Strain Stiffening Induced by Molecular Motors in Active Crosslinked Biopolymer Networks

We have studied the elastic response of actin networks with both compliant and rigid crosslinks by modeling molecular motors as force dipoles. Our finite element simulations show that for compliant crosslinkers such as filamin A, the network can be stiffened by two orders of magnitude while stiffening achieved with incompliant linkers such as scruin is significantly smaller, typically a factor of two, in excellent agreement with recent experiments. We show that the differences arise from the fact that the motors are able to stretch the compliant crosslinks to the fullest possible extent, which in turn causes to the deformation of the filaments. With increasing applied strain, the filaments further deform leading to a stiffened elastic response. When the crosslinks are incompliant, the contractile forces due to motors do not alter the network morphology in a significant manner and hence only small stiffening is observed.Comment: 4 pages, 5 figure

Effect of doping on polarization profiles and switching in semiconducting ferroelectric thin films

This paper proposes a theory to describe the polarization and switching behavior of ferroelectrics that are also wide-gap semiconductors. The salient feature of our theory is that it does not make any a priori assumption about either the space charge distribution or the polarization profile. The theory is used to study a metal-ferroelectric-metal capacitor configuration, where the ferroelectric is n-type doped. The main result of our work is a phase diagram as a function of doping level and thickness that shows different phases, namely, films with polarization profiles that resemble that of undoped classical ferroelectrics, paraelectric, and a new head-to-tail domain structure. We have identified a critical doping level, which depends on the energy barrier in the Landau energy and the built-in potential, which is decided by the electronic structures of both the film and the electrodes. When the doping level is below this critical value, the behavior of the films is almost classical. We see a depleted region, which extends through the film when the film thickness is very small, but is confined to two boundary layers near the electrodes for large film thickness. When the doping level is higher than the critical value, the behavior is classical for only very thin films. Thicker films at this doping level are forced into a tail-to-tail configuration with three depletion layers, lose their ferroelectricity, and may thus be described as nonlinear dielectric or paraelectric. For films which are doped below the critical level, we show that the field required for switching starts out at the classical coercive field for very thin films, but gradually decreases

Depletion Layers and Domain Walls in Semiconducting Ferroelectric Thin Films

Commonly used ferroelectric perovskites are also wide-band-gap semiconductors. In such materials, the polarization and the space-charge distribution are intimately coupled, and this Letter studies them simultaneously with no a priori ansatz on either. In particular, we study the structure of domain walls and the depletion layers that form at the metal-ferroelectric interfaces. We find the coupling between polarization and space charges leads to the formation of charge double layers at the 90° domain walls, which, like the depletion layers, are also decorated by defects like oxygen vacancies. In contrast, the 180° domain walls do not interact with the defects or space charges. Implications of these results to domain switching and fatigue in ferroelectric devices are discussed

Observing System Simulation Experiments Using Small Unmanned Aerial Vehicles in Various Configurations to Improve High-Resolution Forecasts of Convection

There has been a lot of interest and development of small Unmanned Aerial Vehicles (sUAVs) to obtain atmospheric measurements for research and operations. Some have proposed a 3D Mesonet concept to add vertical profiling to mesonets such as the Oklahoma Mesonet. Observation System Simulation Experiments (OSSEs) are an effective tool to measure the impact of a proposed observing system before a complete set of observations are available, and thus are the ideal tool to study different configurations of sUAVs that may be deployed in such a 3D Mesonet. In this OSSE study, a Nature Run is constructed using a short term 3 km and 1 km WRF nested model forecast covering Oklahoma and parts of surrounding states. Simulated sUAV profiles, as well as observations representing standard existing observations, are created from the WRF model forecast. The observations are then assimilated into the ARPS hourly for 6 hours. The case being examined is the May 20, 2013 severe weather outbreak in central and eastern Oklahoma. The sUAV system’s ability to update the background forecast for conditions on May 20, focusing on convective initiation and early storm development in the afternoon, is assessed. To examine the effect of adding simulated sUAV observations, experiments are run to test the impact of sUAV simulated observations at various max heights to 3 km. The number of simulated sUAV observations is also varied up to 108 sites. Additional experiments were run to test the impact of adjusting analysis parameters, changing the time interval of observations down to 30 minute intervals and adjusting the start time of the assimilation of data. From the forecasts of convection in the OSSE experiments, we can clearly see positive impact from the addition of the sUAV observations in the convective initiation and early storm evolution. The quantitative impacts on the forecast state variables show clear positive dependence on the height of the sUAV data assimilated. There is also improvement in timing and placement of convection when the interval of consecutive UAV obs is reduced from hourly to every 30 minutes. However, there is not as much improvement in fitting the UAV sites to the average site density for 50 or fewer UAV sites, nor is there a clear linear relationship between delaying start times of consecutive hourly UAV obs and the areal coverage and placement of convective initiation. It is also found that decreasing the sUAV observation interval to 30 minutes from 1 hour while using 50 sites cannot replicate the results from using 108 sites

Surface terminated germanene as emerging nanomaterials

Using first principle calculations, we propose functionalized germanene (GeX, X = H, F, Cl, Br, I, OH, CH3) as emerging nanomaterials. Although germanene has no band gap, complete functionalization with H induces band gap of ~1.80 eV. A 50% H functionalization shows a dangling band at the Fermi level. Germanene I (GeI) is a 2D Topological Insulators (TI). GeH, GeF, GeCl, and GeBr can be transformed into TI by applying strain.The methyl-functionalized two-dimensional germanium monolayer sheets have been synthesized with a facile, one-step metathesis approach from CaGe2 crystals. We find that tensile strain can induce topological phase transition with band inversion at Gamma point. The band gap opened by spin-orbit coupling in this quantum spin Hall insulator can be as large as 0.1 eV ample for practical applications at room temperature

Remodeling of Fibrous Extracellular Matrices by Contractile Cells: Predictions from Discrete Fiber Network Simulations

Contractile forces exerted on the surrounding extracellular matrix (ECM) lead to the alignment and stretching of constituent fibers within the vicinity of cells. As a consequence, the matrix reorganizes to form thick bundles of aligned fibers that enable force transmission over distances larger than the size of the cells. Contractile force-mediated remodeling of ECM fibers has bearing on a number of physiologic and pathophysiologic phenomena. In this work, we present a computational model to capture cell-mediated remodeling within fibrous matrices using finite element based discrete fiber network simulations. The model is shown to accurately capture collagen alignment, heterogeneous deformations, and long-range force transmission observed experimentally. The zone of mechanical influence surrounding a single contractile cell and the interaction between two cells are predicted from the strain-induced alignment of fibers. Through parametric studies, the effect of cell contractility and cell shape anisotropy on matrix remodeling and force transmission are quantified and summarized in a phase diagram. For highly contractile and elongated cells, we find a sensing distance that is ten times the cell size, in agreement with experimental observations.Comment: Accepted for publication in the Biophysical Journa

Mechanics of Graphene and CNT-polystyrene nanocomposites

We performed molecular dynamics simulation to investigate the mechanics of graphene and CNT-polystyrene nanocomposites. First, we studied the different mechanical properties of polystyrene and matched with experimental data. Then, we have investigated the interfacial and adhesive strength of graphene and CNT with polystyrene. For graphene, we considered the effect of hydrogen functionalization and different defects: stone-wales, vacancy. For CNT, we considered different chirality. Our simulation supported with analytical model give comprehensive insight into mechanism of graphene and CNT-nanocomposite