9,252 research outputs found
Contact lines for fluid surface adhesion
When a fluid surface adheres to a substrate, the location of the contact line
adjusts in order to minimize the overall energy. This adhesion balance implies
boundary conditions which depend on the characteristic surface deformation
energies. We develop a general geometrical framework within which these
conditions can be systematically derived. We treat both adhesion to a rigid
substrate as well as adhesion between two fluid surfaces, and illustrate our
general results for several important Hamiltonians involving both curvature and
curvature gradients. Some of these have previously been studied using very
different techniques, others are to our knowledge new. What becomes clear in
our approach is that, except for capillary phenomena, these boundary conditions
are not the manifestation of a local force balance, even if the concept of
surface stress is properly generalized. Hamiltonians containing higher order
surface derivatives are not just sensitive to boundary translations but also
notice changes in slope or even curvature. Both the necessity and the
functional form of the corresponding additional contributions follow readily
from our treatment.Comment: 8 pages, 2 figures, LaTeX, RevTeX styl
Disentangling positivity constraints for generalized parton distributions
Positivity constraints are derived for the generalized parton distributions
(GPDs) of spin-1/2 hadrons. The analysis covers the full set of eight twist-2
GPDs. Several new inequalities are obtained which constrain GPDs by various
combinations of usual (forward) unpolarized and polarized parton distributions
including the transversity distribution.Comment: 9 pages (REVTEX), typos correcte
Rapid prototyped porous nickel-titanium scaffolds as bone substitutes
While calcium phosphate–based ceramics are currently the most widely used materials in bone repair, they generally lack tensile strength for initial load bearing. Bulk titanium is the gold standard of metallic implant materials, but does not match the mechanical properties of the surrounding bone, potentially leading to problems of fixation and bone resorption. As an alternative, nickel–titanium alloys possess a unique combination of mechanical properties including a relatively low elastic modulus, pseudoelasticity, and high damping capacity, matching the properties of bone better than any other metallic material. With the ultimate goal of fabricating porous implants for spinal, orthopedic and dental applications, nickel–titanium substrates were fabricated by means of selective laser melting. The response of human mesenchymal stromal cells to the nickel–titanium substrates was compared to mesenchymal stromal cells cultured on clinically used titanium. Selective laser melted titanium as well as surface-treated nickel–titanium and titanium served as controls. Mesenchymal stromal cells had similar proliferation rates when cultured on selective laser melted nickel–titanium, clinically used titanium, or controls. Osteogenic differentiation was similar for mesenchymal stromal cells cultured on the selected materials, as indicated by similar gene expression levels of bone sialoprotein and osteocalcin. Mesenchymal stromal cells seeded and cultured on porous three-dimensional selective laser melted nickel–titanium scaffolds homogeneously colonized the scaffold, and following osteogenic induction, filled the scaffold’s pore volume with extracellular matrix. The combination of bone-related mechanical properties of selective laser melted nickel–titanium with its cytocompatibility and support of osteogenic differentiation of mesenchymal stromal cells highlights its potential as a superior bone substitute as compared to clinically used titanium
How to determine local elastic properties of lipid bilayer membranes from atomic-force-microscope measurements: A theoretical analysis
Measurements with an atomic force microscope (AFM) offer a direct way to
probe elastic properties of lipid bilayer membranes locally: provided the
underlying stress-strain relation is known, material parameters such as surface
tension or bending rigidity may be deduced. In a recent experiment a
pore-spanning membrane was poked with an AFM tip, yielding a linear behavior of
the force-indentation curves. A theoretical model for this case is presented
here which describes these curves in the framework of Helfrich theory. The
linear behavior of the measurements is reproduced if one neglects the influence
of adhesion between tip and membrane. Including it via an adhesion balance
changes the situation significantly: force-distance curves cease to be linear,
hysteresis and nonzero detachment forces can show up. The characteristics of
this rich scenario are discussed in detail in this article.Comment: 14 pages, 9 figures, REVTeX4 style. New version corresponds to the
one accepted by PRE. The result section is restructured: a comparison to
experimental findings is included; the discussion on the influence of
adhesion between AFM tip and membrane is extende
Balancing torques in membrane-mediated interactions: Exact results and numerical illustrations
Torques on interfaces can be described by a divergence-free tensor which is
fully encoded in the geometry. This tensor consists of two terms, one
originating in the couple of the stress, the other capturing an intrinsic
contribution due to curvature. In analogy to the description of forces in terms
of a stress tensor, the torque on a particle can be expressed as a line
integral along any contour surrounding the particle. Interactions between
particles mediated by a fluid membrane are studied within this framework. In
particular, torque balance places a strong constraint on the shape of the
membrane. Symmetric two-particle configurations admit simple analytical
expressions which are valid in the fully nonlinear regime; in particular, the
problem may be solved exactly in the case of two membrane-bound parallel
cylinders. This apparently simple system provides some flavor of the remarkably
subtle nonlinear behavior associated with membrane-mediated interactions.Comment: 16 pages, 10 figures, REVTeX4 style. The Gaussian curvature term was
included in the membrane Hamiltonian; section II.B was rephrased to smoothen
the flow of presentatio
Correlated ab-initio calculations for ground-state properties of II-VI semiconductors
Correlated ab-initio ground-state calculations, using relativistic
energy-consistent pseudopotentials, are performed for six II-VI semiconductors.
Valence () correlations are evaluated using the coupled cluster approach
with single and double excitations. An incremental scheme is applied based on
correlation contributions of localized bond orbitals and of pairs and triples
of such bonds. In view of the high polarity of the bonds in II-VI compounds, we
examine both, ionic and covalent embedding schemes for the calculation of
individual bond increments. Also, a partitioning of the correlation energy
according to local ionic increments is tested. Core-valence ()
correlation effects are taken into account via a core-polarization potential.
Combining the results at the correlated level with corresponding Hartree-Fock
data we recover about 94% of the experimental cohesive energies; lattice
constants are accurate to \sim 1%; bulk moduli are on average 10% too large
compared with experiment.Comment: 10 pages, twocolumn, RevTex, 3 figures, accepted Phys. Rev.
Atomic micromotion and geometric forces in a triaxial magnetic trap
Non-adiabatic motion of Bose-Einstein condensates of rubidium atoms arising
from the dynamical nature of a time-orbiting-potential (TOP) trap was observed
experimentally. The orbital micromotion of the condensate in velocity space at
the frequency of the rotating bias field of the TOP was detected by a
time-of-flight method. A dependence of the equilibrium position of the atoms on
the sense of rotation of the bias field was observed. We have compared our
experimental findings with numerical simulations. The nonadiabatic following of
the atomic spin in the trap rotating magnetic field produces geometric forces
acting on the trapped atoms.Comment: 4 pages, 4 figure
Carrageenan Is a Potent Inhibitor of Papillomavirus Infection
Certain sexually transmitted human papillomavirus (HPV) types are causally associated with the development of cervical cancer. Our recent development of high-titer HPV pseudoviruses has made it possible to perform high-throughput in vitro screens to identify HPV infection inhibitors. Comparison of a variety of compounds revealed that carrageenan, a type of sulfated polysaccharide extracted from red algae, is an extremely potent infection inhibitor for a broad range of sexually transmitted HPVs. Although carrageenan can inhibit herpes simplex viruses and some strains of HIV in vitro, genital HPVs are about a thousand-fold more susceptible, with 50% inhibitory doses in the low ng/ml range. Carrageenan acts primarily by preventing the binding of HPV virions to cells. This finding is consistent with the fact that carrageenan resembles heparan sulfate, an HPV cell-attachment factor. However, carrageenan is three orders of magnitude more potent than heparin, a form of cell-free heparan sulfate that has been regarded as a highly effective model HPV inhibitor. Carrageenan can also block HPV infection through a second, postattachment heparan sulfate–independent effect. Carrageenan is in widespread commercial use as a thickener in a variety of cosmetic and food products, ranging from sexual lubricants to infant feeding formulas. Some of these products block HPV infectivity in vitro, even when diluted a million-fold. Clinical trials are needed to determine whether carrageenan-based products are effective as topical microbicides against genital HPVs
Effect of incoherent scattering on shot noise correlations in the quantum Hall regime
We investigate the effect of incoherent scattering in a Hanbury Brown and
Twiss situation with electrons in edge states of a three-terminal conductor
submitted to a strong perpendicular magnetic field. The modelization of
incoherent scattering is performed by introducing an additional voltage probe
through which the current is kept equal to zero which causes voltage
fluctuations at this probe. It is shown that inelastic scattering can lead in
this framework to positive correlations, whereas correlations remain always
negative for quasi-elastic scattering.Comment: 5 pages latex, 5 eps figure
Imaging Inter-Edge State Scattering Centers in the Quantum Hall Regime
We use an atomic force microscope tip as a local gate to study the scattering
between edge channels in a 2D electron gas in the quantum Hall regime. The
scattering is dominated by individual, microscopic scattering centers, which we
directly image here for the first time. The tip voltage dependence of the
scattering indicates that tunneling occurs through weak links and localized
states.Comment: 4 pages, 5 figure
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