Abstract

Plants are under constant threat of attack from pathogens, herbivores, and the environment. The techniques employed by plants to defend themselves are very varied and some involve extremely refined armaments. In this lecture, we present two fascinating examples: First, we discuss the stinging nettle, a plant which employs hollow needle-like stinging hairs to deter herbivores. The hairs are constructed from silica, the mineral from which we make glass, and they are filled with poison. The hairs are remarkably rigid and rarely break. Yet the tip is so sharp that the slightest touch cuts human skin, and so fragile that it breaks at that touch and releases poison into the wound. How the seemingly antagonist mechanical functions of rigidity and fragility are achieved, however, is unknown.

Our second example concerns the movement of water and minerals from plant roots to leaves in the xylem, a network of vascular conduits made from dead cells. When a plant is subjected to drought stress, air pockets can spread inside the xylem, threatening the survival of the plant. Many plants prevent propagation of air by using hydrophobic nano-membranes in the “pit” pores that link adjacent xylem cells. This adds considerable resistance to flow. Interestingly, torus-margo pit pores in conifers are open and offer less resistance. To prevent propagation of air, conifers use a soft gating mechanism, which relies on hydrodynamic interactions between the xylem liquid and the elastic pit. However, it is unclear exactly how it is able to combine high flow permeability with resistance to propagation of air.

We combine experiments on biomimetic model systems with theory to elucidate the physics of these defense mechanism. The designs are compared to other natural systems and optimal strategies are discussed.

Abstract

The nonlinear dynamics of cardiac excitable waves is controlled by ion channels that are the basic molecular building blocks of the heart's electrical circuitry. Variations in gene expression and protein levels can cause the conductance of those channels to vary both between cells of the same heart and between hearts of different individuals in a genetically diverse population.

This talk will discuss the results of recent computational modeling and experimental studies aimed at identifying electrophysiological parameter sets that represent different individuals in a genetically diverse population and at distinguishing intra-heart cell-to-cell from inter-individual variability.

Our main finding is that feedback sensing of the intracellular calcium concentration suffices, remarkably, to constrain parameter sets so as to produce a normal electrophysiological phenotype without any constraint on the electrical signal due to compensation between different ionic currents. Furthermore, parameter sets can differ greatly such that different individuals may respond very differently to environmental stresses and drug therapies. The results have important implications for understanding cardiac homeostasis and developing personalized therapies. 

Abstract

Most cancer deaths arise when then primary tumor metastasizes and cancer takes root in distant organs. From the point of view of cellular behavior, metastatic spread requires many capabilities (motility, chemoresistance, avoidance of cell death due to lack of adhesion, and ability to grow in a foreign location) which seem beyond what is normally possible for cells in typical organs.

To address this issue, we focus on the phenomenon of phenotypic plasticity, the idea that the nonlinear dynamics of cellular genetic networks can lead to transitions to states that are capable of these feats. These new phenotypes can be studied with the help of mathematical models both of the underlying networks and of the resultant biophysical properties (such as motility). By revealing the factors most responsible for the formation of these aggressive cellular types, we hopefully can suggest new targeting therapies for what remains the most recalcitrant aspect of cancer.

Bio

Dr. Levine is a member of the National Academy of Sciences and a fellow of the American Academy of Arts and Sciences. He serves as the co-director for the National Science Foundation sponsored Center for Theoretical Biological Physics (CTBP), located on the campus of Rice University, as a partnership among Rice, Baylor College of Medicine and the University of Houston. In this position, Dr. Levine supervises a large interdisciplinary team of researchers who apply methods from physical science to vexing problems in biology and biomedicine. A particular emphasis in recent years has been on cancer, where pure biology-based approaches have not proven capable of finding effective treatments or cures for metastatic disease. Dr. Levine is a member of the editorial board of the Proceedings of the National Academy of Sciences, editor in chief of the journal Physical Biology and an associate editor of Physical Review Letters. He is first author on over 250 publications in the area of theoretical physics as applied to a wide variety of systems, especially living systems.

 First-year Grad students have expressed an interest in working in astrophysics. The Center for Relativistic Astrophysics(CRA)  would like to invite you to  “Meet the CRA" Friday, Sept 22 from 3pm-4pm, in the Howey Interaction Zone.  

We will introduce the research and activities happening at the CRA and give students an opportunity to meet the Faculty and Staff and enjoy Pizza. 

Abstract
This talk will describe new routes to melting of crystal lattices. The lattices are nearly defect-free, and are formed through repulsive interactions exclusively, thus forming Wigner lattices. The absence of defects inhibits melting, making the crystal much more stable. This reduces the energy difference between crystal and liquid, giving the melting transition a distinct second order character, even though it is strictly a first order transition. This form of melting was first proposed by Born nearly 80 years ago, but it is only now that its study can be carried out.

Bio
Professor Weitz received his PhD in physics from Harvard University and then joined Exxon Research and Engineering Company, where he worked for nearly 18 years. He then became a professor of physics at the University of Pennsylvania and moved to Harvard at the end of the last millennium as professor of physics and applied physics. Professor Weitz leads a group studying soft matter science with a focus on materials science, biophysics and microfluidics. He has co-founded several companies to commercialize some of the microfluidics work developed in his lab.

Here we use model-generated surrogate observations from a numerical experiment to evaluate the performance of the ensemble Kalman filter in reconstructing such time series for a discordant alternans state in one spatial dimension and for scroll waves in three dimensions. We show that our approach is able to recover time series of both observed and unobserved variables that match the truth. Where nearby observations are available, the error is reduced below the synthetic observation error, with a smaller reduction with increased distance from observations. 

Using one-dimensional cases, we provide a deeper analysis showing that limitations in model formulation, including incorrect parameter values and undescribed spatial heterogeneity, can be managed appropriately and that some parameter values can be estimated directly as part of the data assimilation process. Our findings demonstrate that state reconstruction for spatiotemporally complex cardiac electrical dynamics is possible and has the potential for successful application to real experimental data.

Abstract:

The cycling of gas through galactic fountains links disks to halos. Simulations enable astronomers to follow this cycle by tracking gas particles. Here I analyze the role of accretion and outflows in the growth of stellar and metal mass in a suite of twenty high-resolution simulated dwarf and spiral galaxies. These simulations agree with key observables, including the stellar mass-halo mass, Tully-Fisher, and mass-metallicity relations.

This agreement relies on strong feedback-driven outflows that drive large fractions of the metals into the circumgalactic media. In fact, in dwarf galaxies, 90% of the available metals lie outside of the galactic disk at z = 0, a fraction that decreases to ~ 1/3 in Milky Way-mass galaxies.  In general, ejective feedback is increasingly important to the evolution of galaxies as halo mass decreases, and we find an effective mass loading factor that scales as circular velocity to the -2.2 power. However, recycling is common: about half the outflow mass across all galaxy masses is later re-accreted on timescales of about 1 Gyr.  I will discuss how these results together elucidate and quantify how the baryon cycle plausibly regulates star formation and produces the mass-metallicity relationship.

Abstract: It is well-known that most galaxies are missing most of their baryonic mass.  Perhaps more surprisingly, they also seem to be missing most of their metals. 

I will present Chandra observations probing our Milky Way halo in absorption. Together with XMM and Suzaku data on emission, our results show that the Milky Way halo contains a huge reservoir of warm-hot gas that may account for a large fraction of missing baryons and metals. I'll review the current status of this field, discuss implications of our results to models of galaxy formation and evolution and outline paths for future progress.

Abstract

I’ll talk about a joint project with Sabetta Matsumoto to investigate theoretical and (via 3D printed models) practical possibilities for designing three-dimensional auxetic mechanisms. We consider connections between existing two-dimensional designs, and generalize these to three-dimensions.

We propose two general schemes for designing 3D auxetic mechanisms, one using two counterrotating copies of 2D mechanisms such as the diamondplate, kagome and jitterbug mechanisms, and the other using a “branched” version of the scissor linkage. In all cases, these use the Sarrus linkage to ensure that the mechanism has only one degree of freedom.