Single polymer gating of channels under a solvent gradient

We study the effect of a gradient of solvent quality on the coil-globule transition for a polymer in a narrow pore. A simple self-attracting self-avoiding walk model of a polymer in solution shows that the variation in the strength of interaction across the pore leads the system to go from one regime (good solvent) to the other (poor solvent) across the channel. This may be thought analogous to thermophoresis, where the polymer goes from the hot region to the cold region under the temperature gradient. The behavior of short chains is studied using exact enumeration whilst the behavior of long chains is studied using transfer matrix techniques. The distribution of the monomer density across the layer suggests that a gate-like effect can be created, with potential applications as a sensor.Comment: 5 Pages, 7 Figures, Accepted in Phys. Rev. E (2013

Evaluation of Spot Welds by Various Techniques

The Iowa Demonstration Laboratory (IDL) is an outreach arm of the Center for NDE that works primarily with in-state clients. Over a course of time, various clients have approached the IDL with a similar query: how do we nondestructively test spot welds? In our work with these clients, we encountered anecdotal information about the success or failure of various means of evaluating spot welds, and sometimes conflicting interpretations of the ease of use of competing techniques. Additionally, published articles seldom contrast the efficacy of different test methods used on the same samples. It appears that a consensus to interpreting weld conditions does not exist, presumably due to a range in effectiveness of various techniques over a range of possible welded materials. This project, in effect, will attempt to determine the best inspection method over a wide variety of welded materials, and, ideally, generate guidelines on how best to implement the various techniques

Statistical Mechanics of DNA Rupture: Theory and Simulations

We study the effects of the shear force on the rupture mechanism on a double stranded DNA. Motivated by recent experiments, we perform the atomistic simulations with explicit solvent to obtain the distributions of extension in hydrogen and covalent bonds below the rupture force. We obtain a significant difference between the atomistic simulations and the existing results in the iterature based on the coarse-grained models (theory and simulations). We discuss the possible reasons and improve the coarse-grained model by incorporating the consequences of semi-microscopic details of the nucleotides in its description. The distributions obtained by the modified model (simulations and theoretical) are qualitatively similar to the one obtained using atomistic simulations.Comment: 18 pages, 9 figures. Accepted in J. Chem. Phys. (2013). arXiv admin note: text overlap with arXiv:1104.305

Metallic monoclinic phase in VO2_2 induced by electrochemical gating: in-situ Raman study

We report in-situ Raman scattering studies of electrochemically top gated VO$_2$ thin film to address metal-insulator transition (MIT) under gating. The room temperature monoclinic insulating phase goes to metallic state at a gate voltage of 2.6 V. However, the number of Raman modes do not change with electrolyte gating showing that the metallic phase is still monoclinic. The high frequency Raman mode A$_g$(7) near 616 cm$^{-1}$ ascribed to V-O vibration of bond length 2.06 \AA~ in VO$_6$ octahedra hardens with increasing gate voltage and the B$_g$(3) mode near 654 cm$^{-1}$ softens. This shows that the distortion of the VO$_6$ octahedra in the monoclinic phase decreases with gating. The time dependent Raman data at fixed gate voltages of 1 V (for 50 minute, showing enhancement of conductivity by a factor of 50) and 2 V (for 130 minute, showing further increase in conductivity by a factor of 5) show similar changes in high frequency Raman modes A$_g$(7) and B$_g$(3) as observed in gating. This slow change in conductance together with Raman frequency changes show that the governing mechanism for metalization is more likely to the diffusion controlled oxygen vacancy formation due to the applied electric field.Comment: 5 pages, 6 figure

AFLOW-QHA3P: Robust and automated method to compute thermodynamic properties of solids

Accelerating the calculations of finite-temperature thermodynamic properties is a major challenge for rational materials design. Reliable methods can be quite expensive, limiting their applicability in autonomous high-throughput workflows. Here, the three-phonon quasiharmonic approximation (QHA) method is introduced, requiring only three phonon calculations to obtain a thorough characterization of the material. Leveraging a Taylor expansion of the phonon frequencies around the equilibrium volume, the method efficiently resolves the volumetric thermal expansion coefficient, specific heat at constant pressure, the enthalpy, and bulk modulus. Results from the standard QHA and experiments corroborate the procedure, and additional comparisons are made with the recently developed self-consistent QHA. The three approaches—three-phonon, standard, and self-consistent QHAs—are all included within the open-source ab initio framework aflow, allowing the automated determination of properties with various implementations within the same framework