7,084 research outputs found
Healthiness from Duality
Healthiness is a good old question in program logics that dates back to
Dijkstra. It asks for an intrinsic characterization of those predicate
transformers which arise as the (backward) interpretation of a certain class of
programs. There are several results known for healthiness conditions: for
deterministic programs, nondeterministic ones, probabilistic ones, etc.
Building upon our previous works on so-called state-and-effect triangles, we
contribute a unified categorical framework for investigating healthiness
conditions. We find the framework to be centered around a dual adjunction
induced by a dualizing object, together with our notion of relative
Eilenberg-Moore algebra playing fundamental roles too. The latter notion seems
interesting in its own right in the context of monads, Lawvere theories and
enriched categories.Comment: 13 pages, Extended version with appendices of a paper accepted to
LICS 201
Fluids of platelike particles near a hard wall
Fluids consisting of hard platelike particles near a hard wall are
investigated using density functional theory. The density and orientational
profiles as well as the surface tension and the excess coverage are determined
and compared with those of a fluid of rodlike particles. Even for low densities
slight orientational packing effects are found for the platelet fluid due to
larger intermolecular interactions between platelets as compared with those
between rods. A net depletion of platelets near the wall is exhibited by the
excess coverage, whereas a change of sign of the excess coverage of hard-rod
fluids is found upon increasing the bulk density.Comment: 6 pages, 9 figure
Theory of asymmetric non-additive binary hard-sphere mixtures
We show that the formal procedure of integrating out the degrees of freedom
of the small spheres in a binary hard-sphere mixture works equally well for
non-additive as it does for additive mixtures. For highly asymmetric mixtures
(small size ratios) the resulting effective Hamiltonian of the one-component
fluid of big spheres, which consists of an infinite number of many-body
interactions, should be accurately approximated by truncating after the term
describing the effective pair interaction. Using a density functional treatment
developed originally for additive hard-sphere mixtures we determine the zero,
one, and two-body contribution to the effective Hamiltonian. We demonstrate
that even small degrees of positive or negative non-additivity have significant
effect on the shape of the depletion potential. The second virial coefficient
, corresponding to the effective pair interaction between two big spheres,
is found to be a sensitive measure of the effects of non-additivity. The
variation of with the density of the small spheres shows significantly
different behavior for additive, slightly positive and slightly negative
non-additive mixtures. We discuss the possible repercussions of these results
for the phase behavior of binary hard-sphere mixtures and suggest that
measurements of might provide a means of determining the degree of
non-additivity in real colloidal mixtures
On the Detectability of the Hydrogen 3-cm Fine Structure Line from the EoR
A soft ultraviolet radiation field, 10.2 eV < E <13.6 eV, that permeates
neutral intergalactic gas during the Epoch of Reionization (EoR) excites the 2p
(directly) and 2s (indirectly) states of atomic hydrogen. Because the 2s state
is metastable, the lifetime of atoms in this level is relatively long, which
may cause the 2s state to be overpopulated relative to the 2p state. It has
recently been proposed that for this reason, neutral intergalactic atomic
hydrogen gas may be detected in absorption in its 3-cm fine-structure line
(2s_1/2 -> 2p_3/2) against the Cosmic Microwave Background out to very high
redshifts. In particular, the optical depth in the fine-structure line through
neutral intergalactic gas surrounding bright quasars during the EoR may reach
tau~1e-5. The resulting surface brightness temperature of tens of micro K (in
absorption) may be detectable with existing radio telescopes. Motivated by this
exciting proposal, we perform a detailed analysis of the transfer of Lyman
beta,gamma,delta,... radiation, and re-analyze the detectability of the
fine-structure line in neutral intergalactic gas surrounding high-redshift
quasars. We find that proper radiative transfer modeling causes the
fine-structure absorption signature to be reduced tremendously to tau< 1e-10.
We therefore conclude that neutral intergalactic gas during the EoR cannot
reveal its presence in the 3-cm fine-structure line to existing radio
telescopes.Comment: 7 pages, 4 figures, MNRAS in press; v2. some typos fixe
Purification, crystallization and preliminary X-ray crystallographic analysis of 3-ketosteroid Delta(1) -dehydrogenase from Rhodococcus erythropolis SQ1
3-Ketosteroid Delta(1)-dehydrogenase plays a crucial role in the early steps of steroid degradation by introducing a double bond between the C1 and C2 atoms of the A-ring of its 3-ketosteroid substrates. The 3-ketosteroid Delta(1)-dehydrogenase from Rhodococcus erythropolis SQ1, a 56 kDa flavoprotein, was crystallized using the sitting-drop vapour-diffusion method at room temperature. The crystals grew in various buffers over a wide pH range (from pH 5.5 to 10.5), but the best crystallization condition consisted of 2%(nu/nu) PEG 400, 0.1 M HEPES pH 7.5, 2.0 M ammonium sulfate. A native crystal diffracted X-rays to 2.0 angstrom resolution. It belonged to the primitive orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 107.4, b = 131.6, c = 363.2 angstrom, and contained eight molecules in the asymmetric unit. The initial structure of the enzyme was solved using multi-wavelength anomalous dispersion (MAD) data collected from a Pt-derivatized crystal
Phase behavior of hard spheres confined between parallel hard plates: Manipulation of colloidal crystal structures by confinement
We study the phase behavior of hard spheres confined between two parallel
hard plates using extensive computer simulations. We determine the full
equilibrium phase diagram for arbitrary densities and plate separations from
one to five hard-sphere diameters using free energy calculations. We find a
first-order fluid-solid transition, which corresponds to either capillary
freezing or melting depending on the plate separation. The coexisting solid
phase consists of crystalline layers with either triangular or square symmetry.
Increasing the plate separation, we find a sequence of crystal structures from
n triangular to (n+1) square to (n+1) triangular, where n is the number of
crystal layers, in agreement with experiments on colloids. At high densities,
the transition between square to triangular phases are intervened by
intermediate structures, e.g., prism, buckled, and rhombic phases.Comment: 9 pages, 4 figures. Accepted for publication in J. Phys.: Condens.
Matte
Phase behaviour of charged colloidal sphere dispersions with added polymer chains
We study the stability of mixtures of highly screened repulsive charged
spheres and non-adsorbing ideal polymer chains in a common solvent using free
volume theory. The effective interaction between charged colloids in an aqueous
salt solution is described by a screened-Coulomb pair potential, which
supplements the pure hard-sphere interaction. The ideal polymer chains are
treated as spheres that are excluded from the colloids by a hard-core
interaction, whereas the interaction between two ideal chains is set to zero.
In addition, we investigate the phase behaviour of charged colloid-polymer
mixtures in computer simulations, using the two-body (Asakura-Oosawa pair
potential) approximation to the effective one-component Hamiltonian of the
charged colloids. Both our results obtained from simulations and from free
volume theory show similar trends. We find that the screened-Coulomb repulsion
counteracts the effect of the effective polymer-mediated attraction. For
mixtures of small polymers and relatively large charged colloidal spheres, the
fluid-crystal transition shifts to significantly larger polymer concentrations
with increasing range of the screened-Coulomb repulsion. For relatively large
polymers, the effect of the screened-Coulomb repulsion is weaker. The resulting
fluid-fluid binodal is only slightly shifted towards larger polymer
concentrations upon increasing the range of the screened-Coulomb repulsion. In
conclusion, our results show that the miscibility of dispersions containing
charged colloids and neutral non-adsorbing polymers increases, upon increasing
the range of the screened-Coulomb repulsion, or upon lowering the salt
concentration, especially when the polymers are small compared to the colloids.Comment: 25 pages,13 figures, accepted for publication on J.Phys.:Condens.
Matte
Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?
We study sedimentation equilibrium of low-salt suspensions of binary mixtures
of charged colloids, both by Monte Carlo simulations of an effective
colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture.
We show that the theoretically predicted lifting and layering effect, which
involves the entropy of the screening ions and a spontaneous macroscopic
electric field [J. Zwanikken and R. van Roij, Europhys. Lett. {\bf 71}, 480
(2005)], can also be understood on the basis of an effective colloid-only
system with pairwise screened-Coulomb interactions. We consider, by theory and
by simulation, both repelling like-charged colloids and attracting oppositely
charged colloids, and we find a re-entrant lifting and layering phenomenon when
the charge ratio of the colloids varies from large positive through zero to
large negative values
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