Optimal packing of polydisperse hard-sphere fluids II

We consider the consequences of keeping the total surface fixed for a polydisperse system of $N$ hard spheres. In contrast with a similar model (J. Zhang {\it et al.}, J. Chem. Phys. {\bf 110}, 5318 (1999)), the Percus-Yevick and Mansoori equations of state work very well and do not show a breakdown. For high pressures Monte Carlo simulation we show three mechanically stable polydisperse crystals with either a unimodal or bimodal particle-size distributions.Comment: 17 pages, 8 figures, revtex (accepted by J. Chem. Phys.

Exact analytic expression for a subset of fourth virial coefficients of polydisperse hard sphere mixtures

We derive an exact, analytic expression for the fourth virial coefficient of a system of polydisperse spheres under the constraint that the smallest sphere has a radius smaller than a given function of the radii of the three remaining particles.Comment: 10 pages RevTex with EPS figure

Regular binary thermal lattice-gases

We analyze the power spectrum of a regular binary thermal lattice gas in two dimensions and derive a Landau-Placzek formula, describing the power spectrum in the low-wavelength, low frequency domain, for both the full mixture and a single component in the binary mixture. The theoretical results are compared with simulations performed on this model and show a perfect agreement. The power spectrums are found to be similar in structure as the ones obtained for the continuous theory, in which the central peak is a complicated superposition of entropy and concentration contributions, due to the coupling of the fluctuations in these quantities. Spectra based on the relative difference between both components have in general additional Brillouin peaks as a consequence of the equipartition failure.Comment: 20 pages including 9 figures in RevTex

Cubatic phase for tetrapods

We investigate the phase behavior of tetrapods, hard non-convex bodies formed by 4 rods connected under tetrahedral angles. We predict that, depending on the relative lengths of the rods these particles can form a uniaxial nematic phase, and more surprisingly they can exhibit a cubatic phase, a special case of the biaxial nematic phase. These predictions may be experimentally testable, as experimental realizations of tetrapods have recently become available.Comment: 8 pages ReVTeX 4, including 3 EPS figure

Do cylinders exhibit a cubatic phase?

We investigate the possibility that freely rotating cylinders with an aspect ratio $L/D=0.9$ exhibit a cubatic phase similar to the one found for a system of cut-spheres. We present theoretical arguments why a cubatic phase might occur in this particular system. Monte Carlo simulations do not confirm the existence of a cubatic phase for cylinders. However, they do reveal an unexpected phase behavior between the isotropic and crystalline phase.Comment: 24 pages, 12 figures, RevTex (Submitted to J. Chem. Phys.

Reversible gelation and dynamical arrest of dipolar colloids

We use molecular dynamics simulations of a simple model to show that dispersions of slightly elongated colloidal particles with long-range dipolar interactions, like ferrofluids, can form a physical (reversible) gel at low volume fractions. On cooling, the particles first self-assemble into a transient percolating network of cross-linked chains, which, at much lower temperatures, then undergoes a kinetic transition to a dynamically arrested state with broken ergodicity. This transition from a transient to a frozen gel is characterised by dynamical signatures reminiscent of jamming in much denser dispersions.Comment: 6 pages, 7 figure

Continuous phase transition in polydisperse hard-sphere mixture

In a previous paper (J. Zhang {\it et al.}, J. Chem. Phys. {\bf 110}, 5318 (1999)) we introduced a model for polydisperse hard sphere mixtures that is able to adjust its particle-size distribution. Here we give the explanation of the questions that arose in the previous description and present a consistent theory of the phase transition in this system, based on the Percus-Yevick equation of state. The transition is continuous, and like Bose-Einstein condensation a macroscopic aggregate is formed due to the microscopic interactions. A BMCSL-like treatment leads to the same conclusion with slightly more accurate predictions.Comment: 7 pages including 5 figures in revte

The normative practitioner:adding value to organisational learning in education NGOs in Uganda

Development cooperation is no longer considered inherently good. Development programmes are often proven ineffective and there are persistent inequalities between the global North and South, black and white, men and women. Development organisations such as NGOs are expected to take a more critical stance and to seek alignment with local systems and needs.This research looks at organisational learning processes of NGOs that aim at improving lifelong learning in Uganda. Between 2015 and 2019, Marit Blaak has collaborated with practitioners in education NGOs in Uganda to find solutions towards better alignment to local needs and opportunities, especially collectively with communities. By using participatory action research, the team has had the opportunity to try out some of these solutions. During the research, for example, a dialogue was facilitated in a village where over eighteen NGOs operate. In this dialogue, residents explained how NGOs can better shape spaces for collaboration and collective learning. Another example is that one of the participating NGOs adjusted their orientation training for young volunteers to ensure they are better prepared for the complexities of local collaboration and innovation of education activities. Because of its practical nature, this research has offered insight in how NGOs can facilitate organisational learning processes. What seems to be important are value-laden questions such as: Are we doing the right thing? And for whom? The answers to these questions are often ambiguous, demanding continuous reflection and normative accountability