Mapping a Homopolymer onto a Model Fluid

We describe a linear homopolymer using a Grand Canonical ensemble formalism, a statistical representation that is very convenient for formal manipulations. We investigate the properties of a system where only next neighbor interactions and an external, confining, field are present, and then show how a general pair interaction can be introduced perturbatively, making use of a Mayer expansion. Through a diagrammatic analysis, we shall show how constitutive equations derived for the polymeric system are equivalent to the Ornstein-Zernike and P.Y. equations for a simple fluid, and find the implications of such a mapping for the simple situation of Van der Waals mean field model for the fluid.Comment: 12 pages, 3 figure

From density to interface fluctuations: the origin of wavelength dependence in surface tensions

The height-height correlation function for a fluctuating interface between two coexisting bulk phases is derived by means of general equilibrium properties of the corresponding density-density correlation function. A wavelength-dependent surface tension $\gamma(\mathbf{q})$ can be defined and expressed in terms of the direct correlation function $c(\mathbf{r},\mathbf{r}')$, the equilibrium density profile $\rho_{\circ}(\mathbf{r})$ and an operator which relates density to surface configurations. Neither the concept of an effective interface Hamiltonian nor the difference in pressure is needed to determine the general structure of the height-height correlations or $\gamma(\mathbf{q})$, respectively. This result generalizes the Mecke/Dietrich surface tension \gmd (Phys. Rev. E {\bf 59}, p. 6766 (1999)) and modifies recently published criticism concerning \gmd (P. Tarazona, R. Checa and, E.Chac\'{o}n: Phys. Rev. Lett. {\bf 99}, p. 196101 (2007)).Comment: 8 pages, 1 figur

Benzene degradation in a denitrifying biofilm reactor: activity and microbial community composition

Benzene is an aromatic compound and harmful for the environment. Biodegradation of benzene can reduce the toxicological risk after accidental or controlled release of this chemical in the environment. In this study, we further characterized an anaerobic continuous biofilm culture grown for more than 14 years on benzene with nitrate as electron acceptor. We determined steady state degradation rates, microbial community composition dynamics in the biofilm, and the initial anaerobic benzene degradation reactions. Benzene was degraded at a rate of 0.15 μmol/mg protein/day and a first-order rate constant of 3.04/day which was fourfold higher than rates reported previously. Bacteria belonging to the Peptococcaceae were found to play an important role in this anaerobic benzene-degrading biofilm culture, but also members of the Anaerolineaceae were predicted to be involved in benzene degradation or benzene metabolite degradation based on Illumina MiSeq analysis of 16S ribosomal RNA genes. Biomass retention in the reactor using a filtration finger resulted in reduction of benzene degradation capacity. Detection of the benzene carboxylase encoding gene, abcA, and benzoic acid in the culture vessel indicated that benzene degradation proceeds through an initial carboxylation step.</p

Anastomosis groups and pathogenicity of Rhizoctonia solani and binucleate Rhizoctonia from potatoes in South Africa

A survey of anastomosis groups (AGs) of Rhizoctonia species associated with potato diseases was conducted in South Africa. A total of 112 Rhizoctonia solani and 19 binucleate Rhizoctonia (BNR) isolates were recovered from diseased potato plants, characterized for AG and pathogenicity. The AG identity of the isolates was confirmed using phylogenetic analysis of the internal transcribed spacer region of ribosomal DNA. Rhizoctonia solani isolates recovered belonged to AG 3-PT, AG 2- 2IIIB, AG 4HG-I, AG 4HG-III and AG 5, while BNR isolates belonged to AG A and AG R, with frequencies of 74, 6.1, 2.3, 2.3, 0.8, 12.2 and 2.3%, respectively. Rhizoctonia solani AG 3-PT was the most predominant AG and occurred in all the potato growing regions sampled whereas the other AGs occurred in distinct locations. Different AGs grouped into distinct clades with high maximum parsimony and maximum likelihood bootstrap support for both R. solani and BNR. An experiment under greenhouse conditions with representative isolates from different AGs showed differences in aggressiveness between and within AGs. Isolates of AG 2-2IIIB, AG 4HG-III and AG R were the most aggressive in causing stem canker while AG 3-PT, AG 5 and AG R caused black scurf. This is the first comprehensive survey of R. solani and BNR on potatoes in South Africa using a molecularbased approach. This is the first report of R. solani AG 2-2IIIB and AG 4 HG-I causing stem and stolon canker and BNR AG A and AG R causing stem canker and black scurf on potatoes in South Africa.Potatoes South Africa.National Research Foundation of South Africa (UID: 78566 (NRF RISP grant for the ABI3500)).http://apsjournals.apsnet.org/loi/pdishb201

Variation in fungicide sensitivity among Rhizoctonia isolates recovered from potatoes in South Africa

Please read abstract in the article.Potatoes South Africahttp://apsjournals.apsnet.org/loi/pdishj2018Plant Production and Soil Scienc

Droplet minimizers for the Gates-Lebowitz-Penrose free energy functional

We study the structure of the constrained minimizers of the Gates-Lebowitz-Penrose free-energy functional ${\mathcal F}_{\rm GLP}(m)$, non-local functional of a density field $m(x)$, $x\in {\mathcal T}_L$, a $d$-dimensional torus of side length $L$. At low temperatures, ${\mathcal F}_{\rm GLP}$ is not convex, and has two distinct global minimizers, corresponding to two equilibrium states. Here we constrain the average density L^{-d}\int_{{\cal T}_L}m(x)\dd x to be a fixed value $n$ between the densities in the two equilibrium states, but close to the low density equilibrium value. In this case, a "droplet" of the high density phase may or may not form in a background of the low density phase, depending on the values $n$ and $L$. We determine the critical density for droplet formation, and the nature of the droplet, as a function of $n$ and $L$. The relation between the free energy and the large deviations functional for a particle model with long-range Kac potentials, proven in some cases, and expected to be true in general, then provides information on the structure of typical microscopic configurations of the Gibbs measure when the range of the Kac potential is large enough

Time scales in shear banding of wormlike micelles

Transient stress and birefringence measurements are performed on wormlike micellar solutions that "shear band", i.e. undergo flow-induced coexistence of states of different viscosities along a constant stress "plateau". Three well-defined relaxation times are found after a strain rate step between two banded flow states on the stress plateau. Using the Johnson-Segalman model, we relate these time scales to three qualitatively different stages in the evolution of the bands and the interface between them: band destabilization, reconstruction of the interface, and travel of the fully formed interface. The longest timescale is then used to estimate the magnitude of the (unknown) "gradient" terms that must be added to constitutive relations to explain the history independence of the steady flow and the plateau stress selection

Laterally driven interfaces in the three-dimensional Ising lattice gas

We study the steady state of a phase-separated driven Ising lattice gas in three dimensions using computer simulations with Kawasaki dynamics. An external force field F(z) acts in the x direction parallel to the interface, creating a lateral order parameter current j^x(z) which varies with distance z from the interface. Above the roughening temperature, our data for `shear-like' linear variation of F(z) are in agreement with the picture wherein shear acts as effective confinement in this system, thus supressing the interfacial capillary-wave fluctuations. We find sharper magnetisation profiles and reduced interfacial width as compared to equilibrium. Pair correlations are more suppressed in the vorticity direction y than in the driving direction; the opposite holds for the structure factor. Lateral transport of capillary waves occurs for those forms of F(z) for which the current j^x(z) is an odd function of z, for example the shear-like drive, and a `step-like' driving field. For a V-shaped driving force no such motion occurs, but capillary waves are suppressed more strongly than for the shear-like drive. These findings are in agreement with our previous simulation studies in two dimensions. Near and below the (equilibrium) roughening temperature the effective-confinement picture ceases to work, but the lateral motion of the interface persists.Comment: 20 pages, 11 figures, submitted to Phys. Rev.

Evaluation of mefenoxam and fludioxonil for control of Rhizoctonia solani, Pythium ultimum and Fusarium solani on cowpea

Cowpea (Vigna unguiculata) is susceptible to pathogens such as Rhizoctonia solani Pythium ultimum and Fusarium solani causing seedling diseases in cowpea, resulting in low yields. Three commercial synthetic fungicides containing mefenoxam 350 g ai L-1 mefenoxam 240 g ai L-1 and fludioxonil 100 g ai L-1, respectively, were evaluated against these pathogens on cowpea in the greenhouse following promising in vitro results. The fungicides were applied initially as a soil drench to seedling trays at planting and fortnightly as a drench according to manufacturer’s recommendations. All fungicides, except mefenoxam 350 g ai L-1 in one trial, were able to reduce diseases caused by R. solani. With the exception of mefenoxam 350 g ai L-1 in F. solani all fungicides increased seedling emergence and dry shoot and root mass of plants and all fungicide treatments reduced disease of seedlings grown in F. solani and P. ultimum inoculated growth medium. Although all three fungicides reduced the percentage of diseased seedlings, none of them gave complete control of the diseases caused by the three pathogens under the trial conditions.National Research Foundation.http://www.tandfonline.com/loi/tjps202017-06-07hb2016Plant Production and Soil Scienc

On the relation between virial coefficients and the close-packing of hard disks and hard spheres

The question of whether the known virial coefficients are enough to determine the packing fraction $\eta_\infty$ at which the fluid equation of state of a hard-sphere fluid diverges is addressed. It is found that the information derived from the direct Pad\'e approximants to the compressibility factor constructed with the virial coefficients is inconclusive. An alternative approach is proposed which makes use of the same virial coefficients and of the equation of state in a form where the packing fraction is explicitly given as a function of the pressure. The results of this approach both for hard-disk and hard-sphere fluids, which can straightforwardly accommodate higher virial coefficients when available, lends support to the conjecture that $\eta_\infty$ is equal to the maximum packing fraction corresponding to an ordered crystalline structure.Comment: 10 pages, 6 figures; v2: discussion about hard-square and hard-hexagon systems on a lattice added; five new reference