Distinguishing Gas-Phase and Nanoparticle Contributions to Small-Angle X-ray Scattering in Reacting Aerosol Flows.

We have developed a strategy for distinguishing between small-angle X-ray scattering (SAXS) from gas-phase species and newly formed nanoparticles in mixed gas- and particle-phase reacting flows. This methodology explicitly accounts for temperature-dependent scattering from gases. We measured SAXS in a sooting linear laminar partially premixed co-flow ethylene/air diffusion flame.

Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation.

The Breakthrough Starshot Initiative aims to send a gram-scale probe to our nearest extrasolar neighbors using a laser-accelerated lightsail traveling at relativistic speeds. Thermal management is a key lightsail design objective because of the intense laser powers required but has generally been considered secondary to accelerative performance. Here, we demonstrate nanophotonic photonic crystal slab reflectors composed of 2H-phase molybdenum disulfide and crystalline silicon nitride, highlight the inverse relationship between the thermal band extinction coefficient and the lightsail's maximum temperature, and examine the trade-off between minimizing acceleration distance and setting realistic sail thermal limits, ultimately realizing a thermally endurable acceleration minimum distance of 23.

Relativistic Light Sails Need to Billow.

We argue that light sails with nanometer-scale thicknesses that are rapidly accelerated to relativistic velocities by lasers must be significantly curved in order to reduce their intrafilm mechanical stresses and avoid tears. Using an integrated opto-thermo-mechanical model, we show that the diameter and radius of curvature of a circular light sail should be comparable in magnitude, both on the order of a few meters, in optimal designs for gram-scale payloads. Moreover, we demonstrate that, when sufficient laser power is available, a sail's acceleration length decreases as its curvature increases.

Hollow Atomic Force Microscopy Cantilevers with Nanoscale Wall Thicknesses.

In atomic force microscopy, the cantilever probe is a critical component whose properties determine the resolution and speed at which images with nanoscale resolution can be obtained. Traditional cantilevers, which have moderate resonant frequencies and high quality factors, have relatively long response times and low bandwidths. In addition, cantilevers can be easily damaged by excessive deformation, and tips can be damaged by wear, requiring them to be replaced frequently.

Progress Toward High Power Output in Thermionic Energy Converters.

Thermionic energy converters are solid-state heat engines that have the potential to produce electricity with efficiencies of over 30% and area-specific power densities of 100 Wcm. Despite this prospect, no prototypes reported in the literature have achieved true efficiencies close to this target, and many of the most recent investigations report power densities on the order of mWcm or less. These discrepancies stem in part from the low-temperature (<1300 K) test conditions used to evaluate these devices, the large vacuum gap distances (25-100 µm) employed by these devices, and material challenges related to these devices' electrodes.

Controlled levitation of nanostructured thin films for sun-powered near-space flight.

We report light-driven levitation of macroscopic polymer films with nanostructured surface as candidates for long-duration near-space flight. We levitated centimeter-scale disks made of commercial 0.5-micron-thick mylar film coated with carbon nanotubes on one side.

Photoionization Efficiencies of Five Polycyclic Aromatic Hydrocarbons.

We have measured photoionization-efficiency curves for pyrene, fluoranthene, chrysene, perylene, and coronene in the photon energy range of 7.5-10.2 eV and derived their photoionization cross-section curves in this energy range.

Critical Assessment of Photoionization Efficiency Measurements for Characterization of Soot-Precursor Species.

We present a critical evaluation of photoionization efficiency (PIE) measurements coupled with aerosol mass spectrometry for the identification of condensed soot-precursor species extracted from a premixed atmospheric-pressure ethylene/oxygen/nitrogen flame. Definitive identification of isomers by any means is complicated by the large number of potential isomers at masses likely to comprise particles at flame temperatures. This problem is compounded using PIE measurements by the similarity in ionization energies and PIE-curve shapes among many of these isomers.