Unresolved Particles Research Articles

Detectability of unresolved particles in off-axis digital holographic microscopy.

Off-axis digital holographic microscopy (DHM) provides both amplitude and phase images, and so it may be used for label-free 3D tracking of micro- and nano-sized particles of different compositions, including biological cells, strongly absorbing particles, and strongly scattering particles. Contrast is provided by differences in either the real or imaginary parts of the refractive index (phase contrast and absorption) and/or by scattering. While numerous studies have focused on phase contrast and improving resolution in DHM, particularly axial resolution, absent have been studies quantifying the limits of detection for unresolved particles. This limit has important implications for microbial detection, including in life-detection missions for space flight. Here we examine the limits of detection of nanosized particles as a function of particle optical properties, microscope optics (including camera well depth and substrate), and data processing techniques and find that DHM provides contrast in both amplitude and phase for unresolved spheres, in rough agreement with Mie theory scattering cross-sections. Amplitude reconstructions are more useful than phase for low-index spheres and should not be neglected in DHM analysis.

An immersed moving boundary for fast discrete particle simulation with complex geometry

An immersed moving boundary (IMB) scheme in LBM-DEM is proposed for discrete particle simulation with arbitrarily complex geometry. Momentum exchange between fluid and unresolved particles can be exerted by a modified weighting function with drag force and a new solid operator in IMB, which can describe both particle–fluid interaction and complex geometry, and requires only local information in perspective of solution. Combined with stereolithography (STL) format for describing complex geometry, a GPU-based LBM-DEM-IMB is implemented, and its ability and accuracy for complex geometry from STL model is verified by simulating flow past a stationary sphere. Compared with CFD-DEM in discrete particle simulations of both fluidized bed with immersed tube and aerosol transporting in human nasal, proposed method has much higher efficiency and maintains comparable accuracy in dense and dilute particle–fluid systems with complex geometry, which demonstrates it to be a promising computational strategy for particle–fluid two-phase flows.

A seven-equation diffused interface method for resolved multiphase flows

The seven-equation model is a compressible multiphase formulation that allows for phasic velocity and pressure disequilibrium. These equations are solved using a diffused interface method that models resolved multiphase flows. Novel extensions are proposed for including the effects of surface tension, viscosity, multi-species, and reactions. The allowed non-equilibrium of pressure in the seven-equation model provides numerical stability in strong shocks and allows for arbitrary and independent equations of states. Whereas, the allowed non-equilibrium of velocity allows the method to be useful even for unresolved particles where a finite-rate velocity relaxation is observed. A discrete equations method (DEM) models the fluxes. We show that even though stiff pressure- and velocity-relaxation solvers have been used, they are not necessarily needed for all the tests with DEM because the non-conservative fluxes are accurately modeled. An interface compression scheme controls the numerical diffusion of the interface, and its effects on the solution are discussed. Test cases are used to validate the computational method and demonstrate its applicability. They include multiphase shock tubes, shock propagation through a material interface and a dispersed particle curtain, a surface-tension-driven oscillating droplet, an accelerating droplet in a viscous medium, and shock–detonation interacting with a deforming droplet. Simulation results are compared against exact solutions and experiments when possible.

Estimation of the number of counts on a particle counter detector with full time resolution

We present a general method for estimating the number of particles impinging on a segmented counter or, in general, on a counter with sub-units. We account for unresolved particles, i.e., the effect of two or more particles hitting the same sub-unit almost simultaneously. To achieve full time resolution we account for the dead time that occurs after the first time-bin of a particle signal. This general counting method can be applied to counting muons in existing detectors like the Underground Muon Detector of the Pierre Auger Observatory. We therefore use the latter as a study case to test the performance of our method and to compare it to other methods from literature. Our method proves to perform with little bias, and also provides an estimate of the number of particles as a function of time (as seen by the detector) to a single time-bin resolution. In this context, the new method can be useful for reconstructing parameters sensitive to cosmic ray mass, which are key to unveiling the origin of cosmic rays.

Nested soft-collinear subtractions in NNLO QCD computations

We discuss a modification of the next-to-next-to-leading order (NNLO) subtraction scheme based on the residue-improved sector decomposition that reduces the number of double-real emission sectors from five to four. In particular, a sector where energies and angles of unresolved particles vanish in a correlated fashion is redundant and can be discarded. This simple observation allows us to formulate a transparent iterative subtraction procedure for double-real emission contributions, to demonstrate the cancellation of soft and collinear singularities in an explicit and (almost) process-independent way and to write the result of a NNLO calculation in terms of quantities that can be computed in four space-time dimensions. We illustrate this procedure explicitly in the simple case of mathcal O(alpha _mathrm{s}^2) gluonic corrections to the Drell–Yan process of q bar{q} annihilation into a lepton pair. We show that this framework leads to fast and numerically stable computation of QCD corrections.

Ion exchange technique (IET) to characterise Ag+ exposure in soil extracts contaminated with engineered silver nanoparticles

Environmental contextBiosolid-amended soils are likely sinks for manufactured silver nanoparticles, the environmental toxicity of which is believed to be related to the release and accumulation of Ag+ ions. This study demonstrates how an ion exchange technique can be applied to soil extracts to provide Ag+ measurements at low, environmentally relevant levels. The technique is a valuable addition to existing analytical methods for tracking the behaviour of Ag nanoparticles and Ag+ ions in the terrestrial environment. AbstractThe lack of silver speciation exposure data in toxicity studies investigating the effects of manufactured silver nanoparticles (AgNPs) in natural soil media limits the ability to discern nano-specific effects from effects of the toxic Ag+ form, which may be released from the manufactured AgNPs contained in wastewater, biosolids or soil environment. Using samples containing Ag+ or mixtures of Ag+ and AgNPs, ranging in total Ag concentrations of 10–5 to 10–9 M, and prepared in de-ionised water and filtered soil extracts, the validity of the ion exchange technique (IET) to quantify Ag+ was investigated by comparing measurements to those of an Ag+ ion selective electrode (ISE) and to the dissolved fraction from single particle inductively coupled plasma–mass spectrometry (SP-ICP-MS) analysis (SP-dissolved). When analysing samples in the filtered soil extract, IET and ISE gave comparable results down to 10–7 M, below which Ag+ activities were below the ISE detection limit. For water samples, SP-dissolved values were generally comparable or slightly greater (on average 65%) compared with IET-Ag+ at all concentrations. The high bias was likely due to inclusion of unresolved particles below the SP-ICP detection limit of 19nm. However, when analysing samples in the soil extract, SP-dissolved values were on average eight-fold greater than IET-Ag+, highlighting the effect that natural colloidal and dissolved soil constituents have on complexing Ag+, as well as the lack of specificity of the SP-dissolved analysis for the Ag+ species. IET is shown here to be a valid procedure to quantify Ag+ activity in soil extracts, and while the study highlights the limitations of using the SP-dissolved fraction to estimate this biologically relevant Ag fraction, it shows that combined, IET and SP-ICP-MS provide a valuable approach for investigating the behaviour of manufactured AgNPs in different matrixes.

Comparative analysis of subgrid drag modifications for dense gas‐particle flows in bubbling fluidized beds

Many subgrid drag modifications have been put forth to account for the effect of small unresolved scales on the resolved mesoscales in dense gas‐particle flows. These subgrid drag modifications significantly differ in terms of their dependencies on the void fraction and the particle slip velocity. We, therefore, compare the hydrodynamics of a three‐dimensional bubbling fluidized bed computed on a coarse grid using the drag correlations of the groups of (i) EMMS, (ii) Kuipers, (iii) Sundaresan, (iv) Simonin, and the homogenous drag law of (v) Wen and Yu with fine grid simulations for two different superficial gas velocities. Furthermore, we present an (vi) alternative approach, which distinguishes between resolved and unresolved particle clusters revealing a grid and slip velocity dependent heterogeneity index. Numerical results are analyzed with respect to the time‐averaged solids volume fraction and its standard deviation, gas and solid flow patterns, bubble size, number density, and rise velocities. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4077–4099, 2013

Nanometric depth resolution from multi-focal images in microscopy

We describe a method for tracking the position of small features in three dimensions from images recorded on a standard microscope with an inexpensive attachment between the microscope and the camera. The depth-measurement accuracy of this method is tested experimentally on a wide-field, inverted microscope and is shown to give approximately 8 nm depth resolution, over a specimen depth of approximately 6 µm, when using a 12-bit charge-coupled device (CCD) camera and very bright but unresolved particles. To assess low-flux limitations a theoretical model is used to derive an analytical expression for the minimum variance bound. The approximations used in the analytical treatment are tested using numerical simulations. It is concluded that approximately 14 nm depth resolution is achievable with flux levels available when tracking fluorescent sources in three dimensions in live-cell biology and that the method is suitable for three-dimensional photo-activated localization microscopy resolution. Sub-nanometre resolution could be achieved with photon-counting techniques at high flux levels.

Integrated dipoles with MadDipole in the MadGraph framework

Heading towards a full automation of next-to-leading order (NLO) QCD corrections, one important ingredient is the analytical integration over the one-particle phase space of the unresolved particle that is necessary when adding the subtraction terms to the virtual corrections. We present the implementation of these integrated dipoles in the MadGraph framework. The result is a package that allows an automated calculation for the NLO real emission parts of an arbitrary process.

Microscopic Morphology and Size Distribution of Residential Indoor PM10 in Beijing City

To investigate characteristics of airborne PM10 in residences in Beijing City the indoor PM10 concentrations, morphology and size distribution were measured using a scanning electron microscope coupled with an energy dispersive X-ray micro-analyser and image analysis. PM10 samples were collected in four homes and from the nearby outdoor environment during the winter season. The results showed that the mean 12-h indoor PM10 concentration was 109.9 g/m3 and the corresponding outdoor level was 179.8 g/m3. Indoor PM10 single particles were subdivided into 5 microscopic types according to their microscopic characteristics and morphologies and included soot aggregates, coal fly ash, mineral, biological and unresolved particles. Each particle type had a different contribution to the PM10 composition which were dominated by soot aggregates in both indoor and outdoor air. Numbersize distribution of PM10 in the residences displayed a bimodal pattern with peaks in 0.1 0.7 m and 1 2.5 m ranges. Coal fly ash exhibited a unimodal pattern in indoor and outdoor air with a peak in 0.1 0.5 m range. Mineral particles showed a bimodal number-size distribution in homes but in the outdoor air they exhibited a unimodal pattern perhaps because of the air temperature.

Double unresolved approximations to multiparton scattering amplitudes

We present approximations to tree-level multiparton scattering amplitudes which are appropriate when two partons are unresolved. These approximations are required for the analytic isolation of infrared singularities of n + 2 parton scattering processes contributing to the next-to-next-to-leading order corrections to n jet cross sections. In each case the colour ordered matrix elements factorise and yield a function containing the singular factors multiplying the n-parton amplitudes. When the unresolved particles are colour unconnected, the approximations are simple products of the familiar eikonal and Altarelli-Parisi splitting functions used to describe single unresolved emission. However, when the unresolved particles are colour connected the factorisation is more complicated and we introduce new and general functions to describe the triple collinear and soft/collinear limits in addition to the known double soft gluon limits of Berends and Giele. As expected the triple collinear splitting functions obey an N = 1 SUSY identify. To illustrate the use of these double unresolved approximations, we have examined the singular limits of the tree-level matrix elements for e + e − → 5 partons when only three partons are resolved. When integrated over the unresolved regions of phase space, these expressions will be of use in evaluating the O(α s 3) corrections to the three-jet rate in electron-positron annihilation.

Optical contamination measurements on early Shuttle missions

Two camera/photometers were included as part of the Induced Environment Contamination Monitor on STS-2, -3, -4, and -9 orbital missions to record photographically Shuttle-induced particulate and background contamination. The cameras collected stereoscopic data continuously during the on-orbit phases of these missions making exposures every 150 s. The results recorded throughout a 32 degrees field-of-view indicate high particle concentrations during early mission operations. These decay to a quiescent rate of 500 particles of >10-microm radius observed per orbit. Preliminary size and velocity distributions of measured particles are presented as are measurements of background brightness due to unresolved particles in the visible spectral region.

Molecular counting of low-density lipoprotein particles as individuals and small clusters on cell surfaces

We employ the intensely fluorescent analogue diI-LDL (Barak, L. S., and W. W. Webb, 1981, J. Cell Biol. 90:595-604) as a counting marker to determine the numbers of LDL-receptor complexes that are contained in clusters on the surfaces of human fibroblasts and human epidermoid carcinoma cells. The application of quantitative digital intensified video optical microscopy allows the measurement of the fluorescence power collected from individual fluorescent spots on a cell with sufficient accuracy that the number of optically unresolved particles producing the fluorescence in the spot can be estimated. We demonstrate that isolated individual diI-LDL particles are detected on the surface of all cells investigated. Analysis of the LDL cluster size distributions on the various cell lines shows clear differences that correlate with efficiency of LDL metabolism. We find that normal fibroblasts (GM3348) have LDL-receptor complex populations dominated by large cluster sizes (greater than 4 LDL), while internalization-deficient J.D. mutant fibroblasts (GM2408A) and epidermoid carcinoma cells (A-431) show predominantly small clusters (1-3 LDL). No evidence for large-scale ordering or "superclustering" of clusters is found.

Stripping Analysis of theSc45(d,p)Sc46Reaction

The ${\mathrm{Sc}}^{45}(d,p){\mathrm{Sc}}^{46}$ reaction has been studied with a multiple-gap magnetic spectrograph and the MIT-ONR electrostatic accelerator. Eighty-four energy levels were observed, and angular distributions of the corresponding proton groups were analyzed up to an excitation energy of 4.0 MeV. Eighty-five additional energy levels in ${\mathrm{Sc}}^{46}$ were observed between 4.0 and 6.0 MeV where the level density is seen to increase rapidly with excitation energy. Some unresolved particle groups are estimated to have separations of a few keV. The ground-state $Q$ value was measured to be 6.541\ifmmode\pm\else\textpm\fi{}0.008 MeV. The angular distributions were measured in 7.5-deg intervals between 7.5 and 172.5 deg. The assignments of the captured-neutron angular-momentum ${l}_{n}$ were based on the position of the first maximum as predicted by a distorted-wave Born-approximation theoretical calculation. The transition strengths for the groups, for which ${l}_{n}$ assignments were made, were determined from the measured differential cross sections and the predictions of the distorted-wave, deuteron-stripping analysis.

The Measurement of Unresolved, Single Particles of Uniform Thickness by Means of Variable Phase Microscopy*

The adjustments of a variable phase microscope control two variable experimental parameters, specifically the phase difference δ between and the amplitude transmission ratio h of the conjugate and complementary areas of the diffraction plate or its polanret equivalent. It is shown that the center of the sharply focused diffraction image of a single unresolved particle can be brought to zero intensity by a definite choice of the parameters h and δ. The relations which exist among the required choice of h, δ, the area and the optical properties of the particle are derived and specialized to various classes of particles. These relations permit the computation of the area and one or more optical properties of the particle from the observed values of the parameters h, δ. Observations made upon opaque particles with a modified polanret microscope are described and found to be in accord with the theory. The radii of these particles ranged from 0.5 to 0.15 times the Airy limit of the test objective.

The Resolution of Two Particles in a Bright Field by Coated Microscope Objectives

A theory is presented for determining the diffraction image of two, identical, opaque or transmitting, uniform, individually unresolved particles of finite area located in a uniform, transmitting surround. Numerical calculations are included for the classical Airy type objective, for center-blocked Airy type objectives and for simplified Sonine type objectives which have been coated so as to produce diffraction patterns having central disks of reduced diameter. It is shown that marked improvement in the resolution of two particles can be expected by such coated objectives. The resolving power for two particles increases rapidly with the area of the particles and has unexpectedly steep dependence upon the relative amplitude and phase transmission of particle and surround. The theory, subject to experimental verification, forms a basis for explaining why a microscope as used with transmitted light can resolve considerably better than is indicated by the Airy limit.