Introductory Physics: Writing scheme teaches science to non-scientists

DOI: 10.1088/0031-9120/42/6/F05 http://iopscience.iop.org/0031-9120/42/6/F05/pdf/0031-9120_42_6_F05.pdfWriting-intensive activities can be made use of to implement a 'narrow-but-deep' approach in an undergraduate introductory physics course for non-science majors. In this approach, a carefully selected number of topics are treated not only in more detail but also with attention to developing them logically and rigorously. We teach a course that utilizes parts of an interdisciplinary text by Alan Lightman [1] and focuses on three subjects: (i) the conservation of energy, (ii) the second law of thermodynamics and (iii) the special theory of relativity

Molecular dynamics studies of the melting of butane and hexane monolayers adsorbed on the basal‐plane surface of graphite

doi:10.1063/1.465067The effect of molecular steric properties on the melting of quasi‐two‐dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal‐plane surface of graphite. These molecules differ only in their length, being members of the n‐alkane series [CH3(CH2)n−2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule-substrate interactions. Nearest‐neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x‐ray scattering experiments. The melting of the low‐temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and the orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid HB phase transforms to a rectangular‐centered structure with a short coherence length in coexistence with a fluid phase. A significant result of the simulations is that they demonstrate the importance of molecular flexibility on the nature of the melting transition. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. The effect of molecular length on the qualitative nature of the melting process is discussed for both monolayers.This work was supported by The Danish Natural Science Research Council Grant No. M 11-7015, the U.S. NSF Grants No. DMR-8704938 and No. DMR-9011069,and the Pittsburgh Supercomputing Center Grant No. DMR-880008P

Application of inelastic epithermal neutron scattering to the vibrational spectroscopy of adsorbed molecules: Butane physisorbed on graphite (0001) surfaces

doi:10.1063/1.448924Inelastic epithermal neutron scattering (IENS) has been used to investigate the intramolecular vibrations of butane [CH3(CH2)2CH3] physisorbed on the (0001) surfaces of a graphite powder. The purpose of these studies was to assess IENS as a vibrational spectroscopy for adsorbed species by using a relatively well‐characterized substrate (Carbopack B). The experiments were performed on the IN1 spectrometer located on a beam from the ''hot'' source at the Institut Laue‐Langevin reactor in Grenoble. Film coverages of 1.0 and 3.6 layers were investigated at a temperature of 78 K. The IENS spectra are rich in structure, containing eight vibrational bands in the energy‐transfer range from 280-3470 cm−1 (35-430 meV). The similarity in the spectra at the two coverages investigated indicates that in this energy‐transfer range the butane intramolecular modes are not strongly perturbed by physisorption on graphite. A detailed comparison is made between the monolayer IENS spectrum and those calculated from models of a free and adsorbed molecule. Both models employ a previously derived intramolecular force field in the normal mode calculation. A third calculation assumes the displacement eigenvectors of the free molecule but replaces the mode eigenfrequencies by their measured Raman and IR values. It reproduces the observed IENS spectra very well. The large number of vibrational bands observed and the close agreement with the calculated spectra suggest butane as a desirable adsorbate for similar experiments on catalytic substrates.This work was supported in part by National Science Foundation Grants DMR-1905958, INT- 8012228, and DMR-8304366 and by a grant of the Petroleum Research fund, administered by the American Chemical Society

Localized diffusive motion on two different time scales in solid alkane nanoparticles

doi: 10.1209/0295-5075/91/66007High-energy-resolution quasielastic neutron scattering on three complementary spectrometers has been used to investigate molecular diffusive motion in solid nano- to bulk-sized particles of the alkane n-C32H66. The crystalline-to-plastic and plastic-to-fluid phase transition temperatures are observed to decrease as the particle size decreases. In all samples, localized molecular diffusive motion in the plastic phase occurs on two different time scales: a "fast" motion corresponding to uniaxial rotation about the long molecular axis; and a "slow" motion attributed to conformational changes of the molecule. Contrary to the conventional interpretation in bulk alkanes, the fast uniaxial rotation begins in the low- temperature crystalline phase.This work was supported by the U.S. National Science Foundation under Grant No. DMR-0705974 and utilized facilities supported in part by the NSF under agreement No. DMR-0454672. A portion of this research at Oak Ridge National Laboratory's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy

Studies of the structure and growth mode of dotriacontane films by synchrotron x-ray scattering and molecular dynamics simulations

doi: 10.1088/0953-8984/16/29/005We report on synchrotron x-ray scattering experiments and molecular dynamics simulations of the structure and growth mode of dotriacontane (n-C32H66 or C32) films adsorbed on Ag(111) and SiO2-coated Si(100) substrates. On the SiO2 surface, the x-ray measurements confirm a structural model of the solid film inferred from high-resolution ellipsometry measurements in which one or two layers of C32 adsorb with the long axis of the molecule oriented parallel to the interface followed by a monolayer in which the molecules have a perpendicular orientation. At higher C32 coverages, preferentially oriented bulk particles nucleate, consistent with a Stranski-Krastanov growth mode. On the Ag(111) surface, we again observe one or two layers of the 'parallel' film but no evidence of the perpendicular monolayer before nucleation of the preferentially oriented bulk particles. We compare the experimentally observed structures with molecular dynamics simulations of a multilayer film of the homologous C24 molecule.This work was support by US National Science Foundation under Grant Nos. DMR-9802476 and DMR-0109057. The Midwest Universities Collaborative Access Team (MUCAT) sector at the Advanced Photon Source (APS) is supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), through Ames Laboratory under ContractNo.W-7405-Eng-82. Use of the APS was supported by the DOE BES under Contract No. W-31-109-ENG-38

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

doi:10.1063/1.3212095We present evidence from neutron diffraction measurements and molecular dynamics (MD) simulations of three different monolayer phases of the intermediate-length alkanes tetracosane (n-C24H50 denoted as C24) and dotriacontane (n-C32H66 denoted as C32) adsorbed on a graphite basal-plane surface. Our measurements indicate that the two monolayer films differ principally in the transition temperatures between phases. At the lowest temperatures, both C24 and C32 form a crystalline monolayer phase with a rectangular-centered (RC) structure. The two sublattices of the RC structure each consists of parallel rows of molecules in their all-trans conformation aligned with their long axis parallel to the surface and forming so-called lamellas of width approximately equal to the all-trans length of the molecule. The RC structure is uniaxially commensurate with the graphite surface in its [110] direction such that the distance between molecular rows in a lamella is 4.26?� = mathag, where ag = 2.46?� is the lattice constant of the graphite basal plane. Molecules in adjacent rows of a lamella alternate in orientation between the carbon skeletal plane being parallel and perpendicular to the graphite surface. Upon heating, the crystalline monolayers transform to a "smectic" phase in which the inter-row spacing within a lamella expands by ~10% and the molecules are predominantly oriented with the carbon skeletal plane parallel to the graphite surface. In the smectic phase, the MD simulations show evidence of broadening of the lamella boundaries as a result of molecules diffusing parallel to their long axis. At still higher temperatures, they indicate that the introduction of gauche defects into the alkane chains drives a melting transition to a monolayer fluid phase as reported previously.This work was supported by U.S. National Science Foundation under Grant Nos. DMR-0411748 and DMR-0705974

Crystalline-to-plastic phase transitions in molecularly thin n-dotriacontane films adsorbed on solid surfaces

doi:10.1063/1.3213642Crystalline-to-rotator phase transitions have been widely studied in bulk hydrocarbons, in particular in normal alkanes. But few studies of these transitions deal with molecularly thin films of pure n-alkanes on solid substrates. In this work, we were able to grow dotriacontane (n-C32H66) films without coexisting bulk particles, which allows us to isolate the contribution to the ellipsometric signal from a monolayer of molecules oriented with their long axis perpendicular to the SiO2 surface. For these submonolayer films, we found a step in the ellipsometer signal at ~331 K, which we identify with a solid-solid phase transition. At higher coverages, we observed additional steps in the ellipsometric signal that we identify with a solid-solid phase transition in multilayer islands (~333 K) and with the transition to the rotator phase in bulk crystallites (~337 K), respectively. After considering three alternative explanations, we propose that the step upward in the ellipsometric signal observed at ~331 K on heating the submonolayer film is the signature of a transition from a perpendicular monolayer phase to a denser phase in which the alkane chains contain on average one to two gauche defects per molecule.This work was supported by the Chilean government through FONDECYT Grant Nos. 1060628 and 7080105 and by CONICYT scholarships (E.A.C., V.d.C. and P.A.S.), and by the U.S. NSF Grant No. DMR-0705974

Magnetic Field Generation in Stars

Enormous progress has been made on observing stellar magnetism in stars from the main sequence through to compact objects. Recent data have thrown into sharper relief the vexed question of the origin of stellar magnetic fields, which remains one of the main unanswered questions in astrophysics. In this chapter we review recent work in this area of research. In particular, we look at the fossil field hypothesis which links magnetism in compact stars to magnetism in main sequence and pre-main sequence stars and we consider why its feasibility has now been questioned particularly in the context of highly magnetic white dwarfs. We also review the fossil versus dynamo debate in the context of neutron stars and the roles played by key physical processes such as buoyancy, helicity, and superfluid turbulence,in the generation and stability of neutron star fields. Independent information on the internal magnetic field of neutron stars will come from future gravitational wave detections. Thus we maybe at the dawn of a new era of exciting discoveries in compact star magnetism driven by the opening of a new, non-electromagnetic observational window. We also review recent advances in the theory and computation of magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo theory. These advances offer insight into the action of stellar dynamos as well as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field generation in stars to appear in Space Science Reviews, Springe

Lymphocyte recruitment and homing to the liver in primary biliary cirrhosis and primary sclerosing cholangitis

The mechanisms operating in lymphocyte recruitment and homing to liver are reviewed. A literature review was performed on primary biliary cirrhosis (PBC), progressive sclerosing cholangitis (PSC), and homing mechanisms; a total of 130 papers were selected for discussion. Available data suggest that in addition to a specific role for CCL25 in PSC, the CC chemokines CCL21 and CCL28 and the CXC chemokines CXCL9 and CXCL10 are involved in the recruitment of T lymphocytes into the portal tract in PBC and PSC. Once entering the liver, lymphocytes localize to bile duct and retain by the combinatorial or sequential action of CXCL12, CXCL16, CX3CL1, and CCL28 and possibly CXCL9 and CXCL10. The relative importance of these chemokines in the recruitment or the retention of lymphocytes around the bile ducts remains unclear. The available data remain limited but underscore the importance of recruitment and homing

Application of empirical interatomic potentials to the calculation of the structure and dynamics of paraffin molecules adsorbed on graphite

URL:http://link.aps.org/doi/10.1103/PhysRevB.19.6542 DOI:10.1103/PhysRevB.19.6542The adsorption of a single molecule of ethane, propane, and butane on a basal-plane surface of graphite is investigated using empirically derived carbon-hydrogen and carbon-carbon potentials. The principal motivation for this study is the calculation of a number of structural and dynamical properties which can be compared with neutron scattering experiments on monolayer paraffin films adsorbed on graphite. For each molecule the equilibrium orientation and position with respect to the surface and contours of minimum potential energy have been calculated. We find that these molecules prefer to align themselves with their carbon skeleton parallel to the graphite basal plane with the hydrogen atoms closest to the surface occupying the center of the graphite carbon hexagons. We have also found that each of the molecules tends to distort upon adsorption by small antisymmetric rotation of the CH3 groups about the terminal C-C bonds. Two different force-constant models have been used to calculate the frequencies of the surface vibratory modes of the adsorbed molecules. The calculated frequencies and, with few exceptions, the force constants are in reasonable agreement with values inferred previously from inelastic-neutron-scattering spectra of monolayer butane films. Finally, the anharmonicity of the molecule-substrate potential has been briefly studied by examining the temperature dependence of the height of a butane molecule above the graphite basal plane and of the frequency of the molecular vibration normal to the surface.One of us (F.Y.H.) is grateful for partial support of this research by the Danish National Science Research Council. The other of us (H.T.) wishes to acknowledge partial support of this research by the Research Corporation, the University of Missouri Research Reactor Facility, and the University of Missouri Research Council