Interatomic van der Waals potential in the presence of a magneto-electric sphere

On the basis of a general formula obtained earlier via fourth-order erturbation theory within the framework of macroscopic quantum electrodynamics, the van der Waals potential between two neutral, unpolarized, ground-state atoms in the presence of a homogeneous, dispersing and absorbing magnetoelectric sphere is studied. When the radius of the sphere becomes sufficiently large, the result asymptotically agrees with that for two atoms near a planar interface. In the opposite limit of a very small sphere, the sphere can effectively be regarded as being a third ground-state atom, and the nonadditive three-atom van der Waals potential is recovered. To illustrate the effect of a sphere of arbitrary radius, numerical results are presented for the triangular arrangement where the atoms are at equidistance from the sphere, and for the linear arrangement where the atoms and the sphere are aligned along a straight line. As demonstrated, the enhancement or reduction of the interaction potential in the presence of purely electric or magnetic spheres can be physically understood in terms of image charges.Comment: 13 pages, 7 figure

A tale of two networks: Market formation on the Cambodia-Vietnam frontier

Although South‐East Asia's trading networks have existed for millennia, recent decades have seen markets dramatically intensify in the region's frontiers, bringing social and environmental upheaval. Within political ecology, such transitions are framed as frontier incorporation into global capitalism – a process that has been ongoing since European colonisation. This paper, however, responds to recent calls for commodity network studies to better account for specific material and social variances in “actually existing capitalism.” My analysis focuses on Mondulkiri province in north‐eastern Cambodia, where a boom in cassava cultivation has produced two distinct commodity networks. The first network supplies dried cassava chips to trans‐border markets that serve bioethanol and livestock production. The second network supplies fresh cassava to Vietnamese starch and processed food factories. In order to understand why two market networks have evolved from one crop, I analyse the social, material and spatial relationships in each network. Comparative analysis shows that both networks involve land and labour commodification and respond to global demand. Yet subtle geographical variations in transport networks, migration patterns and the availability of uncleared land, support dried cassava production in some areas and fresh cassava in others. The cassava case shows that although frontier markets are propelled by globally connected processes of commodification, they ultimately take form through co‐productive networks that mould to and shape frontier landscapes. Furthermore, market networks are not only mobilised, but also can be demobilised by environmental, economic and social pressures – a point that frontier incorporation perspectives may overlook. The paper therefore argues for an understanding of frontier geographies as dynamic and constitutive in market formation.This research is supported by the Australian Research Council (FT130101495 Frontiers of change: resources, access and political agency on the Cambodia–Vietnam borderland)

Learning in sustainable natural resource management: challenges and opportunities in the Pacific

The importance of learning in natural resource management (NRM) is being recognized by an increasing number of scholars and practitioners. A learning approach to NRM applies principles and theories of adult, organizational and social learning, and is underpinned by three core elements – systems thinking, negotiation and reflection. By combining learning theories with concepts from adaptive management, co-management, and participatory resource management, this article explores how the explicit inclusion of learning principles and processes can strengthen community based natural resource management. Case studies from the South Pacific are used to draw out lessons for the wider application of learning approaches to NRM

Attraction between Neutral Dielectrics Mediated by Multivalent Ions in an Asymmetric Ionic Fluid

We study the interaction between two neutral plane-parallel dielectric bodies in the presence of a highly asymmetric ionic fluid, containing multivalent as well as monovalent (salt) ions. Image charge interactions, due to dielectric discontinuities at the boundaries, as well as effects from ion confinement in the slit region between the surfaces are taken fully into account, leading to image-generated depletion attraction, ion correlation attraction and steric-like repulsive interactions. We investigate these effects by employing a combination of methods including explicit-ion and implicit-ion Monte-Carlo simulations, as well as an effective interaction potential analytical theory. The latter incorporates strong ion-image charge correlations, which develop in the presence of high valency ions in the mixture. We show that the implicit-ion simulations and the proposed analytical theory can describe the explicit simulation results on a qualitative level, while excellent quantitative agreement can be obtained for sufficiently large monovalent salt concentrations. The resultant attractive interaction between the neutral surfaces is shown to be significant, as compared with the usual van der Waals interactions between semi-infinite dielectrics, and can thus play a significant role at the nano scale.Comment: 9 pages, 4 figure

The evolution and evaluation of an online role play through design-based research

This paper presents selected findings from a 5-year design-based research case study of the evolution of an online role play that allows postgraduate students to explore the complexities inherent in land rights negotiations between indigenous peoples and others. In the context of Laurillard’s (2002) conversational framework and a design-based research methodology, diverse private and public discussion forum spaces were created for group negotiations on a learning management system (LMS) platform. Our analysis of the conversational framework structure in the evolved role play showed that all four stages – discursive, adaptive, integrative, and reflective – were evidenced, with the adaptive and integrative stages cycling through multiple times. The online role play, whilst implemented as a simple virtual world, facilitated personal, deep and socialised learning experiences focused on consultation, negotiation and decision-making. We also found that student anonymity was not necessary for full engagement in role play, and that students chose to incorporate communication technologies outside the LMS into their learning activities. This research shows that with a strong pedagogical design, and attention paid to an evidence-based iterative improvement cycle, online role plays can provide powerful collaborative learning experiences

Global consequences of a local Casimir force: Adhered cantilever

Although stiction is a cumbersome problem for microsystems, it stimulates investigations of surface adhesion. In fact, the shape of an adhered cantilever carries information of the adhesion energy that locks one end to the substrate. We demonstrate here that the system is also sensitive to the dispersion forces that are operative very close to the point of contact, but their contribution to the shape is maximum at about one third of the unadhered length. When the force exceeds a critical value the cantilever does not lose stability but it settles at smaller unadhered length, whose relation to adhesion energy is only slightly affected by the force. Our calculations suggest to use adhered cantilevers to measure the dispersion forces at short separations, where other methods suffer from jump-to-contact instability. Simultaneous measurement of the force and adhesion energy allows the separation of the dispersion contribution to the surface adhesion.Comment: 5 pages, 3 figure

Some observations on the renormalization of membrane rigidity by long-range interactions

We consider the renormalization of the bending and Gaussian rigidity of model membranes induced by long-range interactions between the components making up the membrane. In particular we analyze the effect of a finite membrane thickness on the renormalization of the bending and Gaussian rigidity by long-range interactions. Particular attention is paid to the case where the interactions are of a van der Waals type.Comment: 11 pages RexTex, no figure

Electromagnetic fluctuation-induced interactions in randomly charged slabs

Randomly charged net-neutral dielectric slabs are shown to interact across a featureless dielectric continuum with long-range electrostatic forces that scale with the statistical variance of their quenched random charge distribution and inversely with the distance between their bounding surfaces. By accounting for the whole spectrum of electromagnetic field fluctuations, we show that this long-range disorder-generated interaction extends well into the retarded regime where higher-order Matsubara frequencies contribute significantly. This occurs even for highly clean samples with only a trace amount of charge disorder and shows that disorder effects can be important down to the nano scale. As a result, the previously predicted non-monotonic behavior for the total force between dissimilar slabs as a function of their separation distance is substantially modified by higher-order contributions, and in almost all cases of interest, we find that the equilibrium inter-surface separation is shifted to substantially larger values compared to predictions based solely on the zero-frequency component. This suggests that the ensuing non-monotonic interaction is more easily amenable to experimental detection. The presence of charge disorder in the intervening dielectric medium between the two slabs is shown to lead to an additional force that can be repulsive or attractive depending on the system parameters and can, for instance, wash out the non-monotonic behavior of the total force when the intervening slab contains a sufficiently large amount of disorder charges.Comment: 9 pages, 5 figure