Democracia y dise??os institucionales participativos en las pol??ticas urbanas: estudio de la experiencia brasilera contempor??nea

El art??culo analiza las instituciones participativas creadas en Brasil a nivel local a lo largo de las ??ltimas d??cadas, espec??ficamente en el ??mbito de las pol??ticas urbanas. Al inicio, evocamos los fundamentos normativos asociados a la democracia y a las instituciones participativas y abordamos el papel de los dise??os institucionales, sus variaciones y potenciales de radicalizaci??n democr??tica. En un segundo momento, contextualizamos las pol??ticas urbanas, se??alando las nuevas referencias legales que requieren la participaci??n y que encuadran el enfoque y el dise??o de las instituciones. Finalmente, analizamos elementos clave de los dise??os participativos, comparando consejos, conferencias, presupuestos participativos y participaci??n en los planes maestros, problematizando sus alcances y potenciales democratizadores

Role of tilt order in the asymmetric ripple phase of phospholipid bilayers

We present the electron density map of the asymmetric ripple phase of dilauroylphosphatidylcholine. We find that the primary feature characterizing the “asymmetry” of the rippled bilayers is the difference in the bilayer thickness in the two arms, and not the asymmetry of the bilayer height profile as is generally assumed. This difference in the bilayer thickness can be attributed to a mean tilt of the hydrocarbon chains of the lipid molecules along the direction of the ripple wave vector. We propose a Landau theory for this phase which takes into account the anisotropic elastic properties of a bilayer with tilt order

Chiral symmetry breaking in three-dimensional smectic-C liquid-crystal domains

We report an observation of a unique type of spontaneous chiral symmetry breaking in three-dimensional domains of a smectic-C material consisting of achiral molecules. The observed helical structure clearly demonstrates the effect of an elastic coupling between the bend and the twist distortions in the c field. The sign and the magnitude of the coupling coefficient are determined experimentally. We also demonstrate that an external chiral bias field favors domains of one handedness

Elasticity of smectic liquid crystals with in-plane orientational order and dispiration asymmetry

The Nelson-Peliti formulation of the elasticity theory of isolated fluid membranes with orientational order emphasizes the interplay between geometry, topology, and thermal fluctuations. Fluid layers of lamellar liquid crystals such as smectic-C, hexatic smectics, and smectic-C∗ are endowed with in-plane orientational order. We extend the Nelson-Peliti formulation so as to bring these smectics within its ambit. Using the elasticity theory of smectics-C∗, we show that positive and negative dispirations (topological defects in Smectic-C∗ liquid crystals) with strengths of equal magnitude have disparate energies—a result that is amenable to experimental tests

Small-angle grain boundaries in quasicrystals

The Read-Shockley treatment of small-angle grain boundaries in crystals is generalized to the case of quasicrystals. The dependence of the grain-boundary energy on the angle of mismatch between abutting quasicrystalline grains is calculated. It is found that, even for a symmetric tilt boundary in a quasicrystal, dislocations with at least two types of Burgers vectors are required; these dislocations have to be arranged quasiperiodically along the boundary. The possible clumping of these dislocations to form composites is discussed. Explicit calculations are presented for a pentagonal quasicrystal

Laser induced rotation of trapped chiral and achiral nematic droplets

We study the response of optically trapped achiral and chiralised nematic liquid crystal droplets to linear as well as circular polarised light. We find that there is internal dissipation in rotating achiral nematic droplets trapped in glycerine. We also demonstrate that some chiralised droplets rotate under linearly polarised light. The best fit to our data on chiralised droplets indicates that rotational frequency of these droplets with radius R is approximately proportional to1/R^2, rather than to 1/R^3.Comment: 15 pages, 6 figure

A novel modulated phase of liquid crystals: Covariant elasticity in the context of soft, achiral smectic-C materials

Ginzburg-Landau-de Gennes -type covariant theories are extensively used in connection with twist grain boundary (TGB) phases of chiral smectogens. We analyze the stability conditions for the linear, covariant elasticity theory of smectic-C liquid crystals in the context of achiral materials, and predict an equilibrium modulated structure with an oblique wavevector. We suggest that a previous experimental observation of stripes in smectic-C is consistent with the predicted structure.Comment: 4 pages, 3 figure

Small-Angle Grain Boundaries in Quasicrystals

The Read-Shockley treatment of small-angle grain boundaries in crystals is generalized to the case of quasicrystals. The dependence of the grain-boundary energy on the angle of mismatch between abutting quasicrystalline grains is calculated. It is found that, even for a symmetric tilt boundary in a quasicrystal, dislocations with at least two types of Burgers vectors are required; these dislocations have to be arranged quasiperiodically along the boundary. The possible clumping of these dislocations to form composites is discussed. Explicit calculations are presented for a pentagonal quasicrystal. Translationally ordered solids, either crystals or quasicrystals, can have grain boundaries, which are interfaces between regions with different orientations. Such boundaries are well known in periodic crystals,' and they have been reported recently in quasicrystals. 2 Frank 3 and Read and Shockley 4 have shown how small-angle grain boundaries in crystals can be described as arrays of dislocations [planar (linear) arrays in three (two) dimensions]. Read and Shockley use this description to calculate the dependence of the (zero-temperature) grainboundary energy on the angle of mismatch between abutting crystalline grains. In this paper we generalize the Read-Shockley treatment of small-angle grain boundaries to quasicrystals. Our principal results are as follows: (l) Whereas a symmetric tilt boundary (the simplest) in a crystal can be obtained by using an array of only one type of dislocation, 4 in a quasicrystal it can be obtained only by using an array of at least two types of dislocations, alternating quasiperiodically along the boundary. The dislocations in the array must be chosen and arranged in such a way that the sum of their Burgers vectors has a phonon part that scales as L and a phason part that vanishes as L ~[, where L is the linear size of the array. This was shown first for an incommensurate smectic liquid crystal. 5 (2) An array of dislocations that does not satisfy the above conditions leads to strains in the quasicrystals that do not vanish infinitely far from the array. Thus, the energy -T; \UiiUn /j= y /« y -+ fly«,-) is the strain tensor, w// = di\Vj, and we sum over repeated i and j indices. Note that E e \ has rotational invariance built into it: Fields with constant, finite Vxu do not cost any energy. However, w fields with constant, finite Vxw do cost energy, because they lead to relative rotations of the density waves (see below) of which the quasicrystal is constituted. It is easy to see now why in a quasicrystal an array of only one type of dislocation yields an energy proportional to L d : As in a crystal, such an array leads to a relative rotation of the regions of quasicrystal on either side of cost of such an array scales as L d for a ^-dimensional quasicrystal and not as L d~' as required for a true grain boundary. (3) As in a crystal, 4 the intensive energy per unit area (length if d = 2) of a grain boundary is E =C\6 -C2#ln0, where 9 is the tilt angle, and Ci and Ci depend on the orientation of the grain boundary and on the elastic constants of the quasicrystal. We calculate these for a symmetric tilt boundary in a pentagonal quasicrystal, 7 In the remaining part of this paper we give the arguments that lead to the results summarized above. These arguments hold for all quasicrystals with dimensions d>2. We substantiate these arguments with explicit calculations for two-dimensional, pentagonal quasicrystals. The elastic energy density of a quasicrystal depends on derivatives of two fields: u (the phonon part) and w (the phason part). [Spatially uniform u (i.e., uniform translations) and spatially uniform w (this shifts density waves relative to one another in a density-wave description) do not cost any energy.] For example, for pentagonal quasicrystals, the elastic energy density is 7 ' I the array; i.e., far from the array there is a u field with uniform nonzero Vxu, which costs no energy. In a quasicrystal, however, energy dislocation has both u and w parts, and so the same argument yields a constant, nonzero Vx w far from the array. Such a field has a nonzero energy density at infinity. Thus the total energy of an array of only one type of dislocation scales as L d in a quasicrystal. Such an array is clearly not a grain boundary. We now show how, using more than one type of dislocation, it is possible to construct an array which leads t