On the drag of model dendrite fragments at low Reynolds number

An experimental study of low Reynolds number drag on laboratory models of dendrite fragments has been conducted. The terminal velocities of the dendrites undergoing free fall along their axis of symmetry were measured in a large Stokes flow facility. Corrections for wall interference give nearly linear drag vs Reynolds number curves. Corrections for both wall interference and inertia effects show that the dendrite Stokes settling velocities are always less than that of a sphere of equal mass and volume. In the Stokes limit, the settling speed ratio is found to correlate well with primary dendrite arm aspect ratio and a second dimensionless shape paremeter which serves as a measure of the fractal-like nature of the dendrite models. These results can be used to estimate equiaxed grain velocities and distance of travel in metal castings. The drag measurements may be used in numerical codes to calculate the movement of grains in a convecting melt in an effort to determine macrosegregation patterns caused by the sink/float mechanism

Efficient Nanosecond Photoluminescence from Infrared PbS Quantum Dots Coupled to Plasmonic Nanoantennas

Infrared (IR) light sources with high modulation rates are critical components for on-chip optical communications. Lead-based colloidal quantum dots are promising nonepitaxial materials for use in IR light-emitting diodes, but their slow photoluminescence lifetime is a serious limitation. Here we demonstrate coupling of PbS quantum dots to colloidal plasmonic nanoantennas based on film-coupled metal nanocubes, resulting in a dramatic 1300-fold reduction in the emission lifetime from the microsecond to the nanosecond regime. This lifetime reduction is primarily due to a 1100-fold increase in the radiative decay rate owing to the high quantum yield (65%) of the antenna. The short emission lifetime is accompanied by high antenna quantum efficiency and directionality. This nonepitaxial platform points toward GHz frequency, electrically modulated, telecommunication wavelength light-emitting diodes and single-photon sources.United States. Dept. of Energy. Center for Excitonics (Award DE-SC0001088

The ionization produced in nuclear emulsion by very relativistic particles

In order to determine the shape of the ionization curve in nuclear emulsions at values of ϒ˃100, we have measured the blob density of relativistic electron tracks. The pv of the pair produced electrons used was determined by multiple scattering. A total of 84.45 centimeters of electron track was blob taunted and scattered. From ϒ = 100 to ϒ = 5400 the data were combined into 14 points each with a statistical uncertainty on blob density of less than 1%. These points indicate a level "plateau" and show no deviation from this plateau within our statistical accuracy. Pions were used to estimate the minimum of the ionization curve 5 and ratio of blob density plateau to blob density minimum is estimated to be 1.140 ± .020.http://www.archive.org/details/ionizationproduc00hansLieutenant, United States Coast GuardLieutenant, United States NavyLieutenant, United States Nav

Ballistic nanofriction

Sliding parts in nanosystems such as Nano ElectroMechanical Systems (NEMS) and nanomotors, increasingly involve large speeds, and rotations as well as translations of the moving surfaces; yet, the physics of high speed nanoscale friction is so far unexplored. Here, by simulating the motion of drifting and of kicked Au clusters on graphite - a workhorse system of experimental relevance -- we demonstrate and characterize a novel "ballistic" friction regime at high speed, separate from drift at low speed. The temperature dependence of the cluster slip distance and time, measuring friction, is opposite in these two regimes, consistent with theory. Crucial to both regimes is the interplay of rotations and translations, shown to be correlated in slow drift but anticorrelated in fast sliding. Despite these differences, we find the velocity dependence of ballistic friction to be, like drift, viscous

Calculation of Dendrite Settling Velocities Using a Porous Envelope

The convective transport and gravitational settling of unattached equiaxed grains and dendrite fragments can cause macrosegregation and influence the structure of the equiaxed zone in a variety of solidification arrangements. An understanding of how the highly nonspherical geometry of the dendrite influences its settling and transport characteristics is needed to determine the motion of unattached dendrites and predict structure and segregation in castings. The empirical results of previous works have been used to develop a FORTRAN 77 computer program to calculate the settling velocity of various dendritic shapes and a number of other parameters of interest, such as the volume and surface area of the dendrite. Required inputs to the code are the physical properties of the system and some simple geometric parameters of the dendrite being considered, such as the average radius of the primary arm. The predicted settling velocities were on average within -+5 pct of those measured for model dendrites and were consistent and in good agreement with three other experimental investigations. Future development of the code will attempt to overcome many of its present limitations by including particle-particle interactions and the effects of tertiary arms, for example

Incidence and correlates of STIs among black men who have sex with men participating in a US PrEP study

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Tabletop nonlinear optics in the 100-eV spectral region

Nonlinear light-matter interactions in the extreme ultraviolet (XUV) are a prerequisite to perform XUV-pump/XUV-probe spectroscopy of core electrons. Such interactions are now routinely investigated at free-electron laser (FEL) facilities. Yet, electron dynamics are often too fast to be captured with the femtosecond resolution of state-of-the-art FELs. Attosecond pulses from laser-driven XUV-sources offer the necessary temporal resolution. However, intense attosecond pulses supporting nonlinear processes have only been available for photon energy below 50 eV, precluding XUV-pump/XUV-probe investigation of typical inner-shell processes. Here, we surpass this limitation by demonstrating two-photon absorption from inner electronic shells of xenon at photon energies around 93 eV and 115 eV. This advance opens the door for attosecond real-time observation of nonlinear electron dynamics deep inside atoms

Comparative analysis of sequence characteristics of imprinted genes in human, mouse, and cattle

Genomic imprinting is an epigenetic mechanism that results in monoallelic expression of genes depending on parent-of-origin of the allele. Although the conservation of genomic imprinting among mammalian species has been widely reported for many genes, there is accumulating evidence that some genes escape this conservation. Most known imprinted genes have been identified in the mouse and human, with few imprinted genes reported in cattle. Comparative analysis of genomic imprinting across mammalian species would provide a powerful tool for elucidating the mechanisms regulating the unique expression of imprinted genes. In this study we analyzed the imprinting of 22 genes in human, mouse, and cattle and found that in only 11 was imprinting conserved across the three species. In addition, we analyzed the occurrence of the sequence elements CpG islands, C + G content, tandem repeats, and retrotransposable elements in imprinted and in nonimprinted (control) cattle genes. We found that imprinted genes have a higher G + C content and more CpG islands and tandem repeats. Short interspersed nuclear elements (SINEs) were notably fewer in number in imprinted cattle genes compared to control genes, which is in agreement with previous reports for human and mouse imprinted regions. Long interspersed nuclear elements (LINEs) and long terminal repeats (LTRs) were found to be significantly underrepresented in imprinted genes compared to control genes, contrary to reports on human and mouse. Of considerable significance was the finding of highly conserved tandem repeats in nine of the genes imprinted in all three species