Radial Morphologies Research Articles

Molecules with ALMA at Planet-forming Scales (MAPS). III. Characteristics of Radial Chemical Substructures

Abstract The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program provides a detailed, high-resolution (∼10–20 au) view of molecular line emission in five protoplanetary disks at spatial scales relevant for planet formation. Here we present a systematic analysis of chemical substructures in 18 molecular lines toward the MAPS sources: IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. We identify more than 200 chemical substructures, which are found at nearly all radii where line emission is detected. A wide diversity of radial morphologies—including rings, gaps, and plateaus—is observed both within each disk and across the MAPS sample. This diversity in line emission profiles is also present in the innermost 50 au. Overall, this suggests that planets form in varied chemical environments both across disks and at different radii within the same disk. Interior to 150 au, the majority of chemical substructures across the MAPS disks are spatially coincident with substructures in the millimeter continuum, indicative of physical and chemical links between the disk midplane and warm, elevated molecular emission layers. Some chemical substructures in the inner disk and most chemical substructures exterior to 150 au cannot be directly linked to dust substructure, however, which indicates that there are also other causes of chemical substructures, such as snowlines, gradients in UV photon fluxes, ionization, and radially varying elemental ratios. This implies that chemical substructures could be developed into powerful probes of different disk characteristics, in addition to influencing the environments within which planets assemble. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Compression-driven collapse of nanotubes

Nanotubes are prone to collapsing under compression due to the competition between the bending stiffness of the walls and the van der Waals interactions. The different radial morphologies during collapse may affect the electrical properties of nanotube, which may find promising potential applications in strain engineering. In this paper, the finite-deformation model is introduced to determine the radial morphologies, energy barrier and radial deformability of a nanotube under compression, in which the adhesion interactions are analytically obtained. The analytical solutions of the radial morphologies during compression are consistent with the molecular dynamics simulations results, indicating the effectiveness of the finite-deformation model. The analytical results reveal that both the energy barrier and the radial deformability show a decreasing tendency with the increase of the nanotube diameter.

Crystallization of Sodium Percarbonate from Aqueous Solution: Basic Principles of Spherulite Product Design

Recently, there has been an increasing interest in spherulites because of their excellent physicochemical properties and remarkable radial morphologies. However, their formation mechanisms remain largely unresolved and are often overlooked in industrial crystallization. In this work, it was demonstrated that the sodium percarbonate (SPC) spherical particle primarily results from the spherulitic growth effect rather than agglomeration effect. And then, in situ nucleation-controlled experiments in Petri dish were established to study the growth kinetics and morphologies of SPC spherulite. During the process, a critical growth rate was first discovered, which divides the spherulite formation into branching and nonbranching stage. A kinetic model was established to study the growth regime for SPC spherulite formation. Moreover, it is found that sodium hexametaphosphate (SHMP) is not necessary for SPC spherulite, but it can promote the branching of SPC which makes it possible to produce compact SPC spherulite ...

Pre-Conditioning Induces the Precocious Differentiation of Neonatal Astrocytes to Enhance Their Neuroprotective Properties

Hypoxic preconditioning reprogrammes the brain's response to subsequent H/I (hypoxia–ischaemia) injury by enhancing neuroprotective mechanisms. Given that astrocytes normally support neuronal survival and function, the purpose of the present study was to test the hypothesis that a hypoxic preconditioning stimulus would activate an adaptive astrocytic response. We analysed several functional parameters 24 h after exposing rat pups to 3 h of systemic hypoxia (8% O2). Hypoxia increased neocortical astrocyte maturation as evidenced by the loss of GFAP (glial fibrillary acidic protein)-positive cells with radial morphologies and the acquisition of multipolar GFAP-positive cells. Interestingly, many of these astrocytes had nuclear S100B. Accompanying their differentiation, there was increased expression of GFAP, GS (glutamine synthetase), EAAT-1 (excitatory amino acid transporter-1; also known as GLAST), MCT-1 (monocarboxylate transporter-1) and ceruloplasmin. A subsequent H/I insult did not result in any further astrocyte activation. Some responses were cell autonomous, as levels of GS and MCT-1 increased subsequent to hypoxia in cultured forebrain astrocytes. In contrast, the expression of GFAP, GLAST and ceruloplasmin remained unaltered. Additional experiments utilized astrocytes exposed to exogenous dbcAMP (dibutyryl-cAMP), which mimicked several aspects of the preconditioning response, to determine whether activated astrocytes could protect neurons from subsequent excitotoxic injury. dbcAMP treatment increased GS and glutamate transporter expression and function, and as hypothesized, protected neurons from glutamate excitotoxicity. Taken altogether, these results indicate that a preconditioning stimulus causes the precocious differentiation of astrocytes and increases the acquisition of multiple astrocytic functions that will contribute to the neuroprotection conferred by a sublethal preconditioning stress.

The development of small-scale sandy alluvial fans at the base of a modern coastal cliff: process, observations and implications

The development of a series of small-scale alluvial fans has been studied at the base of a sandy coastal cliff on the Torre del Loro Beach near Huelva, southwestern Spain. These fans are small enough to permit the experimental modification of flow, sediment load and morphology. Elongate sandy debris flow deposits are also present. The fans are related to sources with perennial discharges and are fed by solitary, shifting distributary channels. The fans have radial morphologies and grow by localized lobe-building at the intersection point, in a mid-fan or fan-toe position, and/or by bulk aggradation due to overflow (sheet flow). Sediment dispersal is by channelized or unconfined water flow, but also involves slugs of “hyperconcentrated” flows and occasionally sandy debris-flows. The radial fans (channelized flow dominated) have surface slopes of up to 20°, their medial segments are often the steepest, and sediment at their toes and lateral flanks can be coarser than in the inner zones. The elongated sandy debris flow deposits are related to sources with ephemeral discharges and are mass flow-dominated. These deposits are smaller and are built chiefly by sandflows (finger-like sandy debris-flows). Their surface slopes are rather uniform, but only up to 6°, and there is no obvious downfan variation in grain size. When regarded as small-scale analogues for larger alluvial fans, these fans deviate from the textbook models, particularly in grain-size distribution, simultaneity of “quiet” and “energetic” processes, and longitudinal gradient profile. Our study indicates that the relationship between fan morphometry and depositional processes still requires a better knowledge.

INFLUENCE OF CHEMICAL PERTURBATIONS ON THE SURFACE ROUGHNESS OF THIN LAYER ELECTRODEPOSITS

We have studied the origin of macroscopic and microscopic changes of the morphology of metal electrodeposits in thin layer cells. We concentrate on zinc electrodeposition to show that a part of the variety of growth patterns reported in the literature (dense radial, stringy, dendritic and DLA-like fractal morphologies) is probably due to pollution of the electrolyte. One common pollution in these experiments is molecular oxygen, which is dissolved in the electrolyte during its preparation and conservation in contact with air. Its cathodic reduction can seriously perturb the metallic host lattice by formation and intercalation of metal oxide. Other perturbations with equal dramatic effects on the morphology and the roughness of the deposit are alkali cations. By well-controlled addition of one of these ions or molecules, we are able to generate a wide variety of morphologies, from highly ordered dendritic growth to completely disordered growth. Other metals such as copper or tin are shown to be also highly sensitive towards impurities. Based on this sensitivity upon chemical perturbation, we finally propose a new explanation for the morphological changes, called Hecker effect, which can occur during the growth process.

Complications of cylindrical anisotropy on the properties of fibres

Cylindrically orthotropic materials having linear elastic stress-strain relations can possess a unique type of coupling between the radial and tangential directions. This type of anisotropy occurs when fibres have onion skin or radial morphologies which are characteristic of some graphite fibres and poly- p-phenylene terephthalamide fibres. Pulling on a long slender fibre having this type of anisotropy does not produce a uniaxial state of stress because the unequal Poisson ratios also cause both radial and tangential stresses. The magnitude of these predicted transverse stresses in relation to the axial stress is similar to the relationship of strengths in these principal directions. Hence, it is conceivable that fibre failure could be caused by fibre splitting from high transverse stresses rather than the longitudinal tensile stresses. Temperature changes and other factors causing dimensional changes such as swelling and pressure can similarly cause large stresses if the radial and tangential expansion coefficients are different. This type of axial-transverse coupling is known to limit certain designs in composite structures having this type of anisotropy, yet little has been done to understand fibres possessing these characteristics. This paper examines the complex state of stress that can result in fibres having cylindrically orthotropic linear elastic properties for several simple modes of deformation.