Application of the flash method to rods and tubes

The flash method is the most used technique to measure the thermal diffusivity of solids. It consists of heating the front face of an opaque slab by a short light pulse and detecting the temperature evolution at its rear surface, from which the thermal diffusivity is retrieved. In this paper we extend the classical flash method to be used with rods and tubes. First, the temperature evolution of the surface temperature of solid and hollow cylinders is calculated. Then, experimental measurements of a set of stainless steel samples using an infrared camera confirm the validity of the method.     

Onsager model for the structure of rigid rods confined on a spherical surface

Recent studies have suggested that a monolayer of self-avoiding hard rods confined on a spherical surface may display a distribution texture corresponding to splay, tennis-ball, rectangle, or cut-and-rotate splay symmetries. We investigate the system on the basis of a generalized Onsager model which includes both excluded-volume and entropic effects. The numerical solution indicates that the splay state, where on average rods line up in parallel to the longitudinal lines on the spherical surface, is the only stable state.     

Homogenization of two-dimensional clusters of rigid rods in air

The scattering of sound waves by circular-shaped clusters consisting of two-dimensional distributions of rigid cylinders in air is studied in the low-frequency limit (homogenization) both theoretically and experimentally. Analytical formulas for the effective density and sound speed are obtained in the framework of multiple scattering. Here, an experimental demonstration is reported in which a cluster of wooden rods acoustically behaves as a cylinder of argon gas. Moreover, evidence is presented indicating the validity of the homogenization in this cluster at frequencies lower than 3 kHz, which corresponds to a wavelength that is only 4 times the parameter of the embedded lattice and is a quarter of the cluster's diameter.     

Nematic ordering of rigid rods in a gravitational field

The isotropic-to-nematic transition in an athermal solution of long rigid rods subject to a gravitational (or centrifugal) field is theoretically considered in the Onsager approximation. The new feature emerging in the presence of gravity is a concentration gradient that coupled with the nematic ordering. For rodlike molecules this effect becomes noticeable at centrifugal acceleration g approximately 10(3)-10(4) m/s(2), while for biological rodlike objects, such as tobacco mosaic virus, the effect is important even for normal gravitational acceleration conditions. Rods are concentrated near the bottom of the vessel, which sometimes leads to gravity induced nematic ordering. The concentration range corresponding to phase separation increases with increasing g. In the region of phase separation the local rod concentration, as well as the order parameter, follow a step function with height.     

Direct measurement of entropic forces induced by rigid rods

We present the first direct depletion potential measurements for a single colloidal sphere close to a wall in a suspension of rigid colloidal rods. Since all particle interactions are as good as hard-core-like, the depletion potentials are of entirely entropic origin. We developed a density functional theory approach that accurately accounts for this experimental situation. The depletion potentials calculated for different rod number densities are in very good quantitative agreement with the experimental results.     

Formation and interaction of membrane tubes

We show that the formation of membrane tubes (or membrane tethers), which is a crucial step in many biological processes, is highly nontrivial and involves first-order shape transitions. The force exerted by an emerging tube is a nonmonotonic function of its length. We point out that tubes attract each other, which eventually leads to their coalescence. We also show that detached tubes behave like semiflexible filaments with a rather short persistence length. We suggest that these properties play an important role in the formation and structure of tubular organelles.     

Limiting polytope geometry for rigid rods,disks, and spheres

The available configuration space for finite systems of rigid particles separates into equivalent disconnected regions if those systems are highly compressed. This paper presents a study of the geometric properties of the limiting high-compression regions (polytopes) for rods, disks, and spheres. The molecular distribution functions represent cross sections through the convex polytopes, and for that reason they are obliged to exhibit single-peak behavior by the Brünn-Minkowski inequality. We demonstrate that increasing system dimensionality implies tendency toward nearest-neighbor particle-pair localization away from contact. The relation between the generalized Euler theorem for the limiting polytopes and cooperative jamming of groups of particles is explored. A connection is obtained between the moments of inertia of the polytopes (regarded as solid homogeneous bodies) and crystal elastic properties. Finally, we provide a list of unsolved problems in this geometrical many-body theory.     

Configurational entropy of adsorbed rigid rods: Theory and Monte Carlo simulations

The configurational entropy of straight rigid rods of length k (k-mers) adsorbed on square, honeycomb, and triangular lattices is studied by combining theory and Monte Carlo (MC) simulations in grand canonical and canonical ensembles. Three theoretical models to treat k-mer adsorption on two-dimensional lattices have been discussed: (i) the Flory-Huggins approximation and its modification to address linear adsorbates; (ii) the well-known Guggenheim-DiMarzio approximation; and (iii) a simple semi-empirical model obtained by combining exact one-dimensional calculations, its extension to higher dimensions and Guggenheim-DiMarzio approach. On the other hand, grand canonical and canonical MC calculations of the configurational entropy were obtained by using a thermodynamic integration technique. In the second case, the method relies upon the definition of an artificial Hamiltonian associated with the system of interest for which the entropy of a reference state can be exactly known. Thermodynamic integration is then applied to calculate the entropy in a given state of the system of interest. Comparisons between MC simulations and theoretical results were used to test the accuracy and reliability of the models studied.     

The effect of hydrogen on the magnetic properties of quenched Ti-Zr-Ni rods

Ti-Zr-Ni quenched rods of 3, 2 and 1.5 mm diameter, prepared by vacuum-casting, were analyzed by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM) before and after hydrogenation. Samples with two different compositions were prepared, i.e., Ti₄₀Zr₄₀Ni₂₀ and Ti₅₃Zr₂₇Ni₂₀. The as-cast rods were pulverized and hydrogenated from the gas-phase at 45 bar and 300 °C for 1000 min. The mass spectra of the desorbed hydrogen revealed the distribution of the hydrogen desorption temperatures from these alloys, whereas using thermogravimetry we obtained the mass% of desorbed H. We found that the ratio between the i-phase and the C14 Laves phase depends on the master alloy composition and the cooling rate, i.e., the rod diameter. VSM measurements revealed an about 30% decrease in paramagnetic susceptibility upon hydrogenation for all six investigated samples.     

Self-diffusion and sedimentation of tracer spheres in (semi)dilute dispersions of rigid colloidal rods

Long-time self-diffusion and sedimentation of fluorescent tracer spheres in electrostatically stabilized dispersions of rigid colloidal host rods have been measured in situ with fluorescence recovery after photobleaching, and gravitational and ultracentrifugal sedimentation. The dynamics of silica tracer spheres of 39 and 370 nm radius was monitored in dispersions of host rods with aspect ratios 9.6 and 25.7 at various rod volume fractions. The translational and rotational diffusion coefficient of the host rods was obtained independently with dynamic light scattering and birefringence decay measurements. Our results indicate that sedimentation and long-time self-diffusion are determined by the same friction factor. Furthermore we find that, as long as the host rods are relatively mobile, tracer sphere sedimentation and long-time self-diffusion are governed by the macroscopic solution viscosity, regardless of the tracer and host rod size. However, when the host rods are immobilized, due to rod entanglements at higher volume fractions, tracer sphere dynamics depends strongly on the tracer size relative to the pore size of the host rod network. The large tracers are completely trapped in the network whereas the small tracer spheres remain mobile. Current models for tracer sphere motion in rod assemblies do not satisfactorily explain the complete dynamic regime covered by our experimental model system because the effect of host rod mobility is not properly taken into account.     

Pearling instabilities of membrane tubes with anchored polymers

We have studied the pearling instability induced on hollow tubular lipid vesicles by hydrophilic polymers with hydrophobic side groups along the backbone. The results show that the polymer concentration is coupled to local membrane curvature. The relaxation of a pearled tube is characterized by two different well-separated time scales, indicating two physical mechanisms. We present a model, which explains the observed phenomena and predicts polymer segregation according to local membrane curvature at late stages.     

The effect of impurity on transition frequency of bound polaron in quantum rods

The Hamiltonian of a quantum rod with an ellipsoidal boundary is given after a coordinate transformation that changes the ellipsoidal boundary into a spherical one. The properties of the quantum rods constituting the bridge between two-dimensional quantum wells, zero-dimensional quantum dots and one-dimensional quantum wires are explored theoretically using linear combination operator method. The first internal excited state energy, the excitation energy and the transition frequency between the first internal excited and the ground states of the strong-coupled impurity-bound polaron in the rod with Coulomb-bound potential, the transverse effective confinement length, the ellipsoid aspect ratio and the electron–phonon coupling strength are studied. It is found that the first internal excited state energy, the excitation energy and the transition frequency are increasing functions of the Coulomb-bound potential and the electron–phonon coupling strength, whereas they are decreasing functions of the ellipsoid aspect ratio and the transverse effective confinement length. These results can be attributed to the interesting quantum size confining effects.     

The effect of turbulence damping on acoustic wave propagation in tubes

The attenuation of sound due to the interaction between a low Mach number turbulent boundary layer and acoustic waves can be significant at low frequencies or in narrow tubes. In a recent publication by the present authors the acoustics of charge air coolers for passenger cars has been identified as an interesting application where turbulence attenuation can be of importance. Favourable low-frequency damping has been observed that could be used for control of the in-duct sound that is created by the engine gas exchange process. Analytical frequency-dependent models for the eddy viscosity that controls the momentum and thermal boundary layers are available but are restricted to thin acoustic boundary layers. For cases with cross-sections of a few millimetres a model based on thin acoustic boundary layers will not be applicable in the frequency range of interest.In the present paper a frequency-dependent axis-symmetric numerical model for interaction between turbulence and acoustic waves is proposed. A finite element scheme is used to formulate the time harmonic linearized convective equations for conservation of mass, momentum and energy into one coupled system of equations. The turbulence is introduced with a linear model for the eddy viscosity that is added to the shear viscosity. The proposed model is validated by comparison with experimental data from the literature.     

Temperature effect on the evolution and stability of rigid screening spatial soliton in biased photorefractive crystal

Based on the temperature dependence of two-wave mixing, temperature effects on the dynamical evolution and stability of rigid screening (RS) bright solitons in a photorefractive dissipative system based on two-wave mixing have been investigated numerically. Our results indicate that the stability of bright RS solitons depends strongly on the crystal temperature. The RS solitons are stable to small temperature perturbations. They will not evolve into stable rigid screening solitons, however, and their intensity and width vary with the propagation distance if the temperature deviation is large enough. The potential applications of the temperature properties of these RS solitons in optical attenuators or repeaters are discussed.     

表面张力对固壁旁空泡运动特性影响的理论和实验研究

表面张力是影响空泡脉动及空蚀的一个重要因素.对五种不同表面张力液体中空泡脉动（膨胀和收缩）过程进行了研究,并将实验结果与基于空泡生长和溃灭理论的计算结果进行了对比.实验中,用激光作为测试光源,采用光偏转测试系统研究了不同表面张力液体中空泡泡壁运动规律及泡壁速度的变化.结果表明：表面张力对空泡膨胀过程起抑制作用,故液体表面张力愈大,空泡能达到的最大直径越小;表面张力对空泡的收缩过程则起加速作用,液体表面张力愈大,收缩越迅速,空泡泡壁运动速度越大,其所产生的瞬时溃灭压强越大,空化效果越好. 关键词： 表面张力 空泡 光偏转