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Soot is known for its enormous and pervasive negative impacts on human health and the environmental, but there much about soot that is not well known, including the precursors and chemical mechanisms involved in its formation. Many studies have characterized species associated with incipient particles, i.e., the first particles produced during soot formation. These studies provide insight into inception mechanisms, the pathways leading from gas-phase precursors to condensed-phase particles. Potential inception mechanisms involve one (or a combination) of two classes of pathways: physical nucleation, in which precursors undergo a thermodynamic phase change and are bound together by electrostatic forces, and chemical clustering, in which precursors react to form covalently bound clusters. In a recent paper, Shao et al.1 concluded that soot inception occurs through physical nucleation and claimed to have provided direct evidence of such a mechanism. We demonstrate that this conclusion is inconsistent with (1) the consensus of published work, (2) the data, theory, and analysis on which this conclusion is nominally based, and (3) the second law of thermodynamics. We show that, contrary to their conclusions, their experimental and theoretical results provide evidence for a chemical-clustering soot-inception mechanism. 

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. When illuminated with light intensity comparable to natural sunlight, the polymer disk heats up and interacts with incident gas molecules differently on the top and bottom sides, producing a net recoil force. We observed the levitation of 6-mm-diameter disks in a vacuum chamber at pressures between 10 and 30 Pa. Moreover, we controlled the flight of the disks using a shaped light field that optically trapped the levitating disks. Our experimentally validated theoretical model predicts that the lift forces can be many times the weight of the films, allowing payloads of up to 10 milligrams for sunlight-powered low-cost microflyers at altitudes of 50 to 100 km. 

Abstract 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. To address these issues, new cantilever probes that have hollow cross‐sections and walls of nanoscale thicknesses made of alumina deposited by atomic layer deposition are introduced. It is demonstrated that the probes exhibit spring constants up to ≈100 times lower and bandwidths up to ≈50 times higher in air than their typical solid counterparts, allowing them to react to topography changes more quickly. Moreover, it is shown that the enhanced robustness of the hollow cantilevers enables them to withstand large bending displacements more readily and to be more resistant to tip wear.