Bose-Einstein condensation of bouncing balls

Microscopic bouncing balls, i.e., particles confined within a positive one-half-dimensional gravitational potential, display Bose-Einstein condensation (BEC) not only in the thermodynamic limit but also in the case of a finite number of particles, and the critical temperature with a finite number of particles is higher than that in the thermodynamic limit. This system is different from the one-dimensional harmonic potential one, for which the standard result indicates that the BEC is not possible unless the number of particles is finite.     

Dynamics of polymers in flowing colloidal suspensions

Using hydrodynamic simulations we examine the behavior of single polymers in a confined colloidal suspension under flow. We study the conformations of both, collapsed and noncollapsed polymers. Our results show that the presence of the colloids has a pronounced effect on the unfolding and refolding cycles of collapsed polymers, but does not have a large effect for noncollapsed polymers. Further inspection of the conformations reveals that the strong flow around the colloids and the direct physical compressions exerted on a globular polymer diffusing in between colloidal shear bands largely facilitate the initiation and unraveling of the globular chains. These results are important for rheological studies of (bio)polymer-(bio)colloid mixtures.     

Rheology of sheared monodisperse granular suspensions

Sheared granular suspensions can either flow or be jammed. They show as well a ‘melting’ transition: partially ordered flowing states are found which can be melted into fully disordered arrangements of grains by sufficient shear. While these are well documented phenomena, the underlying mechanisms and their control parameters are still far from clear. Via Molecular Dynamics simulations, we study the rheology of a model system of sheared frictional monodisperse granular materials [7, 8]. In particular, we aim to understand the nature of a critical line separating crystallised and melted states and the “jammed” region in the phase diagram. We outline as well connections and differences with thermal glass formers and colloidal suspensions.     

Flow, ordering, and jamming of sheared granular suspensions

We study the rheological properties of a granular suspension subject to constant shear stress by constant volume molecular dynamics simulations. We derive the system "flow diagram" in the volume fraction or stress plane (phi, F): at low phi the flow is disordered, with the viscosity obeying a Bagnold-like scaling only at small F and diverging as the jamming point is approached; if the shear stress is strong enough, at higher phi an ordered flow regime is found, the order-disorder transition being marked by a sharp drop of the viscosity. A broad jamming region is also observed where, in analogy with the glassy region of thermal systems, slow dynamics followed by kinetic arrest occurs when the ordering transition is prevented.     

Three-dimensional clogging structures of granular spheres near hopper orifice

The characteristic clogging structures of granular spheres blocking three-dimensional granular flow through hopper outlet are analyzed based on packing structures reconstructed using magnetic resonance imaging techniques.Spheres in clogging structures are arranged in a way with typical features of load-bearing,such as more contacting bonds close to the horizontal plane and more mutually-stabilized contact configurations than packing structures away from the orifice.The requirement of load-bearing inevitably leads to the cooperativity of clogging structures with a correlation length of several particle diameters.This correlation length being comparable with the orifice diameter suggests that a clogging structure is composed of several mutually-stabilized structural motifs to span the orifice perimeter,instead of a collection of independent individual spheres to cover the whole orifice area.Accordingly,we propose a simple geometric model to explain the unexpected linear dependence of the average size of three-dimensional clogging structures on orifice diameter.     

Three-dimensional high-resolution acoustic imaging of the sub-seabed

Chirp sub-bottom profilers are marine acoustic devices that use a known and repeatable frequency-modulated source signature to produce vertical seismic reflection cross-sections of the sub-seabed. Here a 3D Chirp system is described that operates in the frequency range of 1.5-13 kHz, to produce a three-dimensional image of the sub-seabed, with typical penetration of 10-30 m and decimetric horizontal and vertical resolution. The system design incorporates a rigid frame that contains the Chirp source array together with 60 receiver elements, with positioning provided by an integrated real-time-kinematic (RTK) global positioning system (GPS). The system can be surface towed from a small survey vessel and can be applied to targets of marine geological, engineering, archaeological and defence interest. Data acquisition and processing are described for a case study which images a buried engineering structure in the Port of Southampton.     

Three-dimensional integral imaging of micro-objects

We propose a method for displaying micro-objects in space that is based on three-dimensional (3D) integral imaging, in which elemental images are calculated from a two-dimensional sampling of the optical field along different depths by use of confocal scanning microscopy. Experimental results are presented to demonstrate that a uniformly magnified 3D biological specimen can be displayed in space, and thus integral imaging can be used for 3D display of confocal microscopy. To the best of our knowledge, this is the first report of 3D integral imaging of (semitransparent) micro-objects.     

弹跳球运动的理论分析与数值研究

建立了弹跳球运动的二维映射关系式,利用非线性动力学的原理和方法,在不同的控制参数条件下,研究了系统二维映射式不动点的稳定性;数值模拟的结果显示,系统在不同的控制参数条件下具有不同的运动特征.     

Three-dimensional annihilation imaging of trapped antiprotons

We demonstrate three-dimensional imaging of antiprotons in a Penning trap, by reconstructing annihilation vertices from the trajectories of the charged annihilation products. The unique capability of antiparticle imaging has allowed, for the first time, the observation of the spatial distribution of the particle loss in a Penning trap. The radial loss of antiprotons on the trap wall is localized to small spots, strongly breaking the azimuthal symmetry expected for an ideal trap. Our observations have important implications for detection of antihydrogen annihilations.     

Three-dimensional imaging of direct-written photonic structures

Third-harmonic generation microscopy has been used to analyze the morphology of photonic structures created using the femtosecond laser direct-write technique. Three-dimensional waveguide arrays and waveguide-Bragg gratings written in fused-silica and doped phosphate glass were investigated. A sensorless adaptive-optical system was used to correct the optical aberrations occurring in the sample and microscope system, which had a lateral resolution of less than 500?nm. This nondestructive testing method creates volume reconstructions of photonic devices and reveals details invisible to other linear microscopy and index profilometry techniques.     

Periodic knolls and valleys: coexistence of solid and liquid states in granular suspensions

We report the spontaneous emergence of a doubly periodic train of sedimented knolls in a dense suspension. These solidified knolls rise out of, and coexist alongside, a sea of freely flowing liquid in a slowly rotating horizontal bottle. We apply a variable viscosity model that permits simultaneous analysis of fluidlike and solidlike behaviors that are ubiquitous in a variety of sedimenting flows. The model generates qualitative agreement with experiments, and produces new insights into mechanisms by which sedimented structures form.     

Chemical shift imaging with continuously flowing hyperpolarized xenon for the characterization of materials

In this contribution we report new approaches to the MRI of materials using continuously produced laser-polarized (129)Xe gas. This leads to vastly improved sensitivity and makes new kinds of information available. The hyperpolarized xenon is produced in a continuous flow system that conveniently delivers the xenon at low partial pressure to probes for NMR and MRI experiments. We illustrate applications to the study of micropore and other kinds of void space and show for the first time that with flowing hyperpolarized xenon it is possible to obtain chemical-shift-resolved images in a relatively short time.     

Three-dimensional imaging of microstructure in Au nanocrystals

X-ray diffraction using a coherent beam involves the mutual interference among all the extremities of small crystals. The continuous diffraction pattern so produced can be phased because it can be oversampled. We have thus obtained three-dimensional images of the interiors of Au nanocrystals that show 50 nm wide bands of contrast with [111] orientation that probably arise from internal twinning by dynamic recrystallization during their formation at high temperature.     

Fluorescence confocal polarizing microscopy: Three-dimensional imaging of the director

Much of the modern understanding of orientational order in liquid crystals (LCs) is based on polarizing microscopy (PM). A PM image bears only two-dimensional (2D) information, integrating the 3D pattern of optical birefringence over the path of light. Recently, we proposed a technique to image 3D director patterns by fluorescence confocal polarizing microscopy (FCPM). The technique employs the property of LC to orient the fluorescent dye molecules of anisometric shape, added in small quantities to the LC. In LC, smooth director deformations do not alter mass density of the material. Thus the density of dye is also uniform across the sample, except, perhaps, near the surfaces or at the cores of topological defects. In polarized light, the measured fluorescence signal is determined by the spatial orientation of the molecules rather than by dye concentration (as in regular biological samples stained with tissue-specific dyes). The contrast is enhanced when both excitation and detection of fluorescence light are performed in polarized light. This short review describes the essence of FCPM technique and illustrates some of its applications, including imaging of Frederiks electric-field induced effect in a nematic LC and defects such as dislocations in cholesteric LCs.     

非弹性蹦球的动力学研究

对有空气阻尼的非弹性蹦球的动力学行为进行了数值模拟,通过改变控制参数V₀,蹦球的运动表现出倍周期分岔、混沌等非线性现象,采用0-1检测法和最大Lyapunov指数证实了这种现象.     

Three-dimensional photoacoustic imaging of blood vessels in tissue

We applied photoacoustics as a tissue tomography technique for the detection of blood concentrations, e.g., angiogenesis around tumors. We imaged blood vessels in highly scattering samples, using 532-nm light, to depths of ~1 cm . The samples were real tissue (chicken breast) or 10% dilutions of Intralipid-10%. The blood flowed through nylon capillaries. Polyvinylidene difluoride (PVdF) piezoelectric detectors were used in a surface-scanning mode. We demonstrate the sensitivity of the technique by photoacoustic detection of single red blood cells upon a glass plate. Lateral resolution is limited by the detector diameter (200 microm). The depth resolution is ~10 microm.     

Three-dimensional imaging of colloidal glasses under steady shear

Using fast confocal microscopy we image the three-dimensional dynamics of particles in a yielded hard-sphere colloidal glass under steady shear. The structural relaxation, observed in regions with uniform shear, is nearly isotropic but is distinctly different from that of quiescent metastable colloidal fluids. The inverse relaxation time tau(alpha)(-1) and diffusion constant D, as functions of the local shear rate gamma*, show marked shear thinning with tau(alpha)(-1) proportional to D proportional to gamma*(0.8) over more than two decades in gamma*. In contrast, the global rheology of the system displays Herschel-Bulkley behavior. We discuss the possible role of large scale shear localization and other mechanisms in generating this difference.