Ory Zik

Fingering Instability in Combustion

A thin solid (e.g., paper), burning against an oxidizing wind, develops a fingering instability with two decoupled length scales. The spacing between fingers is determined by the Peclet number (ratio between advection and diffusion). The finger width is determined by the degree two dimensionality. Dense fingers develop by recurrent tip splitting. The effect is observed when vertical mass transport (due to gravity) is suppressed. The experimental results quantitatively verify a model based on diffusion limited transport.

Optical fibers and solar power generation

Abstract A study of the potential use of optical fibers for solar thermal power generation is presented. The main performance characteristics (numerical aperture and attenuation) and typical costs of currently available fibers are discussed. Several approaches to the application of fibers are presented, for centralized (tower, central receiver) and distributed (dish–engine) systems. The overall system design-point efficiency and overall system cost are estimated. A scaling relation between system size and the cost of the fiber component is identified, which severely limits the applicability of fibers to small systems only. The overall system cost for centralized systems is found to be higher than the currently competitive range, even under optimistic assumptions of mass production of major components. A significant reduction in fiber cost is required before the use of fibers for centralized solar power generation can become competitive. In distributed generation using dish/engine systems, however, the use of fibers does achieve competitive performance and costs, comparable to the costs for conventional dish systems.

Fingering instability in solid fuel combustion: The characteristic scales of the developed state

We present new results on the fingering instability in solid fuel combustion. The instability occurs when horizontal fuel is forced to burn against an oxidizing wind, under the suppression of vertical flow (natural convection). Focusing on the qualitative behavior of the developed fingering state, we present experimental results that connect the length scales to the two dominant transport processes (transport of reactants and loss of heat). We first elaborate on the relation between reactant transport and the spacing between fingers and proceed to present experimental evidence that relate the characteristic scale (finger width) to the heat losses near the front. The heat losses are varied by three methods: changing the system height, adding nonreacting (cooling) gas to the flow, and changing the heat conductivity of the bottom plate. We discuss the effect of these methods and conclude that the characteristic scale decreases as the heat losses increase. From a practical viewpoint, the results expose a fingering regime in which it is possible that a slow persistent finger will create and maintain a fire hazard that is below conventional detection. A very high degree of experimental control is achieved in the smoldering regime. However, the phenomenon is fuel independent and occurs in other modes of combustion.

A Novel Method for “Wet” SEM

Progress in the processing of wet tissues, without the need of fixation and complex preparation procedures, may facilitate the microscopic examination of tissues and cells. Microscopic examination of tissues is a central tool in clinical diagnosis as well as in diverse areas of research. The authors present the application of Wet SEM, a technology for imaging fully hydrated samples at atmospheric pressure in a scanning electron microscope (SEM). The technique is based on 2 principles. First, samples are imaged in sealed specimen capsules and are separated from the evacuated interior of the electron microscope by a thin, electron-transparent partition membrane that is strong enough to sustain a 1-atm pressure difference. Second, imaging is done in a SEM, based on detection of backscattered electrons, which penetrate a few microns into the specimen and thus give information on the cellular level.

Wet SEM: A Novel Method for Rapid Diagnosis of Brain Tumors

The authors present the application of wet SEM for histopathological assessment, a technology for imaging fully hydrated samples at atmospheric pressure in a scanning electron microscope (SEM). Both transmission and scanning electron microscopy techniques usually require long and complex sample preparation of the tissues. In marked contrast, a rapid preparation of tissues is described for evaluation by SEM imaging. The wet SEM technology successfully demonstrated both histological and ultrastructural features of several CNS tumors: Rosette formation and intracytoplasmic lumens were observed in ependymoma; numerous fibrillary processes in fibrillary astrocytoma; and focal rosette formation with no intracytoplasmic lumens in medulloblastoma. Application of this method simultaneously with frozen section may improve rapid intraoperative diagnosis of these intracranial tumors.

Wet electron microscopy with quantum dots.

Wet electron microscopy (EM) is a new imaging method with the potential to allow higher spatial resolution of samples. In contrast to most EM methods, it requires little time to perform and does not require complicated equipment or difficult steps. We used this method on a common murine macrophage cell line, IC-21, in combination with various stains and preparations, to collect high resolution images of the actin cytoskeleton. Most importantly, we demonstrated the use of quantum dots in conjunction with this technique to perform light/electron correlation microscopy. We found that wet EM is a useful tool that fits into a niche between the simplicity of light microscopy and the high spatial resolution of EM.

The TROF (tower reflector with optical fibers) : A new degree of freedom for solar energy systems

The integration of optical fibers into solar energy systems requires a trade-off between the cost, attenuation, and a limited flux carrying capability (due to limited numerical aperture) on one hand, and the flexibility in light distribution on the other hand. This paper presents a novel approach that minimizes the length of fibers in the system while fully utilizing the flexibility advantage. Optical fibers have been steadily improving and their cost has been declining as a result of the proliferation of their use in communication, and more recently in the lighting industry. The use of fibers in concentrating solar thermal systems has potential advantages of providing unprecedented flexibility in the final concentration and the receiver design. A central receiver system based on the tower reflector with optical fibers (TROF) is presented as a case study in a comparison between conventional concepts of solar thermal power generation, and new concepts employing optical fibers. Two new approaches to thermal conversion utilizing the flexibility of a fiber-based system, non-isothermal high-temperature receivers and distributed receivers, are presented. An approximate performance and cost analysis that assumes mass-produced solar-optimized fibers is presented. The effects of system size and several fiber types are discussed. The results show that the use of current optical fibers may become competitive for solar-driven electricity generation systems under optimistic assumptions. The analysis points to research and development directions that could lead to cost-effective TROF and other optical fiber-based systems in the future.

Morphological phases and exhaustion-induced Hecker effect in electrodeposition

We report a pattern formation study of zinc electrodeposition in galvanostatic conditions. The microscopic structure is based on two alternative growth mechanisms: dendritic (generic) or tip splitting (disturbed) and is decoupled from the macroscopic structure. We present the corresponding phase diagram, demonstrate the limits of the experimental range, and show that the microscopic selection is dominated by the initial concentration of zinc. The macrostructure consists of stable front and dense branches or unstable front and sparse branches. The dense phase is always followed by the Hecker transition. We show that the transition point coincides with the exhaustion of ions from the solution. Hence, the time to the transition is also determined by the concentration.

WETSEM tm - A New Technology for High Resolution Imaging of Fully Hydrated Biological Samples

Electron microscopy is the prime tool for the study of biological ultra structures. Biological specimens are hydrated in their natural state. They therefore cannot be placed in the vacuumed chamber of a conventional Scanning Electron microscope (SEM), since samples will be dried, water would evaporate and interfere with the specimen-emitted electrons and the detectors. Moreover, its routine use in cell biology and histology is hampered by lengthy and potentially destructive sample preparation procedures especially de-hydration of samples. We report here the QX-capsule, based on WETSEMTM, a new technology that allows direct imaging of fully hydrated biological samples in a SEM. This new imaging technology offers unique advantages: 1. Imaging is preformed at atmospheric pressure with wide variety of the user’s preferred temperature. 2. Sample preparation involves only liquid handling, obviating the need for drying, embedding, sectioning or coating. This makes the wet EM easily accessible for routine and reproducible imaging with deformations and other artifacts. 3. Unstained or live samples could be imaged if sufficient contrast could be obtained. Enhancement of sample contrast with a wide variety of EM stains is straightforward. 4. The limited penetration of electrons into the sample allows visualization of wet, unembedded tissues without thin sectioning: only the outer layer of a few micrometers is visualized. 418 Microsc Microanal 11(Suppl 2), 2005 Copyright 2005 Microscopy Society of America DOI: 10.1017/S1431927605505877