7,834 research outputs found
Density profiles and surface tensions of polymers near colloidal surfaces
The surface tension of interacting polymers in a good solvent is calculated
theoretically and by computer simulations for a planar wall geometry and for
the insertion of a single colloidal hard-sphere. This is achieved for the
planar wall and for the larger spheres by an adsorption method, and for smaller
spheres by a direct insertion technique. Results for the dilute and semi-dilute
regimes are compared to results for ideal polymers, the Asakura-Oosawa
penetrable-sphere model, and to integral equations, scaling and renormalization
group theories. The largest relative changes with density are found in the
dilute regime, so that theories based on non-interacting polymers rapidly break
down. A recently developed ``soft colloid'' approach to polymer-colloid
mixtures is shown to correctly describe the one-body insertion free-energy and
the related surface tension
Laser radiation pressure slowing of a molecular beam
There is substantial interest in producing samples of ultracold molecules for
possible applications in quantum computation, quantum simulation of condensed
matter systems, precision measurements, controlled chemistry, and high
precision spectroscopy. A crucial step to obtaining large samples of ultracold,
trapped molecules is developing a means to bridge the gap between typical
molecular source velocities (~150-600 m/s) and velocities for which trap
loading or confinement is possible (~5-20 m/s). Here we show deceleration of a
beam of neutral strontium monofluoride (SrF) molecules using radiative force.
Under certain conditions, the deceleration results in a substantial flux of
molecules with velocities <50 m/s. The observed slowing, from ~140 m/s,
corresponds to scattering ~10000 photons. We also observe longitudinal velocity
compression under different conditions. Combined with molecular laser cooling
techniques, this lays the groundwork to create slow and cold molecular beams
suitable for trap loading.Comment: 7 pages, 7 figures. Supplementary material updated
Coarse-graining polymers as soft colloids
We show how to coarse grain polymers in a good solvent as single particles,
interacting with density-independent or density-dependent interactions. These
interactions can be between the centres of mass, the mid-points or end-points
of the polymers. We also show how to extend these methods to polymers in poor
solvents and mixtures of polymers. Treating polymers as soft colloids can
greatly speed up the simulation of complex many-polymer systems, including
polymer-colloid mixtures.Comment: to appear in Physica A, special STATPHYS 2001 edition. Content of
invited talk by AA
Accurate effective pair potentials for polymer solutions
Dilute or semi-dilute solutions of non-intersecting self-avoiding walk (SAW)
polymer chains are mapped onto a fluid of ``soft'' particles interacting via an
effective pair potential between their centers of mass. This mapping is
achieved by inverting the pair distribution function of the centers of mass of
the original polymer chains, using integral equation techniques from the theory
of simple fluids. The resulting effective pair potential is finite at all
distances, has a range of the order of the radius of gyration, and turns out to
be only moderately concentration-dependent. The dependence of the effective
potential on polymer length is analyzed in an effort to extract the scaling
limit. The effective potential is used to derive the osmotic equation of state,
which is compared to simulation data for the full SAW segment model, and to the
predictions of renormalization group calculations. A similar inversion
procedure is used to derive an effective wall-polymer potential from the center
of mass density profiles near the wall, obtained from simulations of the full
polymer segment model. The resulting wall-polymer potential turns out to depend
strongly on bulk polymer concentration when polymer-polymer correlations are
taken into account, leading to a considerable enhancement of the effective
repulsion with increasing concentration. The effective polymer-polymer and
wall-polymer potentials are combined to calculate the depletion interaction
induced by SAW polymers between two walls. The calculated depletion interaction
agrees well with the ``exact'' results from much more computer-intensive direct
simulation of the full polymer-segment model, and clearly illustrates the
inadequacy -- in the semi-dilute regime -- of the standard Asakura-Oosawa
approximation based on the assumption of non-interacting polymer coils.Comment: 18 pages, 24 figures, ReVTeX, submitted to J. Chem. Phy
Role of magnetic and orbital ordering at the metal-insulator transition in NdNiO3
Soft x-ray resonant scattering at the Ni L2,3 edges is used to test models of
magnetic and orbital-ordering below the metal-insulator transition in NdNiO3.
The large branching ratio of the L3 to L2 intensities of the (1/2,0,1/2)
reflection and the observed azimuthal angle and polarization dependence
originates from a non collinear magnetic structure. The absence of an orbital
signal and the non collinear magnetic structure show that the nickelates are
materials for which orbital ordering is absent at the metal-insulator
transition.Comment: 10 pages, 4 figures, Physical Review B rapid communication, to be
publishe
Vertical bone augmentation and regular implants versus short implants in the vertically deficient posterior mandible:a systematic review and meta-analysis of randomized studies
Item does not contain fulltextThe aim of this study was to perform a systematic review and meta-analysis of randomized controlled trials (RCTs) comparing the outcomes of short dental implants (≤7mm) versus vertical bone augmentation followed by regular dental implants (>7mm) in the deficient posterior mandible. In total, eight RCTs (six using interpositional sandwich grafting and two using a guided bone regeneration technique) were reported in 17 articles at different time points. In the meta-analysis of the sandwich group, the relative risk (RR) for implant loss at 1year was in favour of short implants (RR 0.41, P=0.02), while no significant difference was found at 3 years (RR 0.65, P=0.43), 5 years (RR 1.08, P=0.86), or 8 years (RR 1.53, P=0.52). The risk of complications was in favour of short implants (RR 0.34, P=0.0002), as was the mean difference in marginal bone resorption after 1 year (-0.09mm, P=0.17), 3 years (-0.32mm, P<0.00001), 5 years (-0.65mm, P<0.00001), and 8 years (-0.88, P<0.00001). The mean residual osseointegration length of the implants was between 2.94mm and 4.44mm in the short implants group and between 7.97mm and 8.62mm in the regular implants group after 5 years. In conclusion, in the deficient atrophic posterior mandible, short implants and regular implants demonstrate comparable outcomes within the first 5 years. Patients who are fit for surgery should be informed about the risks and benefits of both options
On the relationship between spectroscopic constants of diatomic molecules: a machine learning approach
Through a machine learning approach, we show that the equilibrium distance, harmonic vibrational frequency and binding energy of diatomic molecules are related, independently of the nature of the bond of a molecule; they depend solely on the group and period of the constituent atoms. As a result, we show that by employing the group and period of the atoms that form a molecule, the spectroscopic constants are predicted with an accuracy of <5%, whereas for the A-excited electronic state it is needed to include other atomic properties leading to an accuracy of <11%
Imaging Photoelectron Circular Dichroism in the Detachment of Mass‐Selected Chiral Anions
Photoelectron Circular Dichroism (PECD) is a forward-backward asymmetry in the photoemission from a non-racemic sample induced by circularly polarized light. PECD spectroscopy has potential analytical advantages for chiral discrimination over other chiroptical methods due to its increased sensitivity to the chiral potential of the molecule. The use of anions for PECD spectroscopy allows for mass-selectivity and provides a path to simple experimental schemes that employ table-top light sources. Evidence of PECD for anions is limited, and insight into the forces that govern PECD electron dynamics in photodetachment is absent. Here, we demonstrate a PECD effect in the photodetachment of mass-selected deprotonated 1-indanol anions. By utilizing velocity map imaging photoelectron spectroscopy with a tunable light source, we determine the energy-resolved PECD over a wide range of photon energies. The observed PECD reaches up to 11%, similar to what has been measured for neutral species
Phytophthora infestans Has a Plethora of Phospholipase D Enzymes Including a Subclass That Has Extracellular Activity
In eukaryotes phospholipase D (PLD) is involved in many cellular processes. Currently little is known about PLDs in oomycetes. Here we report that the oomycete plant pathogen Phytophthora infestans has a large repertoire of PLDs divided over six subfamilies: PXPH-PLD, PXTM-PLD, TM-PLD, PLD-likes, and type A and B sPLD-likes. Since the latter have signal peptides we developed a method using metabolically labelled phospholipids to monitor if P. infestans secretes PLD. In extracellular medium of ten P. infestans strains PLD activity was detected as demonstrated by the production of phosphatidic acid and the PLD specific marker phosphatidylalcohol
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