Phase Separation of Saturated and Mono-unsaturated Lipids as determined from a Microscopic Model

A molecular model is proposed of a bilayer consisting of fully saturated DPPC and mono unsaturated DOPC. The model not only encompasses the constant density within the hydrophobic core of the bilayer, but also the tendency of chain segments to align. It is solved within self-consistent field theory. A model bilayer of DPPC undergoes a main chain transition to a gel phase, while a bilayer of DOPC does not do so above zero degrees centigrade because of the double bond which disrupts order. We examine structural and thermodynamic properties of these membranes and find our results in reasonable accord with experiment. In particular, order-parameter profiles are in good agreement with NMR experiments. A phase diagram is obtained for mixtures of these lipids in a membrane at zero tension. The system undergoes phase separation below the main-chain transition temperature of the saturated lipid. Extensions to the ternary DPPC, DOPC, and cholesterol system are outlined.Comment: 29 pages, 4 figures, 1 table. revised versio

Wheat forecast economics effect study

A model to assess the value of improved information regarding the inventories, productions, exports, and imports of crop on a worldwide basis is discussed. A previously proposed model is interpreted in a stochastic control setting and the underlying assumptions of the model are revealed. In solving the stochastic optimization problem, the Markov programming approach is much more powerful and exact as compared to the dynamic programming-simulation approach of the original model. The convergence of a dual variable Markov programming algorithm is shown to be fast and efficient. A computer program for the general model of multicountry-multiperiod is developed. As an example, the case of one country-two periods is treated and the results are presented in detail. A comparison with the original model results reveals certain interesting aspects of the algorithms and the dependence of the value of information on the incremental cost function

Strategies for Reducing Voluntary Employee Turnover in Call Centers

Call center business leaders who experience voluntary employee turnover are affected by low productivity and high attrition. Call center business leaders are concerned about voluntary employee turnover, as 35 of every 100 call center employees leave the company within the first 6 months of their start date. The purpose of this qualitative multiple case study was to explore strategies call center business leaders use to reduce voluntary employee turnover among several call center managers located in Southeastern New Mexico. The conceptual framework supporting this study was Burns’ transformational leadership theory. The participants included 3 call center business leaders who successfully implemented strategies reducing voluntary employee turnover. Data collection included face-to-face interviews, open-ended questions, and audio recordings. Data were analyzed using Yin’s 5-stage method of qualitative data analysis. Five themes that emerged from this study were: employee turnover, job satisfaction, training and development, employee compensation, and reward and recognition. Business leaders in call center organizations who positively reduce voluntary employee turnover may influence productivity, improve organizational growth, and increase job satisfaction. Business leaders can use the findings from this study to create a positive social change in call center business leaders’ awareness of retention strategies by focusing on the organizations’ performance. Organization leaders who reduce voluntary employee turnover could potentially lead employees to long-term growth and development career opportunities that can affect social change to benefit the behaviors of the company’s employees and families in the community

Spray automated balancing of rotors: Methods and materials

The work described consists of two parts. In the first part, a survey is performed to assess the state of the art in rotor balancing technology as it applies to Army gas turbine engines and associated power transmission hardware. The second part evaluates thermal spray processes for balancing weight addition in an automated balancing procedure. The industry survey reveals that: (1) computerized balancing equipment is valuable to reduce errors, improve balance quality, and provide documentation; (2) slow-speed balancing is used exclusively, with no forseeable need for production high-speed balancing; (3) automated procedures are desired; and (4) thermal spray balancing is viewed with cautious optimism whereas laser balancing is viewed with concern for flight propulsion hardware. The FARE method (Fuel/Air Repetitive Explosion) was selected for experimental evaluation of bond strength and fatigue strength. Material combinations tested were tungsten carbide on stainless steel (17-4), Inconel 718 on Inconel 718, and Triballoy 800 on Inconel 718. Bond strengths were entirely adequate for use in balancing. Material combinations have been identified for use in hot and cold sections of an engine, with fatigue strengths equivalent to those for hand-ground materials

Molecular theory of hydrophobic mismatch between lipids and peptides

Effects of the mismatch between the hydrophobic length, d, of transmembrane alpha helices of integral proteins and the hydrophobic thickness, D_h, of the membranes they span are studied theoretically utilizing a microscopic model of lipids. In particular, we examine the dependence of the period of a lamellar phase on the hydrophobic length and volume fraction of a rigid, integral, peptide. We find that the period decreases when a short peptide, such that d<D_h, is inserted. More surprising, we find that the period increases when a long peptide, such that d>D_h, is inserted. The effect is due to the replacement of extensible lipid tails by rigid peptide. As the peptide length is increased, the lamellar period continues to increase, but at a slower rate, and can eventually decrease. The amount of peptide which fails to incorporate and span the membrane increases with the magnitude of the hydrophobic mismatch |d-D_h|. We explicate these behaviors which are all in accord with experiment. Predictions are made for the dependence of the tilt of a single trans-membrane alpha helix on hydrophobic mismatch and helix density.Comment: 14 pages, 5 figure

Thermoelastic study of nanolayered structures using time-resolved x-ray diffraction at high repetition rate

We investigate the thermoelastic response of a nanolayered sample composed of a metallic SrRuO3 (SRO) electrode sandwiched between a ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) film with negative thermal expansion and a SrTiO3 substrate. SRO is rapidly heated by fs-laser pulses with 208 kHz repetition rate. Diffraction of x-ray pulses derived from a synchrotron measures the transient out-of-plane lattice constant c of all three materials simultaneously from 120 ps to 5 mus with a relative accuracy up to Delta c/c = 10^-6. The in-plane propagation of sound is essential for understanding the delayed out of plane expansion.Comment: 5 pages, 3 figure

Quantum versus thermal fluctuations in the fcc antiferromagnet: alternative routes to order by disorder

In frustrated magnetic systems with competing interactions fluctuations can lift the residual accidental degeneracy. We argue that the state selection may have different outcomes for quantum and thermal order by disorder. As an example, we consider the semiclassical Heisenberg fcc antiferromagnet with only the nearest-neighbor interactions. Zero-point oscillations select the type 3 collinear antiferromagnetic state at T=0. Thermal fluctuations favor instead the type 1 antiferromagnetic structure. The opposite tendencies result in a finite-temperature transition between the two collinear states. Competition between effects of quantum and thermal order by disorder is a general phenomenon and is also realized in the J1-J2 square-lattice antiferromagnet at the critical point J2 = 0.5 J1.Comment: 6 pages, 3 figures (accepted PRB Rapid Comm.

Wetting on a spherical wall: influence of liquid-gas interfacial properties

We study the equilibrium of a liquid film on an attractive spherical substrate for an intermolecular interaction model exhibiting both fluid-fluid and fluid-wall long-range forces. We first reexamine the wetting properties of the model in the zero-curvature limit, i.e., for a planar wall, using an effective interfacial Hamiltonian approach in the framework of the well known sharp-kink approximation (SKA). We obtain very good agreement with a mean-field density functional theory (DFT), fully justifying the use of SKA in this limit. We then turn our attention to substrates of finite curvature and appropriately modify the so-called soft-interface approximation (SIA) originally formulated by Napi\'orkowski and Dietrich [Phys. Rev. B 34, 6469 (1986)] for critical wetting on a planar wall. A detailed asymptotic analysis of SIA confirms the SKA functional form for the film growth. However, it turns out that the agreement between SKA and our DFT is only qualitative. We then show that the quantitative discrepancy between the two is due to the overestimation of the liquid-gas surface tension within SKA. On the other hand, by relaxing the assumption of a sharp interface, with, e.g., a simple smoothing of the density profile there, markedly improves the predictive capability of the theory, making it quantitative and showing that the liquid-gas surface tension plays a crucial role when describing wetting on a curved substrate. In addition, we show that in contrast to SKA, SIA predicts the expected mean-field critical exponent of the liquid-gas surface tension