Homogenization of glass melts by bubbling on a laboratory scale

In a typical melter, the molten glass tends to be inhomogeneous due to the heterogeneity of the raw materials. One means of yielding more homogeneous glass is bubbling air through the glass melt through nozzles at the base of the melter. The induced fluid flow dissolves cords and homogenizes the glass melt. This bubbling process was investigated on a laboratory scale both from an experimental and a theoretical point of view. A standard soda-lime-silica glass was bubbled with argon in a platinum crucible at 1400 C. The samples treated were tested with regard to their optical homogeneity, using an improved version of the Christiansen-Shelyubskii method. The corresponding fluid flow phenomena were simulated by a suitable mathematical model. Due to the axial symmetry of the bubbling equipment and the high viscosity of the glass melt (creeping flow), the problem can be reduced to the solution of a differential equation of the fourth order with the stream function as independent variable. The numerical treatment superposes Gegenbauer functions matching the given boundary values for the velocity and tension, respectively. The homogeneity strongly increased with bubbling time and its local variation showed good correlation with the calculated flow pattern in the crucible

Konventionelle adhäsive Befestigung und Resin-Coating-Technik (Rasterelektronenmikroskopische Untersuchung zum Verbund von Keramikteilkronen)

In der vorliegenden In-vitro-Studie wurde der Frage nachgegangen, inwieweit die marginale Integrität von Vita Mark II Keramikteilkronen durch verschiedene Befestigungsmaterialien und Befestigungstechniken (Resin-Coating-Technik) vor und nach thermomechanischer Wechselbelastung beeinflusst wird. Für das Untersuchungsgut wurde eine einheitliche Präparationsform gewählt. Hierbei handelt es sich um eine MOD-Kavität mit Überkupplung des palatinalen Höckers um 1,5mm. Es wurden vier verschiedene Adhäsivsysteme zur Befestigung der Keramikteilkronen verwendet: 1) Excite/Variolink II 2) ED Primer/Panavia F 2.0 3) Excite/Variolink II mit Tetric Flow 4) ED Primer/Panavia F 2.0 und Clearfil SE Bond/Clearfil Protect Liner Hierbei stehen die Adhäsivsysteme 1) und 2) für die konventionelle adhäsive Befestigung, 3) und 4) für die Resin-Coating-Technik. Die marginale Integrität wurde mittels quantitativer Randanalyse im Rasterelektronenmikroskop an den Grenzen Keramik/Befestigungssystem, Schmelz/Befestigungssystem und Dentin/Befestigungssystem ermittelt. Die Prüfkörper wurden vorher einem Belastungstest (thermomechanische Wechselbelastung) ausgesetzt. Durch die Anwendung der modifizierten Befestigungssysteme mit der Resin-Coating-Technik konnte keine signifikant bessere Randqualität an den Grenzflächen erreicht werden. Alle vier Befestigungssysteme zeigten eine sehr gute Randqualität. Die konventionelle adhäsive Befestigung ist weniger technik- und zeitintensiv und für den Praktiker leichter anzuwenden

Validating virtual reality for time perception research: Virtual reality changes expectations about the duration of physical processes, but not the sense of time

Immersive virtual reality (VR) provides a versatile method for investigating human time perception, because it allows the manipulation and control of relevant variables (e.g., the speed of environmental changes) that cannot be modified in the real world. However, an important premise for interpreting the results of VR studies, namely that the method itself does not affect time perception, has received little attention. Here we tested this assumption by comparing timing performance in a real environment and a VR scenario. Participants performed two timing tasks, requiring the production of intervals defined either by numerical values ("eight seconds") or by a physical process (“the time it takes for a bottle to run out when turned over"). We found that the experience of immersive VR exclusively altered judgments about the duration of physical processes, whereas judgments about the duration of abstract time units were unaffected. These results demonstrate that effects of VR on timing performance are not driven by changes in time perception itself, but rather by altered expectations regarding the duration of physical processes. The present study validates the use of VR in time perception research and strengthens the interpretation of changed timing behaviour induced by manipulations within VR

Evolution of metabolic networks: a computational frame-work

Background: The metabolic architectures of extant organisms share many key pathways such as the citric acid cycle, glycolysis, or the biosynthesis of most amino acids. Several competing hypotheses for the evolutionary mechanisms that shape metabolic networks have been discussed in the literature, each of which finds support from comparative analysis of extant genomes. Alternatively, the principles of metabolic evolution can be studied by direct computer simulation. This requires, however, an explicit implementation of all pertinent components: a universe of chemical reactions upon which the metabolism is built, an explicit representation of the enzymes that implement the metabolism, a genetic system that encodes these enzymes, and a fitness function that can be selected for. Results: We describe here a simulation environment that implements all these components in a simplified way so that large-scale evolutionary studies are feasible. We employ an artificial chemistry that views chemical reactions as graph rewriting operations and utilizes a toy-version of quantum chemistry to derive thermodynamic parameters. Minimalist organisms with simple string-encoded genomes produce model ribozymes whose catalytic activity is determined by an ad hoc mapping between their secondary structure and the transition state graphs that they stabilize. Fitness is computed utilizing the ideas of metabolic flux analysis. We present an implementation of the complete system and first simulation results. Conclusions: The simulation system presented here allows coherent investigations into the evolutionary mechanisms of the first steps of metabolic evolution using a self-consistent toy universe