Phase separation dynamics in colloid-polymer mixtures: the effect of interaction range

Colloid-polymer mixtures may undergo either fluid-fluid phase separation or gelation. This depends on the depth of the quench (polymer concentration) and polymer-colloid size ratio. We present a real-space study of dynamics in phase separating colloid-polymer mixtures with medium- to long-range attractions (polymer-colloid size ratio q_R=0.45-0.89, with the aim of understanding the mechanism of gelation as the range of the attraction is changed. In contrast to previous studies of short-range attractive systems, where gelation occurs shortly after crossing the equilibrium phase boundary, we find a substantial region of fluid-fluid phase separation. On deeper quenches the system undergoes a continuous crossover to gel formation. We identify two regimes, `classical' phase separation, where single particle relaxation is faster than the dynamics of phase separation, and `viscoelastic' phase separation, where demixing is slowed down appreciably due to slow dynamics in the colloid-rich phase. Particles at the surface of the strands of the network exhibit significantly greater mobility than those buried inside the gel strand which presents a method for coarsening.Comment: 8 page

Professionalisation as a social regularity: The policy process in South Africa's natural science curriculum

Curriculum 2005 (C2005) in 1997 and the Revised National Curriculum Statements (RNCS)in 2002 have bee n two major curriculum policy developments in South Africa. In this study, our aim was to unravel the processes by which they developed as they did, and determine how these policy processes are best researched and understood. In this article we use the concept of professionalisation to analyse the policy process for the two reform periods. In addition we attempt to show how professionalisation acts as a social regularity: professionals brought in to write the policy documents for the two reform periods, through their socialization into the profession, have in many ways worked towards the maintenance of a particular social order, rather than changing the social order. This is evident especially in the concept of 'scientific literacy' that emerged, which is strongly consistent with similar policies in developed countries, even though conditions in South Africa are unique.South African Journal of Education Vol. 26 (4) 2006: pp. 515-52

Suppression of Classical and Quantum Radiation Pressure Noise via Electro-Optic Feedback

We present theoretical results that demonstrate a new technique to be used to improve the sensitivity of thermal noise measurements: intra-cavity intensity stabilisation. It is demonstrated that electro-optic feedback can be used to reduce intra-cavity intensity fluctuations, and the consequent radiation pressure fluctuations, by a factor of two below the quantum noise limit. We show that this is achievable in the presence of large classical intensity fluctuations on the incident laser beam. The benefits of this scheme are a consequence of the sub-Poissonian intensity statistics of the field inside a feedback loop, and the quantum non-demolition nature of radiation pressure noise as a readout system for the intra-cavity intensity fluctuations.Comment: 4 pages, 1 figur

Towards a Molecular Inventory of Protostellar Discs

The chemical environment in circumstellar discs is a unique diagnostic of the thermal, physical and chemical environment. In this paper we examine the structure of star formation regions giving rise to low mass stars, and the chemical environment inside them, and the circumstellar discs around the developing stars.Comment: 9 page PDF, 550 kbyte

Imaging with Diffraction Tomography

The problem of cross sectional (tomographic) imaging bf objects with diffracting sources is addressed. Specifically the area of investigation is the effect of multiple scattering and attenuation phenomena in diffraction imaging. This work reviews the theory and limits of first order diffraction tomography and studies iterative techniques that can be used to improve the quality of tomographic imaging with diffracting sources. Conventional (straight-ray) tomographic algorithms are not valid when used with acoustic or microwave energy. Thus more sophisticated algorithms are needed; First order diffraction tomography uses a linearized version of the wave equation and gives an especially simple reconstruction algorithm. This work reviews first order approximations to the scattered field and studies the quality of the reconstructions when the assumptions behind these approximations are violated. It will be shown that the Born approximation is valid when the phase change across the object is less than it and the Rytov approximation is valid when the refractive index changes by less than two or three percent. Better reconstructions will be based on higher order approximations to the scattered field. This work describes two fixed point algorithms (the Born and the Rytov approximations) and an algebraic approach to more accurately calculate the scattered fields. The limits of each of these approaches is discussed and simulated results are shown. Finally a review of higher order inversion techniques is presented. Each of these techniques is reviewed and some of their limitations are discussed

Western Australian soils store large quantities of salt

Enormous quantities of salt are stored in the soil in the agricultural areas of Western Australia. Small movements in this salt can produce large effects on farms

Paspalum vaginatum : for salty seepages and lawns

PASPALUM VAGINATUM, aptly termed sea shore paspalum has an amazing ability to thrive in wet salty situations. It is also a good lawn grass. Seed of the grass is not available but it may easily be established by planting pieces. This article reviews where and how the grass may be used

Plant collections for saltland revegetation and soil conservation

In 1967, during an overseas plant exploration trip, seeds of plants of reputed salt tolerance and forage value were obtained from seven countries. Since 1971, over 600 plants have been added to the original collection of 343 plants already under observation for suitability for forage production in saline and arid areas. Additions to the test programme since 1971, include shrubs from Algeria; Argentina; Australia; Chile; England; Iran; Israel; Libya; Morocco; Netherlands; Russia; South Africa; Spain; Tunis and the United States of America. This Bulletin includes details of all plant collections from 1966 to December 1983