Electrostatic Disorder-Induced Interactions in Inhomogeneous Dielectrics

We investigate the effect of quenched surface charge disorder on electrostatic interactions between two charged surfaces in the presence of dielectric inhomogeneities and added salt. We show that in the linear weak-coupling regime (i.e., by including mean-field and Gaussian-fluctuations contributions), the image-charge effects lead to a non-zero disorder-induced interaction free energy between two surfaces of equal mean charge that can be repulsive or attractive depending on the dielectric mismatch across the bounding surfaces and the exact location of the disordered charge distribution.Comment: 7 pages, 2 figure

Ground state of two unlike charged colloids: An analogy with ionic bonding

In this letter, we study the ground state of two spherical macroions of identical radius, but asymmetric bare charge ((Q_{A}>Q_{B})). Electroneutrality of the system is insured by the presence of the surrounding divalent counterions. Using Molecular Dynamics simulations within the framework of the primitive model, we show that the ground state of such a system consists of an overcharged and an undercharged colloid. For a given macroion separation the stability of these ionized-like states is a function of the difference ((\sqrt{N_{A}}-\sqrt{N_{B}})) of neutralizing counterions (N_{A}) and (N_{B}). Furthermore the degree of ionization, or equivalently, the degree of overcharging, is also governed by the distance separation of the macroions. The natural analogy with ionic bonding is briefly discussed.Comment: published versio

Bacteria isolated from the airways of paediatric patients with bronchiectasis according to HIV status

Background. Knowledge of which bacteria are found in the airways of paediatric patients with bronchiectasis unrelated to cystic fibrosis (CF) is important in defining empirical antibiotic guidelines for the treatment of acute infective exacerbations.Objective. To describe the bacteria isolated from the airways of children with non-CF bronchiectasis according to their HIV status.Methods. Records of children with non-CF bronchiectasis who attended the paediatric pulmonology clinic at Chris Hani Baragwanath Academic Hospital, Johannesburg, South Africa, from April 2011 to March 2013, or were admitted to the hospital during that period, were reviewed. Data collected included patient demographics, HIV status, and characteristics of the airway samples and types of bacteria isolated.Results. There were 66 patients with non-CF bronchiectasis over the 2-year study period. The median age was 9.1 years (interquartile range 7.2 - 12.1). The majority of patients (78.8%) were HIV-infected. A total of 134 samples was collected (median 1.5 per patient, range 1 - 7), of which 81.3% were expectorated or induced sputum samples. Most bacteria were Gram negatives (72.1%). Haemophilus influenzae was the most common bacterium identified (36.0%), followed by Streptococcus pneumoniae (12.6%), Moraxella catarrhalis (11.1%) and Staphylococcus aureus (10.6%). There were no differences between HIV-infected and uninfected patients in prevalence or type of pathogens isolated.Conclusion. Bacterial isolates from the airways of children with non-CF bronchiectasis were similar to those in other paediatric populations and were not affected by HIV status

An assessment of the needs of women-owned construction enterprises in South Africa for construction empowerment, development and sustainability (CODES)

Purpose of this paper: This paper reports on the findings of a needs assessment study of South African (SA) Women-Owned Enterprises in construction. The study was done in preparation for the rolling out of an empowerment initiative in South Africa. The study builds on the lessons learnt and recommendations from the ʻWomen-helping-Womenʼ study. Design/methodology/approach: Relevant literature was reviewed. A survey instrument was developed and professionally facilitated workshops, supervised by stakeholders, informed the needs assessment. Findings: Fair procurement, women friendly construction sites, a construction bank and relevant career-aligned training with mentoring will enable the respondents to transform their enterprises to be sustainable multi-skilled independent entities that are able to construct the infrastructure needed. Research limitations: Findings are based on views from respondents in SA registered with the Construction Industry development Board (CIDB) and also with legitimate e-mail addresses. Practical implications: Given the professionally facilitated workshops and stakeholder involvement in the instrument developed from start to finish the findings of the study proved to be reliable and valid and could be used for similar interventions. Value of paper: Knowledge sharing for auctioning by government, the building industry, development finance institutions and women associations on the critical skills shortage, gender equity and empowerment added value

Universal Elasticity and Fluctuations of Nematic Gels

We study elasticity of spontaneously orientationally-ordered amorphous solids, characterized by a vanishing transverse shear modulus, as realized for example by nematic elastomers and gels. We show that local heterogeneities and elastic nonlinearities conspire to lead to anomalous nonlocal universal elasticity controlled by a nontrivial infared fixed point. Namely, at long scales, such solids are characterized by universal shear and bending moduli that, respectively, vanish and diverge at long scales, are universally incompressible and exhibit a universal negative Poisson ratio and a non-Hookean elasticity down to arbitrarily low strains. Based on expansion about five dimensions, we argue that the nematic order is stable to thermal fluctuation and local hetergeneities down to d_lc < 3.Comment: 4 RevTeX pgs, submitted to PR

The stuff that motor chunks are made of: Spatial instead of motor representations?

In order to determine how participants represent practiced, discrete keying sequences in the discrete sequence production task, we had 24 participants practice two six-key sequences on the basis of two pre-learned six-digit numbers. These sequences were carried out by fingers of the left (L) and right (R) hand with between-hand transitions always occurring between the second and third, and the fifth and sixth responses. This yielded the so-called LLRRRL and RRLLLR sequences. Early and late in practice, the keypad used for the right hand was briefly relocated from the front of the participants to 90° at their right side. The results indicate that after 600 practice trials, executing a keying sequence relies heavily on a spatial cross-hand representation in a trunk- or head-based reference frame that after about only 15 trials is fully adjusted to the changed hand location. The hand location effect was not found with the last sequence element. This is attributed to the application of explicit knowledge. The between-hand transitions appeared to induce initial segmentation in some of the participants, but this did not consolidate into a concatenation point of successive motor chunks

Charge and orbital order in Fe_3O_4

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe_3O_4) is investigated using LSDA+U. While the difference between t_{2g} minority occupancies of Fe^{2+}_B and Fe^{3+}_B cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory.Comment: 4 pages, 2 figure

Resonant X-ray diffraction studies on the charge ordering in magnetite

Here we show that the low temperature phase of magnetite is associated with an effective, although fractional, ordering of the charge. Evidence and a quantitative evaluation of the atomic charges are achieved by using resonant x-ray diffraction (RXD) experiments whose results are further analyzed with the help of ab initio calculations of the scattering factors involved. By confirming the results obtained from X-ray crystallography we have shown that RXD is able to probe quantitatively the electronic structure in very complex oxides, whose importance covers a wide domain of applications.Comment: 4 pages 4 figures, accepted for publication in PR

Correspondence of A. C. Van Raalte with Abraham Kuyper

Correspondence of A. C. Van Raalte with Abraham Kuyper of this date and July 17 about assisting a Mr. Verwey who has been difficult to deal with.https://digitalcommons.hope.edu/vrp_1870s/1104/thumbnail.jp