Convective flow during dendritic growth

A review is presented of the major experimental findings obtained from recent ground-based research conducted under the SPAR program. Measurements of dendritic growth at small supercoolings indicate that below approximately 1.5 K a transition occurs from diffusive control to convective control in succinonitrile, a model system chosen for this study. The key theoretical ideas concerning diffusive and convective heat transport during dendritic growth are discussed, and it is shown that a transition in the transport control should occur when the characteristic length for diffusion becomes larger than the characteristic length for convection. The experimental findings and the theoretical ideas discussed suggest that the Fluid Experiment System could provide appropriate experimental diagnostics for flow field visualization and quantification of the fluid dynamical effects presented here

Spontaneous deterministic side-branching behavior in phase-field simulations of equiaxed dendritic growth

The accepted view on dendritic side-branching is that side-branches grow as the result of selective amplification of thermal noise and that in the absence of such noise dendrites would grow without the development of side-arms. However, recently there has been renewed speculation about dendrites displaying deterministic side-branching [see, e.g., M. E. Glicksman, Metall. Mater. Trans A 43, 391 (2012)]. Generally, numerical models of dendritic growth, such as phase-field simulation, have tended to display behaviour which is commensurate with the former view, in that simulated dendrites do not develop side-branches unless noise is introduced into the simulation. However, here, we present simulations that show that under certain conditions deterministic side-branching may occur. We use a model formulated in the thin interface limit and a range of advanced numerical techniques to minimise the numerical noise introduced into the solution, including a multigrid solver. Spontaneous side-branching seems to be favoured by high undercoolings and by intermediate values of the capillary anisotropy, with the most branched structures being obtained for an anisotropy strength of 0.03. From an analysis of the tangential thermal gradients on the solid-liquid interface, the mechanism for side-branching appears to have some similarities with the deterministic model proposed by Glicksman

Anisotropic diffusion limited aggregation in three dimensions : universality and nonuniversality

We explore the macroscopic consequences of lattice anisotropy for diffusion limited aggregation (DLA) in three dimensions. Simple cubic and bcc lattice growths are shown to approach universal asymptotic states in a coherent fashion, and the approach is accelerated by the use of noise reduction. These states are strikingly anisotropic dendrites with a rich hierarchy of structure. For growth on an fcc lattice, our data suggest at least two stable fixed points of anisotropy, one matching the bcc case. Hexagonal growths, favoring six planar and two polar directions, appear to approach a line of asymptotic states with continuously tunable polar anisotropy. The more planar of these growths visually resembles real snowflake morphologies. Our simulations use a new and dimension-independent implementation of the DLA model. The algorithm maintains a hierarchy of sphere coverings of the growth, supporting efficient random walks onto the growth by spherical moves. Anisotropy was introduced by restricting growth to certain preferred directions

The Rocky Road to Energy Dominance: The Executive Branch’s Limited Authority to Modify and Revoke Withdrawals of Federal Lands from Mineral Production

The Trump Administration’s implementation of its America First Energy Plan, whose goal is achieving U.S. “energy dominance,” has relied heavily upon public mineral development. Mineral development on federal lands is largely governed by statute. The statutory legal mechanisms by which the Executive Branch can “open” or “close” an area of federal lands to mineral development, whether onshore or offshore, are withdrawal, modification, and revocation.The Federal Land Policy and Management Act (FLPMA) and the Outer Continental Shelf Lands Act (OCSLA) are the primary statutes that govern onshore and offshore mineral development on over 2 billion acres of federal lands. Both FLPMA and OCSLA authorize withdrawals, which the Executive can use to place federal lands off limits to mineral development. FLPMA also authorizes modifications and revocations, which can remove constraints on such development. The Trump Administration has relied on both statutes in its quest to expand the areas that are available for private mineral disposition through modification or revocation of withdrawals by prior administrations.The authority provided by FLPMA and OCSLA to determine the availability of federal lands for mineral development is subject to a series of substantive and procedural constraints. Because it regards those constraints as undesirable shackles on the implementation of its mineral development policies, and consistent with its expansive view of Executive Branch power in almost all contexts, the Administration has not been content to rely on statutory authorization to modify and revoke development-precluding withdrawals. Instead, it has also invoked non-statutory, implied or inherent authority to open vast areas of federal lands to oil and gas, coal, and other mineral development.The paucity of judicial precedent governing the parameters of statutory and non-statutory Executive Branch authority to reopen lands previously placed off limits to mineral development raises significant questions about the legality of the Administration’s efforts to alter the status of protected lands and resources. Although the Administration apparently regards downsizing or revocation of withdrawals by previous administrations as a quick and effective way to open up vast new acreage to mineral development, the legal basis for its actions is tenuous at best.This Article examines both statutory and non-statutory mechanisms for determining the availability of federal onshore and offshore lands for uses such as mineral exploration and development. It identifies the constraints that FLPMA and OCSLA impose on revocation or modification of previous withdrawals. It also explores the parameters of non-statutory Executive mineral disposition authority and assesses the legality of the significant Trump Administration withdrawal modification and revocation efforts to date. It concludes that Congress has eliminated any implied or inherent withdrawal, revocation, or modification authority that may once have existed. It also finds that the most prominent and controversial of the Trump withdrawal modifications and revocations exceeded the authority the Executive Branch retains under FLPMA and OCSLA. As a result, that the Trump Administration’s unauthorized pursuit of energy dominance should result in judicial invalidation

Evidance for an Oxygen Diffusion Model for the Electric Pulse Induced Resistance Change Effect in Oxides

Electric pulse induced resistance (EPIR) switching hysteresis loops for Pr0.7Ca0.7MnO3 (PCMO) perovskite oxide films were found to exhibit an additional sharp "shuttle peak" around the negative pulse maximum for films deposited in an oxygen deficient ambient. The device resistance hysteresis loop consists of stable high resistance and low resistance states, and transition regions between them. The resistance relaxation of the "shuttle peak" and its temperature behavior as well as the resistance relaxation in the transition regions were studied, and indicate that the resistance switching relates to oxygen diffusion with activation energy about 0.4eV. An oxygen diffusion model with the oxygen ions (vacancies) as the active agent is proposed for the non-volatile resistance switching effect in PCMO.Comment: 7 pages, 5 figure

Kinetics of Diffusional Droplet Growth in a Liquid/Liquid Two-Phase System

We address the problem of diffusional interactions in a finite sized cluster of spherical particles for volume fractions, V(sub v) in the range 0-0.01. We determined the quasi-static monopole diffusion solution for n particles distributed at random in a continuous matrix. A global mass conservation condition is employed, obviating the need for any external boundary condition. The numerical results provide the instantaneous (snapshot) growth or shrinkage rate of each particle, precluding the need for extensive time-dependent computations. The close connection between these snapshot results and the coarsegrained kinetic constants are discussed. A square-root dependence of the deviations of the rate constants from their zero volume fraction value is found for the higher V(sub v) investigated. This behavior is consistent with predictions from diffusion Debye-Huckel screening theory. By contrast, a cube-root dependence, reported in earlier numerical studies, is found for the lower V(sub v) investigated. The roll-over region of the volume fraction where the two asymptotics merge depends on the number of particles, n, alone. A theoretical estimate for the roll-over point predicts that the corresponding V(sub v) varies as n(sup -2), in good agreement with the numerical results

Finite-Difference Lattice Boltzmann Methods for binary fluids

We investigate two-fluid BGK kinetic methods for binary fluids. The developed theory works for asymmetric as well as symmetric systems. For symmetric systems it recovers Sirovich's theory and is summarized in models A and B. For asymmetric systems it contributes models C, D and E which are especially useful when the total masses and/or local temperatures of the two components are greatly different. The kinetic models are discretized based on an octagonal discrete velocity model. The discrete-velocity kinetic models and the continuous ones are required to describe the same hydrodynamic equations. The combination of a discrete-velocity kinetic model and an appropriate finite-difference scheme composes a finite-difference lattice Boltzmann method. The validity of the formulated methods is verified by investigating (i) uniform relaxation processes, (ii) isothermal Couette flow, and (iii) diffusion behavior.Comment: RevTex, 3 figures. Phys. Rev. E (2005, in press

Attitudes, Beliefs, and Perceptions on Cannabis Among Older Adults Aged 65 and Older: A cross-sectional Survey.

OBJECTIVE: With the rise of medical and recreational cannabis use among older adults, concerns about the potential risks and benefits of cannabis use are increasing. This pilot study aimed to determine the attitudes, beliefs, and perceptions of older adults toward cannabis as a medicinal product, to establish a foundation for future research on how healthcare providers communicate with this population regarding cannabis. METHODS: A cross-sectional survey was conducted on adults aged 65 and older living in Philadelphia. The survey included questions related to participants\u27 demographics, knowledge, attitudes, beliefs, and perceptions of cannabis. Participants were recruited via flyer distributions, publications in newsletters, and a local newspaper. Surveys were conducted from December 2019 through May 2020. Quantitative data were presented using counts, mean, median, and percentages, and qualitative data were analyzed by categorizing common responses. RESULTS: The study aimed to recruit 50 participants, of whom 47 met the requirements, and had their data analyzed, resulting in an average age of 71 years. The majority of participants were male (53%) and Black (64%). Seventy-six percent of participants considered cannabis a highly important treatment for older adults, and 42% considered themselves highly knowledgeable about cannabis. Over half of the participants reported being asked about tobacco (55%) or alcohol (57%) use by their PCP compared to 23% being asked about cannabis. Participants indicated that they most commonly turned to the internet and social media for information about cannabis, while only a small number mentioned relying on their primary care physician (PCP) for such information. DISCUSSION: The results of this pilot study highlight the need for accurate and reliable information about cannabis for older adults and their healthcare providers. As the use of cannabis as a therapy continues to rise, it is crucial for healthcare providers to address misconceptions and encourage older adults to seek out evidence-based research. Further research is needed to explore healthcare providers\u27 perceptions of cannabis therapy and how they can better educate older adults

Extending the scope of microscopic solvability: Combination of the Kruskal-Segur method with Zauderer decomposition

Successful applications of the Kruskal-Segur approach to interfacial pattern formation have remained limited due to the necessity of an integral formulation of the problem. This excludes nonlinear bulk equations, rendering convection intractable. Combining the method with Zauderer's asymptotic decomposition scheme, we are able to strongly extend its scope of applicability and solve selection problems based on free boundary formulations in terms of partial differential equations alone. To demonstrate the technique, we give the first analytic solution of the problem of velocity selection for dendritic growth in a forced potential flow.Comment: Submitted to Europhys. Letters, No figures, 5 page

Undercooling of acoustically levitated molten drops

It was observed that the acoustically levitated molten SCN (succinonitrile) drops can generally be undercooled to a degree where the impurities in the drop are responsible for the nucleation of the solid phase. However, it was also observed that ultrasound occasionally terminates undercooling of the levitated drops by initiating the nucleation of the solid at an undercooling level which is lower than that found for the nucleation catalyzed by the impurities in the drop. This premature nucleation can be explained by thermodynamic considerations which predict an increase in effective undercooling of the liquid upon the collapse of cavities. Pre-existing gas microbubbles which grow under the influence of ultrasound are suggested as the source of cavitation. The highly undercooled SCN drops can be utilized to measure the growth velocity of the solid in the deeply undercooled region including the hypercooled region