Coupled Transport/Hyperelastic Model for High Energy Density Nastic Materials

A new development in aerospace technology involves the creation of aircraft that can undergo large changes in the shape of their wings and control surfaces. This technology, called morphing aircraft, does away with several performance compromises by allowing the aircraft to adapt to a wide variety of speed and altitude conditions. One of the challenges associated with the development of morphing aircraft is the creation of a skin that can allow for the in-plane stretching necessary for morphing but possess enough out-of-plane flexural rigidity to handle aerodynamic forces.A new class of high energy density active materials based upon biological processes is being developed to address this problem. These materials utilize the controlled transport of charge and fluid into micron-scale inclusions. The inclusions are phase separated from the surrounding matrix by a selectively-permeable membrane. Selective stimulation of the membrane enables bulk deformation in a process referred to in the plant kingdom as nastic movements. The particular material considered in this work utilizes biological transport mechanisms to generate an osmotic gradient across the membrane.The purpose of this work is to develop a physics-based computational model of the nastic material that couples ion and solvent fluxes generated by the biological transporters to a finite element analysis of the surrounding matrix. This model is to act as a feedback loop for material synthesis efforts. The processes occurring in the biotransport system are complex and highly coupled to one another. The numerical solution of the resulting transport model and its coupling with the finite element analysis are key challenges in creating a viable model. The resulting model has been compared to experiment and is capable of predicting material response over a wide range of configurations and transport components. A series of parametric studies is performed to determine the relative importance of the material parameters and provide guidance to experimental efforts

Hybrid gene misregulation in multiple developing tissues within a recent adaptive radiation of Cyprinodon pupfishes.

Genetic incompatibilities constitute the final stages of reproductive isolation and speciation, but little is known about incompatibilities that occur within recent adaptive radiations among closely related diverging populations. Crossing divergent species to form hybrids can break up coadapted variation, resulting in genetic incompatibilities within developmental networks shaping divergent adaptive traits. We crossed two closely related sympatric Cyprinodon pupfish species-a dietary generalist and a specialized molluscivore-and measured expression levels in their F1 hybrids to identify regulatory variation underlying the novel craniofacial morphology found in this recent microendemic adaptive radiation. We extracted mRNA from eight day old whole-larvae tissue and from craniofacial tissues dissected from 17-20 day old larvae to compare gene expression between a total of seven F1 hybrids and 24 individuals from parental species populations. We found 3.9% of genes differentially expressed between generalists and molluscivores in whole-larvae tissues and 0.6% of genes differentially expressed in craniofacial tissue. We found that 2.1% of genes were misregulated in whole-larvae hybrids whereas 19.1% of genes were misregulated in hybrid craniofacial tissues, after correcting for sequencing biases. We also measured allele specific expression across 15,429 heterozygous sites to identify putative compensatory regulatory mechanisms underlying differential expression between generalists and molluscivores. Together, our results highlight the importance of considering misregulation as an early indicator of genetic incompatibilities in the context of rapidly diverging adaptive radiations and suggests that compensatory regulatory divergence drives hybrid gene misregulation in developing tissues that give rise to novel craniofacial traits

Actual causation and the art of modeling

We look more carefully at the modeling of causality using structural equations. It is clear that the structural equations can have a major impact on the conclusions we draw about causality. In particular, the choice of variables and their values can also have a significant impact on causality. These choices are, to some extent, subjective. We consider what counts as an appropriate choice. More generally, we consider what makes a model an appropriate model, especially if we want to take defaults into account, as was argued is necessary in recent work.Comment: In Heuristics, Probability and Causality: A Tribute to Judea Pearl (editors, R. Dechter, H. Geffner, and J. Y. Halpern), College Publications, 2010, pp. 383-40

The Hot Hand, Competitive Experience, and Performance Differences by Gender

Using data on junior golf tournaments, we find evidence that the “hot hand” does exist, and that its prevalence decreases as golfers gain experience. This provides an explanation as to why studies that consider professional athletes conclude that the hot hand does not exist. We also show that females are much more likely to experience the hot hand compared with similar males, and provide evidence that this disparity is driven by differences in competitive experience. As golfers’ experience increases, gender dissimilarities disappear. We argue that exposure to competition may also drive other gender differences identified in competitive environments.