Estudio mesoscópico de la adsorción de fluidos simples sobre sustratos microestructurados

Programa de Doctorado en Ciencia y Tecnología de Coloides e InterfasesEn esta Memoria se han estudiado modelos mesoscópicos de adsorción en fluidos simples sobre sustratos microestructurados, tanto analíticamente, numéricamente y mediante simulación por ordenador. Hemos considerado dos tipos prototípicos de sustratos microestructurados: la hendidura en cuña y el sustrato sinusoidal. El primer caso se ha estudiado por simulación por ordenador para el modelo de Ising, centrándonos en las transiciones de rellenado crítica y tricrítica. Estos resultados se han analizado usando resultados analíticos de un modelo fenomenológico que tiene en cuenta las fluctuaciones interfaciales del tipo modo de respiración. Hemos usado como parámetro de control una interacción modificada espín-sustrato en una región alrededor del fondo de la hendidura, que favorece la adhesión de la interfase a dicha región. Si esta interacción no es lo suficientemente intensa para inducir dicha adhesión, hemos visto que nuestros resultados de simulación tienen un buen acuerdo con las predicciones teóricas del modelo fenomenológico para la transición crítica. Si se aumenta su intensidad, se observa un cambio en el comportamiento interfacial consistente con una transición de rellenado de primer orden. Ajustando el valor de la interacción, se observa un cierto valor que es consistente con la predicción de la teoría fenomenológica para la transición tricrítica. Para el sustrato sinusoidal, hemos obtenido el diagrama de fases de adsorción en la aproximación de campo medio tanto en condiciones de coexistencia en volumen como fuera de ella, centrándonos en la transición de rellenado y mojado. Para ello hemos usado distintos modelos microscópicos de grano grueso: el modelo de Landau-Ginzburg y el modelo de doble parábola. En el primer caso, hemos obtenido el diagrama de fases usando técnicas de elementos finitos cuando el sustrato presenta una transición de mojado de primer orden o una transición de mojado crítica. Dependiendo del orden de la transición, y para una rugosidad dada, la temperatura de transición de mojado se reduce al disminuir el periodo del sustrato para el caso de mojado de primer orden, mientras que permanece inalterada para el caso de mojado crítico. Por otro lado, cuando la amplitud de la corrugación es del orden de la longitud de correlación del líquido, la transición de rellenado desaparece y sólo se observa la de mojado. El límite entre ambos escenarios es un punto triple en el caso de primer orden, y un punto crítico de rellenado para mojado crítico. Finalmente, tanto la transición de rellenado como el mojado de primer orden se extienden a las transiciones de rellenado y de premojado fuera de la coexistencia en volumen, que terminan en sendos puntos críticos. El segundo modelo considerado es el modelo de doble parábola, que por un lado puede considerarse como una aproximación al modelo de Landau-Ginzburg, pero por otro puede relacionarse formalmente con modelos de Hamiltoniano interfacial, permitiendo establecer un nexo entre las descripciones microscópica y mesoscópica de los fenómenos interfaciales. Hemos estudiado el diagrama de fases usando el método de los elementos de contorno para el caso de mojado crítico, obteniendo un buen acuerdo con los resultados del modelo de Landau-Ginzburg en términos del ángulo de contacto y la curvatura interfacial.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y Naturale

Thermodynamic properties of the parabolic-well fluid

Article number 627017The thermodynamic properties of the parabolic-well fluid are considered. The intermolecular interaction potential of this model, which belongs to the class of the so-called van Hove potentials, shares with the square-well and the triangular well potentials the inclusion of a hard-core and an attractive well of relatively short range. The analytic second virial coefficient for this fluid is computed explicitly and an equation of state is derived with the aid of the second-order thermodynamic perturbation theory in the macroscopic compressibility approximation and taking the hard-sphere fluid as the reference system. For this latter, the fully analytical expression of the radial distribution function, consistent with the Carnahan-Starling equation of state as derived within the rational function approximation method, is employed. The results for the reduced pressure of the parabolic-well fluid as a function of the packing fraction and two values of the range of the parabolic-well potential at different temperatures are compared with Monte Carlo and Event-driven molecular dynamics simulation data. Estimates of the values of the critical temperature are also provided.Junta de Andalucía FQM20

Dynamics in field-induced biaxial nematic liquid crystals of board-like particles

Biaxial nematic (NB ) liquid crystals have been indicated as promising candidates for the design of next-generation displays with novel electro-optical properties and faster switching times. While at the molecular scale their existence is still under debate, experimental evidence, supported by theory and simulation, has unambiguously proved that suitable colloidal particles can indeed form NB fluids under specific conditions. While this discovery has sparked a widespread interest in the characterisation of the phase behaviour of NB liquid crystals, significantly less attention has been devoted to the study of their transport properties. To bridge this gap, by Dynamic Monte Carlo simulations we have investigated the equilibrium dynamics of field-induced NB phases comprising monodisperse hard cuboids. In particular, we calculated the long-time self-diffusion coefficients of cuboids over a wide range of anisotropies, spanning prolate to oblate geometries. Additionally, we have compared these diffusivities with those that, upon switching the external field off, are measured in the thermodynamically-stable isotropic or uniaxial nematic phases at the same density. Our results indicate that while prolate cuboids diffuse significantly faster in biaxial nematics than in less ordered fluids, we do not observe such an increase with oblate cuboids at high packing fractions. We show that these changes are most likely due to the field-induced freezing of the axes perpendicular to the nematic director, along with a substantial increase in the ordering of the resulting NB phase.Consejería de Transformación Económica, Industria, Conocimiento y Universidades de la Junta de Andalucı́a/FEDER (P20-00816)Spanish Ministerio de Ciencia, Innovación y Universidades and FEDER (PGC2018-097151-B-I00)Consejería de Transformación Económica, Industria, Conocimiento y Universidades de la Junta de Andalucı́a through post-doctoral grant no. DC 00316 (PAIDI 2020), co-funded by the EU Fondo Social Europeo (FSE)Maria Zambrano Senior distinguished researcher fellowship, financed by the European Union within the NextGenerationEU progra

FRAGILIDAD FINANCIERA DE LAS FIRMAS EN COLOMBIA, 2000-2006: UN ANÁLISIS DISCRIMINANTE DE UN MODELO

Con base en Foley (2003) se establecen las situaciones financieras de las firmas, clasificándolas en cubiertas, especulativas y Ponzi. Se aplica también un análisis discriminante, técnica multivariante que permite una clasificación empírica basada en los índices financieros. Como resultado, la clasificación teórica es coherente con la empírica y evidencian que en la fase creciente de un auge hay un buen porcentaje de firmas cubiertas, pero a su vez, surge un leve deterioro de su estabilidad financiera. Así, se da evidencia empírica de consideraciones relevantes de la Hipótesis de Inestabilidad Financiera, se testea el segmento microeconómico de Foley (2003) y se plantea un indicador alternativo de la fragilidad financiera.Hipótesis de inestabilidad financiera, dinámica de financiamiento, análisis discriminante.

Filling and wetting transitions on sinusoidal substrates: a mean-field study of the Landau-Ginzburg model

We study the interfacial phenomenology of a fluid in contact with a one-dimensional array of infinitely long grooves of sinusoidal section, characterized by the periodicity length L and amplitude A. The system is modelled by the Landau–Ginzburg–Wilson functional, with fluid-substrate couplings which control the wettability of the substrate. We investigate the filling and wetting phenomena within the mean-field approximation, and compare with the predictions of the macroscopic and interfacial Hamiltonian theories. For large values of L and under bulk coexistence conditions, we observe first-order filling transitions between dry (D) and partially filled (F) interfacial states, and wetting transitions between partially filled F and completely wet (W) interfacial states of the same order as for the flat substrate. Depending on the order of the wetting transition, the transition temperature is either shifted towards lower temperatures for first-order wetting or it coincides with the wetting temperature on the flat substrate for continuous wetting. On the other hand, if the groove height is of order of the correlation length, only wetting transitions between D and W states are observed under bulk coexistence conditions. For this case, the transition temperature shift obeys approximately Wenzel’s phenomenological law if the substrate favors first-order wetting, but it remains unshifted for continuous wetting. The borderline between the small and large L regimes correspond to a D − F − W triple point if wetting is first-order, and a D − F critical point for continuous wetting. Beyond bulk coexistence conditions, filling and first-order wetting transitions continue into off-coexistence filling and prewetting lines, which end up at critical points. Our findings show that the macroscopic theory only describes accurately the filling transition close to bulk coexistence and large L, while microscopic structure of the fluid is essential to understand wetting and filling away from bulk coexistence.España, Ministerio de Economía y Competitividad FIS2009-09326España, Ministerio de Economía y Competitividad FIS2012-32455Junta de Andalucía, P09-FQM-493

Study of PMT target specificity in Ustilago maydis

Ustilago maydis is a pathogenic fungi responsible for the corn smut fungus disease, which causes a significant loss in mayze production every year. The PMT is a family of well-conserved O-mannosylating proteins. In Ustilago maydis, the deletion of Pmt2 has been shown to be deleterious and the deletion of Pmt4 disrupts completely the infectious process; on the other hand, the deletion of Pmt1 doesn't manifest any phenotype. All PMT proteins have three domains, named PMT, MIR and 4TMC, and several transmembrane regions. We hipothesized that one of these domains of Pmt4 is responsable for the substrate specificity that confers the virulence phenotype in Ustilago maydis Pmt4 and, if this would be the case, we could develop an antifungal drug specific to this domain. To check this, we built three chimerical protein strains, chanching one domain in Pmt4 for the same domain of Pmt1 at a time, and measure the tumor formation in 3 independent experiments. We also built a full Pmt4 length strain as a positive control. The results indicate that either the protein requires all three Pmt4 domains to infect or the chimerical protein is not working properly.Furthermore, we also wanted to check what region gives Pmt2 its specificity for growth. In Schizosaccharomyces pombe, a ortholog of Pmt2, named Ogm2, present the same essencial-for-growth phenotype than in U. maydis. Since U. maydis doesn't have any repressable promoter, we built a S. pombe strain with the shut-off promoter nmt81 in the endougenous Ogm2 gene and will complement the phenotype with mutagenized plasmids containing the U. maydis Pmt2 and measure its viability.Finally, we also plan to build other three strains interchanching Pmt4 domains in the Pmt1 protein

Chiral flow in a binary mixture of two-dimensional active disks

We study, experimentally, the dynamics of a binary mixture of air-fluidized disks. The disks are chiral since they incorporate a set of blades with constant tilt. Both species are identical except for their blades tilt angle, which is rotated by 180o in the second species. We analyze the phase behavior of the system. Our analysis reveals a wide range of different fluid dynamics, including chiral flow. This chiral flow features in its base state a large vortex. We report, for certain ranges of relative particle density of each species, inversion of the vorticity of this vortex. We discuss on the possible mechanisms behind these chiral flow transitions.Comment: Additional data can be found in: https://doi.org/10.5281/zenodo.664770

Dynamics in field-induced biaxial nematic liquid crystals of board-like particles

Biaxial nematic (NB) liquid crystals have been indicated as promising candidates for the design of next- generation displays with novel electro-optical properties and faster switching times. While at the molecular scale their existence is still under debate, experimental evidence, supported by theory and simulation, has unambiguously proved that suitable colloidal particles can indeed form NB fluids under specific conditions. While this discovery has sparked a widespread interest in the characterisation of the phase behaviour of NB liquid crystals, significantly less attention has been devoted to the study of their transport properties. To bridge this gap, by Dynamic Monte Carlo simulations we have investigated the equilibrium dynamics of field-induced NB phases comprising monodisperse hard cuboids. In particular, we calculated the long-time self-diffusion coefficients of cuboids over a wide range of anisotropies, spanning prolate to oblate geometries. Additionally, we have compared these diffusivities with those that, upon switching the external field off, are measured in the thermodynamically-stable isotropic or uniaxial nematic phases at the same density. Our results indicate that while prolate cuboids diffuse significantly faster in biaxial nematics than in less ordered fluids, we do not observe such an increase with oblate cuboids at high packing fractions. We show that these changes are most likely due to the field-induced freezing of the axes perpendicular to the nematic director, along with a substantial increase in the ordering of the resulting NB phase.Universidad Pablo Olavide. Departamento de Sistemas Físicos, Químicos y Naturale