Using large scale molecular dynamics simulations, we study the non-linear rheology of a model glass (a 80:20 binary Lennard-Jones mixture). In a glassy state, this model exhibits shear banding at low imposed shear rates. This behavior seems to be closely related to the existence of a static yield stress, σy, and to the observation that, under imposed shear rate, σ(γ→0) < σy. We measure the static yield stress via simulations under imposed stress and investigate its dependence on temperature. For temperatures far below Tc (the mode coupling critical temperature of the model), σy hardly decreases upon heating whereas it changes much faster as Tc is approached. Furthermore, at extremely low shear rates, large stress fluctuations are observed which is reminiscent of the stick-slip motion observed in friction simulations. Our results, which open the possibility of exploring complex rheological behavior using simulations, are compared to recent experiments on various soft glasses.
The EUREKA-project CHOCLAB (Characterization of Optical Components and Laser Beams), which was initiated in 1995, aims to investigate and develop instruments and standard measurement procedures for laser beam and laser optics characterization. From the start of the project, one of the main work items in the field of laser beam characterization has been the determination of the beam propagation parameters (beam width, divergence angle and beam propagation factor). The most common measurement devices for these purposes are 2d-matrix-cameras (i.e. CCD-cameras), 1d or 2d scanning devices (i.e. scanning pinholes and/or moving knife- edges)--both of them requiring further evaluation of the measured power density distribution to obtain laser beam parameters--and variable transmission/reflectivity optical elements, which allow the direct measurement of beam properties. The advantages and drawbacks of the different types of measurement devices, as well as measures to reduce systematical and statistical errors will be discussed.
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