Search

Barchans are eolian dunes of crescent shape found on Earth, Mars and other celestial bodies. Among the different types of barchan‐barchan interaction, there is one, known as chasing, in which the dunes remain close but without touching each other. In this paper, we investigate the origins of this barchan‐barchan dune repulsion by carrying out grain‐scale numerical computations in which a pair of granular heaps is deformed by the fluid flow into barchan dunes that interact with each other. In our simulations, data such as position, velocity and resultant force are computed for each individual particle at each time step, allowing us to measure details of both the fluid and grains that explain the repulsion. We show the trajectories of grains, time‐average resultant forces, and mass balances for each dune, and that the downstream barchan shrinks faster than the upstream one, keeping, thus, a relatively high velocity although in the wake of the upstream barchan. In its turn, this fast shrinkage is caused by the flow disturbance, which induces higher erosion on the downstream barchan and its circumvention by grains leaving the upstream dune. Our results help explaining the mechanisms behind the distribution of barchans in dune fields found on Earth and Mars. Plain Language Summary: Barchans are crescent‐shaped wind‐formed dunes that grow when the wind is roughly unidirectional and the amount of sand is limited (typically, over a non‐erodible ground). They are found in dune fields on Earth and Mars, where barchan dunes interact with each other, forming different field patterns. One first question that arises is why, in some cases, two nearby barchans do not touch each other. Is there a repulsion mechanism? In this paper, we investigate the barchan‐barchan dune repulsion by carrying out numerical computations of the motion of the fluid and each grain forming a pair of barchans. The outputs are detailed measurements of both fluid and grains, such as trajectories, forces, and mass balances, and they show that the downstream barchan shrinks faster than the upstream one. The results also show that the faster shrinkage is due to the fluid flow disturbance caused by the upstream barchan: the fluid impacts the downstream dune, increasing its erosion at the same time that it entrains further downstream part of grains coming from the upstream dune. As a result, the downstream barchan moves at the same pace or faster than the upstream dune, though receiving grains from the latter (which only loses grains). Key Points: We investigate numerically both the fluid flow and motion of grains for understanding the mechanisms behind barchan‐barchan dune repulsionWe measure the flow rate of grains and show that there is, indeed, greater erosion on the downstream barchanThe disturbed flow impacting the downstream barchan induces higher erosion (of its grains) and low accumulation (of upstream grains) [ABSTRACT FROM AUTHOR]

Selenium (Se) biofortification in crops provides a valuable strategy to enhance human Se intake. However, crops vary greatly with their capacity in tolerating and metabolizing/accumulating Se, and the basis underlying such variations remains to be fully understood. Here, we compared the effects of Se and its analog S treatments on plant growth and biochemical responses between a Se accumulator (arugula) and a non-accumulator (lettuce). Arugula exhibited an increased biomass production in comparison with untreated controls at a higher selenate concentration than lettuce (20 μM vs. 10 μM Na 2 SeO 4 ), showing better tolerance to Se. Arugula accumulated 3-folds more Se and S than lettuce plants under the same treatments. However, the Se/S assimilation as assessed by ATP sulfurylase and O-acetylserine (thiol)lyase activities was comparable between arugula and lettuce plants. Approximately 4-fold higher levels of Se in proteins under the same doses of Se treatments were observed in arugula than in lettuce, indicating that Se accumulators have better tolerance to selenoamino acids in proteins. Noticeably, arugula showed 6-fold higher ascorbate peroxidase activity and produced over 5-fold more glutathione and non-protein thiols than lettuce plants, which suggest critical roles of antioxidants in Se tolerance. Taken together, our results show that the elevated Se tolerance of arugula compared to lettuce is most likely due to an efficient antioxidant defense system. This study provides further insights into our understanding of the difference in tolerating and metabolizing/accumulating Se between Se accumulators and non-accumulators., (Published by Elsevier Masson SAS.)

The nursing shortage of the late 80s necessitated development of an alternative to primary nursing to meet patient needs. After assessing the strengths and limitations of functional nursing, team nursing and primary nursing, a team designed a new model which incorporated the best features of the three. After a pilot period, "primary team nursing" has proven effective and feedback from nurses has been positive.

For the past several years, an orientation program for novice educators at Long Island College Hospital has had a measure of success. A process was developed to encourage qualified applicants to participate in a program designed to develop their competency in the teaching-learning process. This article describes the strategies used as well as challenges in hiring novice educators.