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Seminario DFI: Jets capilares: Formación controlada de gotas |
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Dra. María Luisa Cordero (DFI-FCFM, Universidad de Chile)
Viernes 12 de Marzo -2010, 16:00 hrs.
Sala F-12, Edificio de Fìsica, Av. Blanco Encalada 2008 La estabilidad de un jet capilar sujeto a la acción de la tensión superficial es uno de los problemas más clásicos de dinámica de fluidos. Los primeros estudios se remontan a fines de 1800 con los estudios de Plateau y Rayleigh quienes consideraron un fluido perfecto rodeado de un medio no viscoso e infinito. Desde entonces múltiples trabajos han expandido estos resultados, considerando diferentes parámetros tales como la densidad, viscosidad y velocidad de ambos fluidos y el confinamiento en una geometría finita. Basándose en la ruptura de un jet en gotas por esta inestabilidad, el uso de jets capilares se ha expandido a variadas aplicaciones tales como impresión de tinta, atomización de aerosoles y producción de emulsiones. Los jets líquidos exhiben dos tipos de comportamiento, dependiendo esencialmente de la competición entre fuerzas viscosas y capilares. Estos dos comportamientos son los que se observan al abrir progresivamente un grifo en la cocina: a bajo caudal el agua se rompe en gotas inmediatamente a la salida del grifo, un fenómeno denominado "dripping". Al aumentar el caudal el agua forma una larga columna que se desestabiliza en gotas, a lo cual llamamos "jetting". Estudios recientes relacionan estos fenómenos en dos tipos de inestabilidad hidrodinámica: mientras que el comportamiento de dripping correspondería a una inestabilidad absoluta, el comportamiento de jetting sería un reflejo de una inestabilidad convectiva. En este seminario mostraré la caracterización experimental de un jet de fluido en geometría confinada en su comportamientos tipo dripping y jetting, y la comparación con las teorías existentes, confirmando la relación entre los tipos de inestabilidad y los diferentes comportamientos observados. La teoría permite predecir, en el régimen de jetting, la formación de gotas de tamaño controlado mediante el forzamiento a una frecuencia dada, lo cual se demuestra experimentalmente mediante el uso de un láser focalizado de potencia modulada sinusoidalmente que desestabiliza localmente el sistema. |
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| Seminarios Anteriores |
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Seminario DFI: Stochastic modeling of active biological transport in inhomogeneous environments |
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Sr. Philip Greulich (University of Saarbruecken, Germany) Viernes 5 de Marzo -2010, 16:00 hrs. Sala F-12, Edificio de Fìsica, Av. Blanco Encalada 2008 In this talk I present new results for systems of actively driven particles on biased tracks in inhomogeneous environments. Examples for such systems are vehicular- and pedestrian traffic. The main focus, however, is on biological systems: We model collective directed motion of molecular motors involved in protein production and the transport of cargo on intracellular filaments. Collective transport on inhomogeneous tracks exhibits a jamming transition that emerges if the particle current attains the transport capacity of a bottleneck, which marks the maximum current. Jamming can be observed in traffic, but also for molecular motors on cell filaments or nucleic acid strands. I present an analytical scheme to predict the transport capacity and critical parameters of this transition. The presented stochastic models apply to tracks with slow sites (defects). These can, for example, be induced by biomedical drugs. In the context of intracellular traffic, defects are presently discussed as a cause of several diseases, e.g. Alzheimer's disease. Also particular sites on RNA can slow down molecular motors that polymerize proteins. Furthermore, a model for transport on (cell filament-) networks is presented. It is shown that particle clusters emerge. In contrast to regular networks or diffusion limited (reversible) aggregation, inhomogeneous networks exhibit a scale-free distribution of cluster sizes. This result can help to distinguish microscopic dynamics and structures by analyzing macroscopic particle cluster patterns. Applied to clusters of membrane proteins that promote the internalization of toxins, an analysis of clusters might improve the understanding of toxic pathways. |
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Seminario No Lineal: How convective instability drastically affects dissipative solitons |
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Dr. Saliya Coulibaly (Laboratoire de Physique des Lasers, Atomes et Molécules PhLAM, Lille, France) Martes 26 de Enero - 2010, 16:30 hrs. Laboratorio de Materia Fuera del Equilibrio, Av. Blanco Encalada 2008, Zòcalo Poniente, Edificio de Fìsica In spatially-extended convective systems, reflection-symmetry-breaking; i.e. induced by drift effects, leads to a striking nonlinear effect that drastically affects the shape, frequency, and velocity of dissipative solitons. The utmost importance of convection, in the case of optical parametric oscillators, is revealed via the appearance of an original nonlinear gradient term in the complex quintic Ginzburg-Landau amplitude equation, which describes the weakly nonlinear dynamics of the system. This makes it possible to explain the self-frequency shift, the slowing down and the nonlinear symmetry breaking observed in the envelope of dissipative solitons emitted by optical parametric oscillators. |
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Seminario DFI: Optical rogues waves: Theory and Experiments |
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Dr. Majid Taki (Université des Sciences et Technologies de Lille, Laboratoire de Physique des Lasers, Atomes et Molécules PhLAM) Viernes 22 de Enero -2010, 16:00 hrs. Sala F-12, Edificio de Fìsica, Av. Blanco Encalada 2008 In the ocean, giant waves (also called killer waves, freak or rogue waves) are extremely rare and strong events. They are not well understood yet and the conditions which favour their emergence are unclear. Very recently, it was shown that the governing equations as well as the statistical properties of an optical pulse propagating inside an optical fibre mimic very well these gigantic surface waves in the ocean. Here we generate both experimentally and numerically optical rogue waves in a photonic crystal fiber (microstructured fiber) with continuous wave (CW) pumps. This is relevant for establishing an analogy with rogue waves in an open ocean. After recalling fundamental rogue waves known as Akhmediev breathers that are solutions of pure nonlinear Schrödinger (NLS) equation, we analytically demonstrate that a generalized NLS equation, which governs the propagation of light in the fiber, exhibits convective modulationnal instability. The latter provides one of the main explanations of the optical rogue wave extreme sensitivity to noisy initial conditions at the linear stage of their formation. In the highly nonlinear regime, we provide the evidence that optical rogue waves result from soliton collisions leading to the rapid appearance/disappear |
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Seminario DFI: Astronomical Instrumentation at U de Chile: The Terahertz Photonics Laboratory |
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Dr. Ernest Michael (Departamento de Ingenería Eléctrica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile) Viernes 15 de Enero -2010, 16:00 hrs. Sala F-12, Edificio de Fìsica, Av. Blanco Encalada 2008
This talk summarizes the projects in the Astronomical Instrumentation Group, which are roughly the development of1.) Micro-devices for photonic THz-generation and (also non-cryogenic) submm/THz-detection2.) Improved critical photonic components for ALMA3.) A photonic 30 – 950 GHz test source for ALMA4.) Fiber-based near-infrared heterodyne interferometry5.) A prototype receiver cartridge for ALMA Band and will concentrate on the micro-device development.
First experience in this direction is collected by our two projects on improving efficiency and bandwidth of photonic THz generation, by developing the concept of vertically illuminated traveling-wave (TW)NIR-photomixers. Through the associated thorough training of students in micro-fabrication at versed international groups, those projects could therefore contribute to the formation of Chilean research on micro-devices. Our collaboration with ALMA gives hands-on experience on all the aspects of radio-astronomical instrumentation.
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