A digital oscilloscope setup for the measurement of a transistor's characteristics

The measure of the characteristics of a transistor is an important step in an introductory electronics course. We propose to use a digital oscilloscope with a USB connection to perform a measurement of the characteristic curves with no additional custom circuitry. The setup is presented alongside with code that allows the importation and analysis of the results with open-source software.Comment: 4 pages, 7 figures. Data files and program files available upon request, contact info at: http://homepages.ulb.ac.be/~pdebuyl

Solvent evaporation of spin cast films: crust effects

When a glassy polymer film is formed by evaporation, the region near the free surface is polymer rich and becomes glassy first, as noticed long ago by Scriven et al. We discuss the thickness of this "crust" and the time interval where it is present -before freezing of the whole film. We argue that the crust is under mechanical tension, nd should form some cracks. This may be the source of the roughness observed on the final, dry films, when the solvent vapor pressure is high (and leads to thin crusts).Comment: 8 pages, 2 EPS figures. submitted to Europ. Phys. J. (E

On simplicity of reduced C*-algebras of groups

A countable group is C*-simple if its reduced C*-algebra is a simple algebra. Since Powers recognised in 1975 that non-abelian free groups are C*-simple, large classes of groups which appear naturally in geometry have been identified, including non-elementary Gromov hyperbolic groups and lattices in semisimple groups. In this exposition, C*-simplicity for countable groups is shown to be an extreme case of non-amenability. The basic examples are described and several open problems are formulated.Comment: 23 page

Instabilities during the evaporation of a film: non glassy polymer + volatile solvent

We consider solutions where the surface tension of the solvent is smaller than the surface tension of the polymer. In an evaporating film, a plume of solvent rich fluid, then induces a local depression in surface tension, and the surface forces tend to strengthen the plume. We give an estimate (at the level of scaling laws) for the minimum thickness required to obtain this instability. We predict that the thickness a) is a decreasing function of the solvent vapor pressure b) should be very small (<1 micron) provided that the initial solution is rather dilute. The overall evaporation time for the film should be much longer than the growth time of the instability. The instability should lead to distortions of the free surface and may be optically observable. It should dominate over the classical Bernard-Marangoni instability induced by cooling.Comment: 8 pages, 2 EPS figures. submitted to Eur. Phys. J. (E