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Carbon nanotubes (CNTs) possess many unique properties that make them ideal for field emission. However, screening due to high density and poor substrate adhesion limits their application. We tested the field emission of various patterned vertically aligned carbon nanotube (VACNT) arrays adhered to copper substrates using carbon paste. After many fabrication steps to improve uniformity, we found that the field emission was dominated by individual CNTs that were taller than the bulk VACNT arrays. After testing a sample with silver epoxy as the binder, we found that the failure mechanism was adhesion to the substrate. Using energy dispersive xray spectroscopy (EDX), we found that the carbon paste migrated into the VACNT bulk volume while the silver epoxy did not. The migration of carbon paste into the volume may explain why the carbon paste had greater adhesion than the silver epoxy.

Acoustic waves are a possible reusable method to extinguish flames. Previous studies have placed the sound source near the flame or have used standing waves to reach large enough acoustic amplitudes to extinguish it. In this study, a new method is explored: using time reversal in a room to focus transient acoustic waves to the flame to extinguish it. The peak acoustic overpressure level needed to extinguish a candle flame in the middle of the room is 191 dB

 re 20 

μ

Pa when using a frequency range of 300 to 15000 Hz. The sound level at other locations of the room during the focusing was 130 dB. The required peak level is lower when using a less stable flame, or when the flame is near a room boundary. The momentary focus of high-amplitude sound waves subsequently causes acoustic streaming or a flow of air at the flame location that extinguishes the flame. By analyzing high-speed video, it is shown that acoustic streaming extinguishes the flame when using this method, not the acoustic particle displacement. It is also shown that the streaming does not occur when the flame is not present.

The characteristics of the spiral structure of galaxies in compact groups (from the HCG and SDSSCGA catalogs) and in isolation (from the CIG catalog) have been obtained and analyzed. The dependence of the type of spiral pattern on the spatial environment of galaxies has been studied. The conclusions about how a tidal interaction affects the spiral structure have been drawn. The fraction of grand design spirals in compact groups is shown to be considerably higher than that in a sample of isolated objects. Grand design galaxies in the field, on average, have bluer and narrower spiral arms. This may probably due to the absence of a tidal interaction that has a strong effect on the structure of galaxies in a crowded environment.

This paper examines the connection of convective Mach number definitions to maximum noise radiation angle for a T-7A-installed GE F404 jet engine. Definitions include those corresponding to Kelvin–Helmholtz (K-H) and supersonic instability (SI) Mach waves, and an empirical formulation. Under convectively supersonic conditions without an afterburner (AB), only K-H waves are present. At AB, SI Mach waves may exist, but at shallow angles outside the main radiation lobe. Evidence suggests that Mach wave radiation from faster-than-ordinary K-H waves could stem from shock-cell velocity fluctuations. The empirical convective Mach number indicates decreasing effective convective velocity from ∼80 to ∼60\% of fully expanded velocity as engine power increases to AB. This convective velocity decreases with frequency, especially for those whose maximum source locations occur between the potential and supersonic core tips. Additionally, a new definition of supersonic-jet convective Mach number, dependent solely on the jet acoustic Mach number, ∼Mac, has been derived from wide-ranging jet data. This definition describes the F404 maximum noise radiation angle from intermediate thrust through AB within 2°. Relating this expression to K-H Mach waves for an isothermal jet indicates the relative unimportance of temperature in determining maximum radiation angle for heated supersonic jets, including military jet aircraft and rockets.

Undergraduate students on track for medical school are often required to take general physics lab courses. Many of these students carry an attitude of obligation into these courses which can make it challenging for instructors to engage students in course material. We address the question: How does engaging with medically based models in introductory physics labs affect pre-med undergraduate perceptions of the modeling process and their perceptions of science? We redesigned an electricity and magnetism lab in an introductory physics lab course, where approximately 70% of the undergraduates reported plans to attend medical school. We situated the lab in the mechanics of MRI magnetic resonance and collected data on the participants’ experiences through surveys and lab submissions. As a part of the analysis, we modified a rubric to evaluate engagement in modeling and applied grounded coding theory to the survey responses to develop themes of the participants’ understanding of scientific modeling. The participants’ understanding and engagement in scientific modeling increased during the newly developed lab and remained high for subsequent labs. We recommend that instructors of undergraduate nonmajor labs consider the demographic of their student population and design lab experiences situated within their interests and focus on central science practices like modeling.

Single-shot two-dimensional (2D) phase retrieval (PR) can recover the phase shift distribution within an object from a single 2D x-ray phase contrast image (XPCI). Two competing XPCI imaging modalities often used for single-shot 2D PR to recover material properties critical for predictive performance capabilities are: speckle-based (SP-XPCI) and propagation-based (PB-XPCI) XPCI imaging. However, PR from SP-XPCI and PB-XPCI images are, respectively, limited to reconstructing accurately slowly and rapidly varying features due to noise and differences in their contrast mechanisms. Herein, we consider a combined speckle- and propagation-based XPCI (SPB-XPCI) image by introducing a mask to generate a reference pattern and imaging in the near-to-holographic regime to induce intensity modulations in the image. We develop a single-shot 2D PR method for SPB-XPCI images of pure phase objects without imposing restrictions such as object support constraints. It is compared against PR methods inspired by those developed for SP-XPCI and PB-XPCI on simulated and experimental images of a thin glass shell before and during shockwave compression. Reconstructed phase maps show improvements in quantitative scores of root-mean-square error and structural similarity index measure using our proposed method.