Towards a Landau-Ginzburg-type Theory for Granular Fluids

In this paper we show how, under certain restrictions, the hydrodynamic equations for the freely evolving granular fluid fit within the framework of the time dependent Landau-Ginzburg (LG) models for critical and unstable fluids (e.g. spinodal decomposition). The granular fluid, which is usually modeled as a fluid of inelastic hard spheres (IHS), exhibits two instabilities: the spontaneous formation of vortices and of high density clusters. We suppress the clustering instability by imposing constraints on the system sizes, in order to illustrate how LG-equations can be derived for the order parameter, being the rate of deformation or shear rate tensor, which controls the formation of vortex patterns. From the shape of the energy functional we obtain the stationary patterns in the flow field. Quantitative predictions of this theory for the stationary states agree well with molecular dynamics simulations of a fluid of inelastic hard disks.Comment: 19 pages, LaTeX, 8 figure

On the dependence of the avalanche angle on the granular layer thickness

A layer of sand of thickness h flows down a rough surface if the inclination is larger than some threshold value theta which decreases with h. A tentative microscopic model for the dependence of theta with h is proposed for rigid frictional grains, based on the following hypothesis: (i) a horizontal layer of sand has some coordination z larger than a critical value z_c where mechanical stability is lost (ii) as the tilt angle is increased, the configurations visited present a growing proportion $_s of sliding contacts. Instability with respect to flow occurs when z-z_s=z_c. This criterion leads to a prediction for theta(h) in good agreement with empirical observations.Comment: 6 pages, 2 figure

UM Soluçar de Vida! Cantos Ecoando Com Projetos Sociais de Barra do Riacho

RESUMO Um soluçar de vida: cantos ecoando com os projetos sociais de Barra do Riacho é um dos estudos do Núcleo de Pesquisa Contornos de Cidades: movimentos e composições (PPGPSIUFES). Esta pesquisa toma como terreno analítico os movimentos sociais de uma comunidade de periferia de Aracruz/ES. Um bairro que pode ser entendido como bolsão de pobreza, convergência de efeitos e contingências de uma região altamente industrializada e portuária com fortes tradições pesqueiras e indígenas. A pesquisa trabalhou na perspectiva metodológica aberta pela história oral, ouvindo a voz de moradores do bairro, fazendo das narrativas uma produção humana e da narração uma atitude. Um recurso político diante do contar uma história, diante da vida, de seus impasses e conflituosidades. Constrói, a partir desse eixo, narrativas de histórias de vida com moradores que se encontram com os projetos sociais, formalizados ou não por ONGs no bairro. A aposta e a experimentação das construções narrativas, com base nas lutas sociais e sonhos dos moradores, inserem-se na panorâmica acadêmica que contextualiza projetos sociais como uma ação no cerne da confluência perversa que caracterizaria o Terceiro Setor. Assim, ideias como cidadania, participação popular, ação e movimento social ganham contornos fundamentados agora por uma política do tempo e pelo entendimento de uma realidade social que é inventiva, plural e cheia de paradoxos, necessariamente como a condição humana

Domain wall description of superconductivity

In the present work we shall address the issue of electrical conductivity in superconductors in the perspective of superconducting domain wall solutions in the realm of field theory. We take our set up made out of a dynamical complex scalar field coupled to gauge field to be responsible for superconductivity and an extra scalar real field that plays the role of superconducting domain walls. The temperature of the system is interpreted through the fact that the soliton following accelerating orbits is a Rindler observer experiencing a thermal bath.Comment: 9 pages, 5 figures, Latex. Version to appear in PL

On the rigidity of a hard sphere glass near random close packing

We study theoretically and numerically the microscopic cause of the mechanical stability of hard sphere glasses near their maximum packing. We show that, after coarse-graining over time, the hard sphere interaction can be described by an effective potential which is exactly logarithmic at the random close packing $\phi_c$. This allows to define normal modes, and to apply recent results valid for elastic networks: mechanical stability is a non-local property of the packing geometry, and is characterized by some length scale $l^*$ which diverges at $\phi_c$ [1, 2]. We compute the scaling of the bulk and shear moduli near $\phi_c$, and speculate on the possible implications of these results for the glass transition.Comment: 7 pages, 4 figures. Figure 4 had a wrong unit in abscissa, which was correcte

Dissipative collapse of the adiabatic piston

An adiabatic piston, separating two granular gases prepared in the same macroscopic state, is found to eventually collapse to one of the sides. This new instability is explained by a simple macroscopic theory which is furthermore in qualitative agreement with hard disk molecular dynamics.Comment: 7 pages, 5 figure