A comparison of pendulum experiments and discrete-element simulations of oblique collisions of wet spheres

Oblique collisions of two spherical particles coated with a thin layer of viscous liquid are considered. Experimental measurements are performed using particle tracking velocimetry and the results are compared to viscous force and capillary force models via numerical simulation using the discrete-element method. Comprehensive experimental data for collisions with an impact angle between 0° and 60° are presented to ensure future models can be rigorously validated. Collisions are characterized by the normal Stokes' number (a dimensionless ratio of normal inertial forces to normal viscous forces), and the normal coefficient of restitution (a dimensionless ratio of postcollisional normal velocity to precollisional normal velocity). Good agreement was found between the models and the experiments at high Stokes' number, where the models are dominated by the normal components. As the tangential forces become more significant (i.e., at low to medium Stokes' number, and high collision angle), agreement between the simulations and experiments is poorer. Furthermore, the models predict a decreasing rotational velocity with increasing Stokes' number past some critical Stokes' number. A coefficient of friction term was implemented and found to improve agreement for the rotational velocity postcollision.     

Characterization of unsteady flow in a 3D-printed Schwarz Diamond monolith using magnetic resonance velocimetry

Process engineering applications such as heat transfer, reactions, and separations involve passing fluid through a porous medium. Historically, random-channel porous media have been used for these operations. Such systems do not represent optimal configurations for process performance because of poor flow distribution and high-pressure drop. It is now possible to fabricate porous monoliths with tailored morphology and regular channel structure using 3D-printing. In this work, we use magnetic resonance imaging to study flow through a Schwarz Diamond triply periodic minimal surface (TPMS) monolith for Reynolds numbers up to 350. A transition to unsteady flow was observed experimentally for the first time. The channel structure diverts flow such that free shear layers form in the channel centers that contribute to flow instability. These measurements serve to inform the design of novel transport processes with enhanced performance.     

Population connectivity of aquatic insects in a dam-regulated,desert river

Humans have exaggerated natural habitat fragmentation, negatively impacting species dispersal and reducing population connectivity. Habitat fragmentation can be especially detrimental in freshwater populations, whose dispersal is already constrained by the river network structure. Aquatic insects, for instance, are generally limited to two primary modes of dispersal: downstream drift in the aquatic juvenile life stages and flight during the terrestrial winged adult stage. Yet the impacts of large hydropower dams can make rivers uninhabitable for incoming (drifting) juvenile insects, with remaining refugia found only in tributaries. The ability of adult aquatic insects to traverse such river stretches in search of suitable tributary habitat likely depends on factors such as species-specific dispersal ability and distance between tributaries. To explore the intersection of natural and human-induced habitat fragmentation on aquatic insect dispersal ability, we quantified population genetics of three taxa with varying dispersal abilities, a caddisfly (Hydropsychidae, Hydropsyche oslari), a mayfly (Baetidae: Fallceon quilleri), and a water strider (Veliidae: Rhagovelia distincta), throughout tributaries of the Colorado River in the Grand Canyon, Arizona, USA. Using 2bRAD reduced genome sequencing and landscape genetics analyses, we revealed a strong pattern of isolation by distance among mayfly populations. This contrasts with caddisfly and water strider populations, which were largely panmictic. Analysis of thousands of informative single nucleotide polymorphisms showed that realized dispersal ability may not be accurately predicted by species traits for these widespread species. Principal components analysis revealed a strong division between caddisfly populations upstream and downstream of Havasu Creek (279 km through the 390 km study reach), suggesting that the geography of the Grand Canyon imposes a dispersal barrier for this species. Our use of genetic tools in the Grand Canyon to understand population structure has enabled us to elucidate dispersal barriers for aquatic insects. Ultimately, these data may be useful in informing effective conservation management plans for understudied organisms of conservation interest.     

Twisted Crystalline Organic Semiconductor Photodetectors

Optoelectronic properties of anisotropic crystals vary with direction requiring that the orientation of molecular organic semiconductor crystals is controlled in optoelectronic device active layers to achieve optimal performance. Here, a generalizable strategy to introduce periodic variations in the out-of-plane orientations of 5,11-bis(triisopropylsilylethynyl)anthradithiophene (TIPS ADT) crystals is presented. TIPS ADT crystallized from the melt in the presence of 16 wt.% polyethylene (PE) forms banded spherulites of crystalline fibrils that twist in concert about the radial growth direction. These spherulites exhibit band-dependent light absorption, photoluminescence, and Raman scattering depending on the local orientation of crystals. Mueller matrix imaging reveals strong circular extinction (CE), with TIPS ADT banded spherulites exhibiting domains of positive or negative CE signal depending on the crystal twisting sense. Furthermore, orientation-dependent enhancement in charge injection and extraction in films of twisted TIPS ADT crystals compared to films of straight crystals is visualized in local conductive atomic force microscopy maps. This enhancement leads to 3.3- and 6.2-times larger photocurrents and external quantum efficiencies, respectively, in photodetectors comprising twisted crystals than those comprising straight crystals.