Crystallization of PNIPAM-hydrogel nanospheres inferred by static light scattering

In this paper we present a kinetic study on crystallization of a colloidal dispersion formed by thermosensitive Poly-N- Isopropylacrylamide (PNIPAM) nanospheres. The liquid-crystal transition is induced by decreasing the temperature of the dispersion and the crystallization process is monitored by measuring the evolution of the static structure factor of the dispersion by means of light scattering techniques. The measured static structure factor confirms the formation of a mesoscopic crystal array at the final equilibrium state, which is preceded by two stages: nucleation and crystal growth

Thickness effect on electric resistivity on polystyrene and carbon black- based composites

Changes on electrical resistivity were experimentally studied for polystyrene and carbon black-based composites respect to the temperature. 22% w/w carbon black composite films at 30 μm, 2mm y 1cm thick were submitted to thermal heating-cooling cycles from room temperature to 100°C, slightly up to Tg of the composite. For each cycle changes on electrical resistivity constituent a hysteresis loop that depends on the sample thickness. The changes during the heating stage could be explained as a consequence of the thermal expansion and mobility of the polymer chains at Tg, producing a disconnecting of the electrical contacts among carbon black particles and an important increasing (200%) of the electrical resistivity. For each cycle, the hysteresis loop was observed in thicker samples, whereas for 30 ? m thickness sample the hysteresis loop was lost after four cycles.CONACY

Laser ablated carbon nanodots for light emission

The synthesis of fluorescent carbon dots-like nanostructures (CNDs) obtained through the laser ablation of a carbon solid target in liquid environment is reported. The ablation process was induced in acetone with laser pulses of 1064, 532, and 355 nm under different irradiation times. Close-spherical amorphous CNDs with sizes between 5 and 20 nm, whose abundance strongly depends on the ablation parameters were investigated using electron microscopy and was confirmed using absorption and emission spectroscopies. The π- π* electronic transition at 3. 76 eV dominates the absorption for all the CNDs species synthesized under different irradiation conditions. The light emission is most efficient due to excitation at 3.54 eV with the photoluminescence intensity centered at 3. 23 eV. The light emission from the CNDs is most efficient due to ablation at 355 nm. The emission wavelength of the CNDs can be tuned from the near-UV to the green wavelength region by controlling the ablation time and modifying the ablation and excitation laser wavelength

Caracterización óptica de sistemas coloidales

En el capítulo se ha descrito brevemente la técnica de dispersión de luz utilizada comúnmente para caracterizar las propiedades estáticas y dinámicas de sistemas coloidales. En particular, se ha mostrado su utilidad para caracterizar el proceso de cristalización de una dispersión de microgeles de PNIPAM a partir de la evolución temporal del factor de estructura del sistema.Secretaría de Investigación y Estudios Avanzados-UAEMex

La ciudad desde el aire: vistas aéreas y “vedutas” del centro de Bogotá

El retrato de ciudad desde el aire, a vuelo de pájaro o a través del vedutismo, es un género de “fotografía urbana dibujada” que surge en Europa a principios del siglo XVI con la Tavola Strozzi de Nápoles de 1472. Las vedutas tendrán una época de esplendor con destacados pintores urbanos del siglo XVIII como Gaspar van Wittel (1653-1736), precursor de la tradición del Vedutismo y de pintores como Canaletto (1697-1768) o Francesco Guardi (1712-1793). Las representaciones urbanas de ciudad, panorámicas o parciales, han permitido mostrar según las distancias y alturas, las relaciones con la geografía y el paisaje, las estructuras, tramas, vida urbana, calles, edificios y habitantes. Con el tiempo, la fotografía ha ido complementando y también sustituyendo las vistas oblicuas, a vuelo de pájaro y las vedutas urbanas dibujadas, por la facilidad de capturar imágenes desde puntos altos a través de una ventana, desde una terraza, un globo, un avión, o incluso Google Earth. En Bogotá, las vistas realizadas por el Instituto Geográfico Agustín Codazzi (IGAG), las tomas de los fotógrafos-aviadores Saúl Ordúz y Rudolph, y las vedutas urbanas de fotógrafos como Armando Matiz, entre 1950 y 1970, conforman un conjunto de miradas desde el aire y a diferentes alturas. Una colección de “vistas y vedutas modernas”, que testimonian la evolución urbana y además revelan ante una mirada atenta, como la arquitectura moderna reconfiguro el centro de la ciudad, con transformaciones donde se agrupan nuevos edificios y espacios urbanos con históricos y emblemáticos. Un caso destacable de esto, son los espacios centrales a lo largo de la carrera Séptima: la Plaza de Bolívar, el Parque Santander y el Centro Internacional.Portrait city from aerial views, through “vedutismo”, is a genre of “urban photography drawn” that emerged in Europe in the early sixteenth century with the Strozzi Tavola city of Naples in 1472. The vedutas will have a heyday with outstanding urban eighteenth-century painters as Caspar van Wittel (1653-1736), forerunner of the tradition of vedutismo and painters such as Canaletto (1697-1768) and Francesco Guardi (1712-1793). Urban representations city, panoramic or partial, have enabled display for distances and heights, city relations with geography and landscape, structures and urban plots or urban life, the streets and buildings and its inhabitants. Over time, photography has been complementing and replacing the oblique views, a bird and urban vedutas drawn by the ability to capture images from high points through a window, a terrace, a balloon, a plane , and even Google Earth. In Bogota, the views held by the IGAC (Instituto Geográfico Agustín Codazzi), takes photographers-aviators Saul Orduz and Rudolph, and urban vedutas of photographers like Leo and Armando Matiz, between 1950 and 1970, make up a set of looks from the air at different heights. A collection of “modern vedutas” which testify to the urban evolution and also reveal to a careful look as modern architecture reconfigure the city center, with transformations where new buildings and urban spaces with historical and emblematic are grouped. A case of this are the central spaces along the Seventh race: Bolivar square, Santander Park and International Center

Coupled resonator acoustic waveguides-based acoustic interferometers designed within two-dimensional phononic crystals: experiment and theory

The acoustic response of defect-based acoustic interferometer-like designs, known as Coupled Resonator Acoustic Waveguides (CRAWs), in two-dimensional phononic crystals (PnCs) is reported. The PnC is composed of steel cylinders arranged in a square lattice within a water matrix with defects induced by selectively removing cylinders to create Mach-Zehnder-like (MZ) defect-based interferometers. Two defect-based acoustic interferometers of MZ-type are fabricated, one with arms oriented horizontally and another one with arms oriented diagonally, and their transmission features are experimentally characterized using ultrasonic spectroscopy. The experimental data are compared with finite element method (FEM) simulations and with tight-binding (TB) calculations in which each defect is treated as a resonator coupled to its neighboring ones. Significantly, the results exhibit excellent agreement indicating the reliability of the proposed approach. This comprehensive match is of paramount importance for accurately predicting and optimizing resonant modes supported by defect arrays, thus enabling the tailoring of phononic structures and defect-based waveguides to meet specific requirements. This successful implementation of FEM and TB calculations in investigating CRAWs systems within phononic crystals paves the way for designing advanced acoustic devices with desired functionalities for various practical applications, demonstrating the application of solid-state electronics principles to underwater acoustic devices description

Bolivia and the paradoxes of democratic consolidation

In Bolivia from the 1990s on, two presidents were ousted by popular protests, and protests were rampant. The protests expressed a growing discontent not only with successive administrations and their policies but with politics itself. The polity failed to built trust in democracy, ignored or repressed protests, and thus contributed to a process of democratic "deconsolidation." The main factors were corruption and the reluctance of the traditional political parties to discuss the neoliberal economic model. As a result, the current administration of Evo Morales faces two challenges: to change economic policies and to repair the support for democracy

Introducción

La memoria es ese espacio de hilos delgados y gruesos por el que es complejo transitar. Los días, los meses y los años la convierten en un terreno impredecible de zonas fangosas; pero sobre ella, venciendo obstáculos, siempre hay que volver.De eso trata este libro, de volver sobre el pasado de 12 colombianos que hoy superan los 80 años, y que fueron testigos de la época de La Violencia, que se establece desde 1930 y que se complejiza el 9 de abril de 1948, con el asesinato de líder político Jorge Eliecer Gaitán

Estudio de ecosistemas terrestres y acuáticos ubicados en el parque nacional natural cueva de los guácharos (acevedo, huila)

Los estudios realizados durante el tiempo en el que se realizó la salida de campo de la asignatura Ecología Regional Continental del 3 al 17 de septiembre de 2010 permitieron determinar ciertas características y estados de calidad en los sistemas acuáticos y vegetales (bosque subandino) del Parque Nacional Natural Cueva de los Güácharos(PNNCG). Para el estudio limnológico se realizaron muestreos en varios ecosistemas acuáticos: Laguna Encantada, Quebrada Chánchiras, Quebrada La Lindosa, Cascada Cristales, Rí­o Suaza, Cueva de los Guacharos y Cueva del Indio. Se tomaron muestras a de variales fisicoquímicas y de comunidades biológicas (perifiton, plancton, macroinvertebrados). Las aguas de la región son de temperaturas medias y se mineralizan al pasar por el sistema de cuevas; son ligeramente alcalinas y bajas en nutrientes (oligotróficas) y transportan bajas a moderadas cantidades de sólidos suspendidos. Los datos fisicoquímicos que caracterizan a los ecosistemas acuáticos del PNNCG corresponden a sistemas típicos neotropicales, con baja intervención humana y condiciones propias de ambientes cársticos. La materia orgánica aportada por la vegetación riparia y por las macrófitas sustenta diferentes órdenes de macroinvertebrados, siendo los más representativos Diptera, Ephemeroptera, Hemíptera y Trichoptera. El bosque subandino estudiado se encuentra en proceso de desarrollo debido a que presenta una mayor cantidad de arboles del conjunto del futuro; no obstante, el conjunto del presente, aunque en una menor proporción, se encuentra bien establecido. Esto también se puede observar en los mapas de cobertura vegetal del estrato arbóreo, donde el dosel es continuo y solamente se interrumpe por los disturbios ocasionados por la muerte y caída de árboles del conjunto del presente, lo que genera claros en el bosque y permite que los arboles del conjunto del futuro desarrollen su cobertura