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Search Results: 1 - 10 of 544492 matches for " N. M. Silvestre "
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Liquid crystals boojum-colloids
M. Tasinkevych,N. M. Silvestre,M. M. Telo da Gama
Physics , 2012, DOI: 10.1088/1367-2630/14/7/073030
Abstract: Colloidal particles dispersed in a liquid crystal lead to distortions of the director field. The distortions are responsible for long-range effective colloidal interactions whose asymptotic behaviour is well understood. The short distance behaviour of the interaction, however, is sensitive to the structure and dynamics of the topological defects nucleated near the colloidal particles in the strong anchoring regime. The full non-linear theory is required in order to determine the interaction at short separations. Spherical colloidal particles with sufficiently strong planar degenerate anchoring nucleate a pair of antipodal surface topological defects, known as boojums. We use the Landau-de Gennes formalism in order to resolve the mesoscopic structure of the boojum cores and to determine the pairwise colloidal interaction. We compare the results in three (3D) and two (2D) spatial dimensions. The corresponding free energy functionals are minimized numerically using finite elements with adaptive meshes. Boojums are always point-like in 2D, but acquire a rather complex structure in 3D which depends on the combination of the anchoring potential, the radius of the colloid, the temperature and the LC elastic anisotropy. We identify three types of defect cores in 3D which we call single, double and split core boojums, and investigate the associated structural transitions. In the presence of two colloidal particles there are substantial re-arrangements of the defects at short distances, both in 3D and 2D. These re-arrangements lead to qualitative changes in the force-distance profile when compared to the asymptotic quadrupole-quadrupole interaction. In line with the experimental results, the presence of the defects prevents coalescence of the colloidal particles in 2D, but not in 3D systems.
The key-lock mechanism in nematic colloidal dispersions
N. M. Silvestre,P. Patricio,M. M. Telo da Gama
Physics , 2003, DOI: 10.1103/PhysRevE.69.061402
Abstract: We consider the interaction of two-dimensional colloids, in nematic liquid crystals, with walls or geometrical boundaries. The interactions between colloidal disks and flat walls, with homeotropic boundary conditions, are always repulsive. The repulsions may be turned into strong attractions at structured or sculpted walls with cavities, matching closely the shape and size of the colloids. This key-lock type of interaction is analyzed in detail for spherocylindrical cavities of various length to breath ratios, by minimizing the Landau-de Gennes free energy functional of the orientational order parameter. We find that the attractions occur only for walls with cavities within a small range of the colloidal size and for a narrow range of orientations around the wall's symmetry axis.
On elliptical soft colloids in smectic-C films
N. M. Silvestre,P. Patricio,M. M. Telo da Gama
Physics , 2006, DOI: 10.1103/PhysRevE.74.021706
Abstract: We investigate theoretically the elliptical shapes of soft colloids in freely standing smectic C films, that have been reported recently. The colloids favour parallel alignment of the liquid crystal molecules at their surfaces and, for sufficiently strong anchoring, will generate a pair of defects at the poles of the colloidal particles. The elastic free energy of the liquid crystal matrix will, in turn, affect the shape of the colloids. In this study we will focus on elliptical soft colloids and determine how their equilibrium shapes depend on the elastic constants of the liquid crystal, the anchoring strength, the surface tension and the size of the colloids. A shape diagram is obtained analytically, by minimizing the Frank elastic free energy, in the limit of small eccentricities. The analytical results are verified, and generalized to arbitrary eccentricities, by numerical minimization of an appropriate Landau free energy. The latter is required for an adequate description of the topological defects when the liquid crystal correlation length is comparable to the size of the colloidal particles.
Concordancia oftalmólogo-enfermera en la primera exploración post-quirúrgica de la cirugía de cataratas
Soto,M.N.; Silvestre,M. C.; Andonegui,J.;
Anales del Sistema Sanitario de Navarra , 2008,
Abstract: introduction. nursing activity in the cataract extraction process extends from carrying out biometry in the preoperative stage and collaboration in surgery, to providing the initial care in the postoperative stage and keeping the patient and family informed throughout the whole process. this article analyses agreement between ophthalmologists and nurses in the findings of the first post-surgical exploration of patients of cataract extraction using facoemulsification and without complications in surgery. methods. the study was carried out between may and june of 2005 on 129 patients who had been operated on for cataracts, 78 women and 51 men with an average age of 73.1 years. the variables to be evaluated in the first post-surgical exploration were defined, and a study was made of the agreement between ophthalmologists and nurses in identification of the most frequent complications in cataract surgery. results. agreement was 100% in the cases of intraocular pressure >22 mm hg, pupil irregularity, inflammatory reaction of the anterior chamber and iris prolapse; it was 65.9% on the presence of seidel; and 60.5% on severe corneal edema. conclusions. the high rate of agreement between ophthalmologists and nurses in 4 of the variables might offer the possibility of the nurse carrying out this post-surgical exploration. however, a moderate rate of agreement in the other variables makes a longer training period for nursing personnel necessary in order to obtain higher values.
Concordancia oftalmólogo-enfermera en la primera exploración post-quirúrgica de la cirugía de cataratas Agreement between ophthalmologist and nurse in the first postoperative exploration following cataract extraction
M.N. Soto,M. C. Silvestre,J. Andonegui
Anales del Sistema Sanitario de Navarra , 2008,
Abstract: Introducción. La actividad de enfermería en el proceso quirúrgico de la catarata engloba, desde la realización de la biometría en el preoperatorio y la colaboración en la cirugía, hasta la administración de los primeros cuidados en el postoperatorio y la información al paciente y a la familia durante todo el proceso. El objetivo del presente trabajo es analizar la concordancia entre oftalmólogo y enfermera en los hallazgos de la primera exploración postquirúrgica de los pacientes intervenidos de cataratas mediante facoemulsificación y sin complicaciones en la cirugía. Metodología. El estudio se realizó entre mayo y junio de 2005 en 129 pacientes intervenidos de cataratas, 78 mujeres y 51 hombres con edad media de 73,1 a os. Se definieron las variables a valorar en la primera exploración postquirúrgica y se estudió la concordancia entre oftalmólogos y enfermeras en la identificación de las complicaciones más frecuentes de la cirugía de cataratas. Resultados. La concordancia fue del 100% de los casos en la presión intraocular >22 mm de Hg, la irregularidad pupilar, la reacción inflamatoria de cámara anterior y el prolapso de iris; del 65,9% en la presencia de Seidel, y del 60,5% en el edema corneal severo. Conclusiones. El alto índice de concordancia médico-enfermera encontrado en 4 de las variables, puede ofrecer la posibilidad de que la enfermera realice esta primera exploración postquirúrgica. Sin embargo, una concordancia moderada en el resto hace necesario un mayor periodo de formación del personal de enfermería, para conseguir aumentar estos valores. Introduction. Nursing activity in the cataract extraction process extends from carrying out biometry in the preoperative stage and collaboration in surgery, to providing the initial care in the postoperative stage and keeping the patient and family informed throughout the whole process. This article analyses agreement between ophthalmologists and nurses in the findings of the first post-surgical exploration of patients of cataract extraction using facoemulsification and without complications in surgery. Methods. The study was carried out between May and June of 2005 on 129 patients who had been operated on for cataracts, 78 women and 51 men with an average age of 73.1 years. The variables to be evaluated in the first post-surgical exploration were defined, and a study was made of the agreement between ophthalmologists and nurses in identification of the most frequent complications in cataract surgery. Results. Agreement was 100% in the cases of intraocular pressure >22 mm Hg, pupil irregularity, inflamm
Structure of the cholesteric-isotropic interface
N. R. Bernardino,M. C. F. Pereira,N. M. Silvestre,M. M. Telo da Gama
Physics , 2014,
Abstract: The interface of a cholesteric liquid crystal with an isotropic fluid can display a range of unusual properties, such as a layer of topological defects close to an undulated interface. These properties have been know for a long time and have been explored for technological applications as a tunable substrate for colloidal self-assembly. However, from a fundamental point of view, this interface remains poorly understood and even basic properties, such as the dependence of the surface tension on the attributes of the liquid crystal, remain unknown. Here, we present a systematic calculation of the structure and surface tension of the cholesteric-isotropic interface and how these vary with the properties of the liquid crystal. We also suggest the intriguing possibility of wetting of this interface by a blue phase.
Interactions of distinct quadrupolar nematic colloids
Z. Eskandari,N. M. Silvestre,M. Tasinkevych,M. M. Telo da Gama
Physics , 2012,
Abstract: The effective interaction between spherical colloids in nematic liquid crystals is investigated in the framework of the Landau-de Gennes theory. The colloids differ through their interaction with the nematic. While both particles induce quadrupolar far-field distortions in the nematic matrix, with unlike quadrupole moments, one favours homeotropic and the other degenerate planar anchoring of the nematic director. In the strong anchoring regime the colloids with homeotropic anchoring are accompanied by an equatorial disclination line defect, known as "Saturn-ring", while the colloids with degenerate planar anchoring nucleate a pair of antipodal surface defects, called "Boojums". In the linear (large-distance) regime the colloidal interactions are of the quadrupolar type, where the quadrupoles have opposite signs. These are attractive when the colloids are aligned either parallel or perpendicular to the far-field director. At short distance, non-linear effects including "direct" interactions between defects give rise to a repulsion between the particles, which prevents them from touching. This finding supports the stability of nematic colloidal square crystallites the assembly of which has been reported recently.
Colloidal interactions in two dimensional nematics
M. Tasinkevych,N. M. Silvestre,P. Patricio,M. M. Telo da Gama
Physics , 2002, DOI: 10.1140/epje/i2002-10087-y
Abstract: The interaction between two disks immersed in a 2D nematic is investigated (i) analitically using the tensor order parameter formalism for the nematic configuration around isolated disks and (ii) numerically using finite element methods with adaptive meshing to minimize the corresponding Landau-de Gennes free energy. For strong homeotropic anchoring, each disk generates a pair of defects with one-half topological charge responsible for the 2D quadrupolar interaction between the disks at large distances. At short distance, the position of the defects may change, leading to unexpected complex interactions with the quadrupolar repulsive interactions becoming attractive. This short range attraction in all directions is still anisotropic. As the distance between the disks decreases their preferred relative orientation with respect to the far-field nematic director changes from oblique to perpendicular.
Nematic droplets at fibres
V. M. O. Batista,N. M. Silvestre,M. M. Telo da Gama
Physics , 2015,
Abstract: The emergence of new techniques for the fabrication of nematic droplets with nontrivial topology provides new routes for the assembly of responsive devices. Here we perform a numerical study of spherical nematic droplets on fibres. We analyse the equilibrium textures and find that, under certain conditions,the nematic can avoid the nucleation of topological defects. We consider in detail a homeotropic nematic droplet wrapped around a fibre with planar anchoring. We investigate the effect of an electric field on the texture of this droplet such type of system. In the presence of a DC field, the system undergoes a Freederickzs-like transition above a given threshold $E_c$. We also consider AC fields, at high and low frequencies, and find that the textures are similar to those observed for static fields, in contrast with recently reported experiments.
Particle selection through topographic surface patterns in nematic colloids
Z. Eskandari,N. M. Silvestre,M. M. Telo da Gama,M. R. Ejtehadi
Physics , 2014, DOI: 10.1039/C4SM02231A
Abstract: We propose the use of topographic modulation of surfaces to select and localize particles in nematic colloids. By considering convex and concave deformations of one of the confining surfaces we show that the colloid-flat surface repulsion may be enhanced or switched into an attraction. In particular, we find that when the colloidal particles have the same anchoring conditions as the patterned surfaces, they are strongly attracted to concave dimples, while if they exhibit different anchoring conditions they are pinned at the top of convex protrusions. Although dominated by elastic interactions the first mechanism is reminiscent of the depletion induced attraction or of the key-lock mechanism, while the second is specific to liquid crystal colloids. These long-ranged, highly tunable, surface-colloid interactions contribute for the development of template-assisted assembly of large colloidal crystals, with well defined symmetries, required for applications.
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