豫数据求出扩散系数.同时还讨论了两个应用实例.     

Study of K*(892) mass and width splitting caused by model difference

According to the new K*(892)0 and K*(892)- masses reported by the BELLE experiment and the K*(892)0 mass reported by the FOCUS experiment, mass splitting between neutral and charged K*(892) becomes very small. This is significantly different from the current world average values given by the Particle Data Group 2008. We find that there are differences between models used to fit the K*(892) decay invariant mass spectra in different measurements and study the model dependence in the measurement of K*(892) parameters. We refit the K*(892)0 mass spectra of the BELLE and FOCUS experiments with the formula used by BELLE in fitting K*(892)- to get new mass and width. After refitting, the K*(892)0 mass of the BELLE experiment becomes 1.4 MeV/c2 larger than the initial value and that of the FOCUS experiment is 1 MeV/c2 smaller than the initial value. We also fit the spectra of some other experiments to extract the K*(892) parameters using the BELLE K* (892)- parametrization.     

Band head spin assignment of superdeformed bands in 86Zr

Two parameter expressions for rotational spectra viz. variable moment of inertia (VMI), ab formula and three parameter Harris ω² expansion are used to assign the band head spins (I₀) of four rotational superdeformed bands in ⁸⁶Zr. The least-squares fitting method is employed to obtain the band head spins of these four bands in the A~80 mass region. Model parameters are extracted by fitting of intraband γ-ray energies, so as to obtain a minimum root-mean-square (rms) deviation between the calculated and the observed transition energies. The calculated transition energies are found to depend sensitively on the assigned spins. Whenever an accurate band head spin is assigned, the calculated transition energies are in agreement with the experimental transition energies. The dynamic moment of inertia is also extracted and its variation with rotational frequency is investigated. Since a better agreement of band head spin with experimental results is found using the VMI model, it is a more powerful tool than the ab formula and Harris ω² expansion.     

Origins of nonexponential decay in single molecule measurements of rotational dynamics

Recent reports have demonstrated that the correlation function of the fluorescence dichroism signal, measured as a probe of single molecule rotational dynamics, should not manifest a single exponential decay even for isotropic diffusion. This has called into question the attribution of observed nonexponential behavior in supercooled fluids and polymer systems to dynamical heterogeneity. We show here that, for the case of a high numerical aperture objective, the dichroism decay becomes indistinguishable from a single exponential. As a consequence, observed nonexponential decays can be associated with complex rotational dynamics. These effects are illustrated via simulated rotational trajectories for isotropic diffusion of a dipole.     

Measurement of the second hyperpolarizability of linear conjugated polymer based on attenuated-total-reflection technique

Based on the attenuated-total-reflection technique, a new method has been proposed to determine the second hyperpolarizability gamma(-omega(4);omega(3),omega(2),omega(1)) of the linear conjugated polymer in the off-resonant region by means of quadratic electro-optic effect. An important feature of this method is the absence of a high-power pulse laser, resulting in more convenience and cost effectivity than other techniques.     

Elastic and anelastic anomalies associated with the antiferromagnetic ordering transition in wüstite, FexO

The elastic and anelastic properties of three different samples of Fe(x)O have been determined in the frequency range 0.1-2 MHz by resonant ultrasound spectroscopy and in the range 0.1-50 Hz by dynamic mechanical analysis in order to characterize ferroelastic aspects of the magnetic ordering transition at T(N) ~ 195 K. No evidence was found of separate structural and magnetic transitions but softening of the shear modulus was consistent with the involvement of bilinear coupling, λe(4)q, between a symmetry-breaking strain, e(4), and a structural order parameter, q. Unlike a purely ferroelastic transition, however, C(44) does not go to zero at the critical temperature, T*(c), due to the intervention of the magnetic ordering at a higher temperature. The overall pattern of behaviour is nevertheless consistent with what would be expected for a system with separate structural and magnetic instabilities, linear-quadratic coupling between the structural (q) and magnetic (m) driving order parameters, λqm(2), and T(N) > T*(c). Comparison with data from the literature appears to confirm the same pattern in MnO and NiO, with a smaller difference between T(N) and T*(c) in the former and a larger difference in the latter. Strong attenuation of acoustic resonances at high frequencies and a familiar pattern of attenuation at low frequencies suggest that twin walls in the rhombohedral phase have typical ferroelastic properties. Acoustic dissipation in the stability field of the cubic phase is tentatively attributed to anelastic relaxations of the defect ordered structure of non-stoichiometric wüstite or of the interface between local regions of wüstite and magnetite, with a rate controlling step determined by the diffusion of iron.