Orientation Of Individual Particles Research Articles

Ultrasound Manipulation and Extrusion of Active Nanorods.

Synthetic self-propelled nano and microparticles have a growing appeal for targeted drug delivery, collective functionality, and manipulation at the nanoscale. However, it is challenging to control their positions and orientations under confinement, e.g., in microchannels, nozzles, and microcapillaries. This study reports on the synergistic effect of acoustic and flow-induced focusing in microfluidic nozzles. In a microchannel with a nozzle, the balance between the acoustophoretic forces and the fluid drag due to streaming flows generated by the acoustic field controls the microparticle's dynamics. This study manipulates the positions and orientations of dispersed particles and dense clusters inside the channel at a fixed frequency by tuning the acoustic intensity. The main findings are: first, this study successfully manipulates the positions and orientations of individual particles and dense clusters inside the channel at a fixed frequency by tuning the acoustic intensity. Second, when an external flow is applied, the acoustic field separates and selectively extrudes shape-anisotropic passive particles and self-propelled active nanorods. Finally, the observed phenomena are explained by multiphysics finite-element modeling. The results shed light on the control and extrusion of active particles in confined geometries and enable applications for acoustic cargo (e.g., drug) delivery, particle injection, and additive manufacturing via printed self-propelled active particles.

A deep learning approach to multi-track location and orientation in gaseous drift chambers

Accurate measuring the location and orientation of individual particles in a beam monitoring system is of particular interest to researchers in multiple disciplines. Among feasible methods, gaseous drift chambers with hybrid pixel sensors have the great potential to realize long-term stable measurement with considerable precision. In this paper, we introduce deep learning to analyze patterns in the beam projection image to facilitate three-dimensional reconstruction of particle tracks. We propose an end-to-end neural network based on segmentation and fitting for feature extraction and regression. Two segmentation branches, named binary segmentation and semantic segmentation, perform initial track determination and pixel-track association. Then pixels are assigned to multiple tracks, and a weighted least squares fitting is implemented with full back-propagation. Besides, we introduce a center-angle measure to judge the precision of location and orientation by combining two separate factors. The initial position resolution achieves 8.8 μm for the single track and 11.4 μm (15.2 μm) for the 1–3 tracks (1–5 tracks), and the angle resolution achieves 0.15°and 0.21°(0.29°) respectively. These results show a significant improvement in accuracy and multi-track compatibility compared to traditional methods.

The stress generated by non-Brownian fibers in turbulent channel flow simulations

Turbulent fiber suspension channel flow is studied using direct numerical simulation. The effect of the fibers on the fluid mechanics is governed by a stress tensor, involving the distribution of fiber position and orientation. Properties of this function in channel flow are studied by computing the trajectories and orientations of individual particles, referred to as the particle method. It is shown that, due to computer restrictions, the instantaneous stress in channel flow cannot be simulated directly with the particle method. To approximate the stress we compute the second-order moment of the fiber distribution function. This method involves an unknown subgrid term, which is modeled as diffusion. The accuracy of the moment approximation is studied by comparing Reynolds averaged stress to results obtained from the particle method. It is observed that the errors are ∼1% for y+>20, and ∼20% for y+<20. The model is improved by applying a wall damping function to the diffusivity. The moment approximation is used to simulate drag-reduced channel flow. A simplified model for fiber stress is introduced as fiber viscosity times rate of strain, where fiber viscosity is defined as the ratio of Reynolds averaged dissipation due to fiber stress and Reynolds averaged dissipation due to Newtonian stress. Fluid velocity statistics predicted by the simple model compare very well to those obtained from the moment approximation. This means that the effect of fibers on turbulent channel flow is equivalent to an additional Reynolds averaged viscosity.

Comparing simulation and experiment of a 2D granular Couette shear device.

We present experiments along with molecular-dynamics (MD) simulations of a two-dimensional (2D) granular material in a Couette cell undergoing slow shearing. The grains are disks confined between an inner, rotating wheel and a fixed outer ring. The simulation results are compared to experimental studies and quantitative agreement is found. Tracking the positions and orientations of individual particles allows us to obtain density distributions, velocity and particle rotation rates for the system. The key issue of this paper is to show the extent to which quantitative agreement between an experiment and MD simulations is possible. Besides many differences in model details and the experiment, the qualitative features are nicely reproduced. We discuss the quantitative agreement/disagreement, give possible reasons, and outline further research perspectives.

Kinematics of a two-dimensional granular Couette experiment at the transition to shearing

We describe experiments on a two-dimensional granular Couette system consisting of photoelastic disks undergoing slow shearing. The disks rest on a smooth surface and are confined between an inner wheel and an outer ring. Only shearing from the inner wheel is considered here. We obtain velocity, particle rotation rate (spin), and density distributions for the system by tracking positions and orientations of individual particles. At a characteristic packing fraction, ${\ensuremath{\gamma}}_{c}\ensuremath{\simeq}0.77,$ the wheel just engages the particles. In a narrow range of $\ensuremath{\gamma},$ $0.77l~\ensuremath{\gamma}l~0.80$ the system changes from just able to shear to densely packed. The transition at ${\ensuremath{\gamma}}_{c}$ has a number of hallmarks of a critical transition, including critical slowing down, and an order parameter. For instance, the mean stress grows from $0$ as $\ensuremath{\gamma}$ increases above ${\ensuremath{\gamma}}_{c},$ and hence plays the role of an order parameter. Also, the mean particle velocity vanishes at the transition point, implying slowing down at ${\ensuremath{\gamma}}_{c}.$ Above ${\ensuremath{\gamma}}_{c},$ the mean azimuthal velocity decreases roughly exponentially with distance from the inner shearing wheel, and the local packing fraction shows roughly comparable exponential decay from a highly dilated region next to the wheel to a denser but frozen packing further away. Approximate but not perfect shear rate invariance occurs; variations from perfect rate invariance appear to be related to small long-time rearrangements of the disks. The characteristic width of the induced ``shear band'' near the wheel varies most rapidly with distance from the wheel for $\ensuremath{\gamma}\ensuremath{\simeq}{\ensuremath{\gamma}}_{c},$ and is relatively insensitive to the packing fraction for the larger $\ensuremath{\gamma}$'s studied here. The mean particle spin oscillates near the wheel, and falls rapidly to zero away from the shearing surface. The distributions for the tangential velocity and particle spins are wide and show a complex shape, particularly for the disk layer nearest to the shearing surface. The two-variable distribution function for tangential velocity and spin reveals a separation of the kinematics into a slipping state and a nonslipping state consisting of a combination of rolling and translation.

High Spatial Resolution EBSD Study of Nanosized Epitaxial Particles

Abstract Traditionally, the structure and orientation relationship of individual catalyst particles on various oxide substrates have been studied by transmission electron microscopy. However, the combination of high resolution scanning electron microscopes (HRSEM) equipped with Schottky field emission sources with CCD cameras for recording electron backscatter diffraction (EBSD) paterns, it is now possible to obtain both morpholgy and orientation of individual particles with high spatial resolution. In this paper, we present results on the application of combined EBSD with HRSEM to determine the epitaxial orientation relationship of 80 nm Au particles on TiO2 (110). An evaluation of the spatial resolution limit of EBSD using Monte Carlo simulation of backscattered electron trajectories is also presented. The TiO2 (110) single crystal surfaces used in this study were prepared in UHV using surface science tools followed by in-situ metallization. After deposition of 15 nm Au at 300K followed by annealing at 800K, the samples were transferred in air to the Field Emission Scanning Electron microscope (LEO 982 Gemini) for high resolution imaging.

The potential and limitations of neutrons, electrons and X-rays for atomic resolution microscopy of unstained biological molecules.

Radiation damage is the main problem which prevents the determination of the structure of a single biological macromolecule at atomic resolution using any kind of microscopy. This is true whether neutrons, electrons or X-rays are used as the illumination. For neutrons, the cross-section for nuclear capture and the associated energy deposition and radiation damage could be reduced by using samples that are fully deuterated and 15N-labelled and by using fast neutrons, but single molecule biological microscopy is still not feasible. For naturally occurring biological material, electrons at present provide the most information for a given amount of radiation damage. Using phase contrast electron microscopy on biological molecules and macromolecular assemblies of approximately 10(5) molecular weight and above, there is in theory enough information present in the image to allow determination of the position and orientation of individual particles: the application of averaging methods can then be used to provide an atomic resolution structure. The images of approximately 10,000 particles are required. Below 10(5) molecular weight, some kind of crystal or other geometrically ordered aggregate is necessary to provide a sufficiently high combined molecular weight to allow for the alignment. In practice, the present quality of the best images still falls short of that attainable in theory and this means that a greater number of particles must be averaged and that the molecular weight limitation is somewhat larger than the predicted limit. For X-rays, the amount of damage per useful elastic scattering event is several hundred times greater than for electrons at all wavelengths and energies and therefore the requirements on specimen size and number of particles are correspondingly larger. Because of the lack of sufficiently bright neutron sources in the foreseeable future, electron microscopy in practice provides the greatest potential for immediate progress.

Structure of complement poly-C9 determined in projection by cryo-electron microscopy and single particle analysis

The ring-like complement ‘lesions’ found on membranes of complement lysed cells comprise a complex of components C5b through C9 that coalesce to form hollow cylinders which penetrate the membrane bilayer and create lytic pores. Walls of these C5b-9 membrane attack complex cylinders may consist primarily of the C9 component, since samples of purified, isolated C9 can polymerize into cylindrical structures which appear identical with the fully assembled C5b-9 complex. The structure of these poly-C9 molecules has been investigated using the techniques of cryo-electron microscopy and single particle analysis. Sets of single poly-C9 particles viewed as rings were selected from cryo-EM images, then particles were aligned and treated by correspondence analysis to identify the principle interparticle similarities and variations. The highest ranking variation found was the presence or absence of a dense inner ring of protein density. Other important variations were interpreted as different types of particle tilt. These results were used in selecting a subgroup of untilted particles for averaging and symmetry analysis. The rotational power spectrum of the initial average suggested 13-fold symmetry. The 13-fold symmetry was used to select and group particles for further analysis. Individual particles were 13-fold rotational averaged and those with enhanced peripheral features were placed into either a right-handed subgroup or into a left-handed subgroup based on orientation of the peripheral features. Particles within each group were aligned and averaged, and a poly-C9 structure was produced which shows important structural details and from which the C9 monomer structure can be deduced. The poly-C9 structure contains a dense inner ring of diameter between 113–181 Å and which is modulated into 13 discrete peaks with peak-to-peak separation of approx. 35 Å. The dense inner ring is surrounded by a less dense, concentric outer rim extending to 254 Å diameter. The outer rim contains projections that are contiguous with the inner peaks but are skewed relative to the ring radius to produce the appearance of a pin-wheel. These projections correspond with the peripheral features picked up in the rotationally averaged individual particles; the left- or right-handed orientation of projections may result from the up/down orientation of individual particles in ice. The C9 monomer structure within the cylinder is suggested by the density distribution. The monomer would be a rod with diameter of 35 Å, oriented parallel to the cylinder axis and would be roughly perpendicular to a membrane. A portion of the C9 monomer would form the projecting outer rim. In side views of the complex the outer rim density would be the torus structure seen distal to the membrane-inserting region. These results indicating an extended C9 structure are consistent with C9 unfolding in the transition from monomer to poly-C9 complex.

Nucleosome reconstruction by electron spectroscopic imaging of phosphorus

The three dimensional structure of a biological macromolecule or complex can be reconstructed from electron micrographs of different macromolecules or complexes lying in random orientation, provided that the relative orientations of individual particles can be determined (figure 1). In the case of the nucleosome core particle, a complex comprising 146 base pairs of DNA and 8 histone proteins, such orientational information can be obtained using a new technique called electron spectroscopic imaging which permits not only electron microscopy of the structure, but also the visualisation at high spatial resolution of the phosphorus distribution within the particle. Since phosphorus is a predominant constituent of DNA and not of protein, this elemental map represents an image of the nucleosomal DNA component. A comparison of a projection of a DNA supercoil model with images of the phosphorus distribution allowed us to orient individual distributions (figure 2). Using a direct convolution back-projection algorithm, the entire nucleosome containing both protein and DNA information was then reconstructed to a resolution of about 1.5 nm.