Seminarios

Proximo Seminario

Seminario DFI Extraordinario: Surface Studies of Ice

Expositor: Enge Wang

School of Physics

Peking University

CHINA

Martes 22 de enero, 16 h **NOTAR LA FECHA**

Av Blanco Encalada 2008, tercer piso

Sala de Seminarios

Despite ice being a ubiquitous and well-studied substance, it is surprising that some basic questions about its surface properties are still debated. Here computer simulations are used to study the unusual structure and dynamics of ice surface at atomic scale. An order parameter, which defines the ice surface energy, is identified for the first time. A classical electrostatic model proves useful to explain the physics inside. We predict that the proton order-disorder transition, which occurs in the bulk at ~72 K, will not occur at the surface at any temperature below surface melting. In addition, we find that the surface of crystalline ice exhibits a remarkable variance in vacancy formation energies that is more characteristic of an amorphous material. A fraction of surface molecules are bound by less than the strength of a single hydrogen bond, yet other sites are more strongly bound than those in the crystal interior. Vacancy energies are found to be as low as ~0.1eV at the surface, leading to a higher than expected concentration of vacancies at the external layer. Once a surface vacancy is formed, the energetic cost of forming neighbouring vacancies is greatly reduced, facilitating pits on the surface and other processes that may contribute to the phenomenon of pre-melting and quasi-liquid layer formation. Finally, we show that the distribution of local arrangement of dangling atoms, characterized by a surface proton order parameter, is also of crucial importance for the adsorption of water monomer on ice surface. The positive correlation of adsorption energy of water monomer with surface proton ordering suggests that the adsorption may prefer to firstly occur in the inhomogeneous surface, which sheds light on our understanding of the ice nucleation and growth as well as other physical/chemical reactivity in high altitude clouds.

Detalles

Seminarios Anteriores

SEMINARIO DFI: Fluctuaciones capilares de una interfase sólido-líquido en un sistema granular no coh

Expositor:

Nicolás Mujica

DFI

Viernes 30 de noviembre, 16 h

Av. Blanco Encalada 2008, 3er piso

Sala de Seminarios

Uno de los comportamientos colectivos mas notables de un sistema granular no cohesivo es que bajo ciertas condiciones se observa la formación de grumos densos, e incluso de fases sólidas. En esta charla se presentará un estudio experimental y teórico de las fluctuaciones de tipo termo-capilares que se observan en un sistema granular somero compuesto por partículas no cohesivas. Cuando el sistema es forzado por sobre un umbral crítico de vibración, el medio granular se separa en un dominio de fase sólida en coexistencia con una fase liquida, con una interfase bien definida pero fuertemente fluctuante. El mecanismo físico de la formación del grumo sólido es entendido por una presión efectiva del medio que presenta una comprensibilidad negativa de tipo van der Waals. Desde el punto de vista microscópico, esto ocurre por las fuertes interacciones colisionales que existen, las cuales son disipativas, y no debido a interacciones de tipo cohesivas. En promedio el grumo sólido se parece a una gota, con una forma sorprendentemente circular. En esta charla se presentará el estudio de las fluctuaciones entorno a esta interfase circular, mediante su caracterización con funciones de correlación estáticas y dinámicas en el espacio de Fourier. Éstas resultan estar bien descritas por un modelo térmico-capilar, lo que permite medir la tensión de superficie y mobilidad efectivas del sistema una vez que la energía cinética ("térmica") es determinada. Por último, en analogía con los sistemas atómicos, se mostrará que tanto la tensión de superficie como la mobilidad pueden ser estimadas mediante un análisis dimensional considerando las escalas típicas de energía, de longitud y de tiempo del sistema.

Detalles

SEMINARIO DFI: Imágenes de Difusión por Resonancia Magnética Cuantitativa: Principios y Técnicas

Expositor: Hernán Jara

Department of Radiology

Boston University School of Medicine

Boston, MA (USA)

Viernes 16 de Noviembre, 16 h

Blanco Encalada 2008, 3er piso

Sala de Seminarios

Resumen: Se discutirán los principios teóricos y las técnicas experimentales para el mapeamiento del coeficiente de difusión por resonancia magnética. Específicamente, se discutirán las técnicas basadas en la metodología de gradientes magnéticos pulsantes (Stejskal-Tanner) así como nuevas metodologías basadas en relaxometría. Se proveerán ejemplos comparativos en el cerebro humano.

Detalles

Seminario DFI: Mechanics and Dynamics of Plant Cell Division

Expositor: Jacques Dumais

Facultad de Ingeniería y Ciencias

Universidad Adolfo Ibáñez

Viña del Mar

Viernes 9 de noviembre, 16 h

Av. Blanco Encalada 2008, 3er piso

sala de seminarios

The division of eukaryotic cells involves the assembly of complex cytoskeletal

structures to exert the forces required for chromosome segregation and

cytokinesis. In plants, tensional forces within the cytoskeleton appear to

constrain cells to divide according to a small number of area minimizing

configurations. We have shown that the probability of observing a particular

division configuration increases inversely with its relative area according to an

exponential probability distribution known as the Gibbs measure. The distribution

is universal up to experimental accuracy with a unique constant that applies for

all plants studied irrespective of the shape and size of their cells. Using a

maximum entropy formulation, we were able to demonstrate that the empirically

derived division rule is predicted by the dynamics of the tense cytoskeletal

elements controlling the positioning of the division plane. Finally, by framing this

division rule as a dynamical system we were able to identify several attractors

that are predictive of characteristic cell patterns observed in plants. Plant cell

division thus offers a remarkable example of how simple mechanical interactions

at the subcellular level can lead to subtle behaviors at the cellular or multicellular

levels.

Detalles

SEMINARIO DFI: The Cosmic History of Black Hole Growth

Expositor:

Ezequiel Treister

Departamento de Astronomia

Universidad de Concepcion

Viernes 26 de Octubre, 16 h

Blanco Encalada 2008, 3er piso

Sala de Seminarios

Abstract: It is now clear that supermassive black holes (M>1e6 Msun) live in the center of most (all) galaxies, including our own Milky Way. Furthermore, it is now clear that the energy released during the growth of this black hole is a critical ingredient in understanding galaxy formation and evolution. In this talk, I will show what we know about how, when and where these supermassive black holes are acquiring their masses. In particular, I will focus on the effects of obscuration, as it is now clear that the majority of this black hole growth is hidden from our view by large amounts of gas and dust. I will present observational evidence for the link between major galaxy mergers and black hole growth. Future data obtained combining observations with the ALMA radio telescope and NuSTAR X-ray observatory will allow us to finally understand the physical details of this connection.

Detalles

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