Abstract. The scientific investigation of the nature of shock waves started 130 years ago with the advent of the schlieren method which was developed in the period 1859-1864 by August Toepler. At the very beginning applied to the visualization of heat and flow phenomena, he immediately turned to air shock waves generated by electric sparks, and subjectively studied the propagation, reflection and refraction of shock waves. His new delay circuit in the microsecond time regime for the first time made it possible to vary electrically the delay time between a spark generating a shock wave and a second spark acting as a flash light source in his schlieren setup. In 1870 Toepler, together with Boltzmann, applied Jamin's interferometric refractometer and extended the visualization to very weak sound waves at the threshold of hearing. Toepler's pioneering schlieren method stimulated Ernst Mach and his team to objectively investigate the nature of shock waves: they improved Toepler's time delay circuit; continued the study on the reflection of shock waves; introduced shadowgraphy as a modification of the schlieren method; photographed the propagation of shock waves generated by an electric spark and by supersonic projectiles, and improved interferometry. Based on a large number of original documents the paper illuminates the concomitant circumstances of the invention of the schlieren method and its first applications by others.
A comprehensive testing of microactuators is decisive (i) to prove the workability, (ii) to characterize the output parameters, (iii) to provide kinematic and temperature data for a revaluation of numerical modeling, (iv) to ensure the expected response with the macroscopic world, and (v) to study the mechanism of a premature failure. All this information helps to develop well-aimed strategies for improvements and promote the marketing process of new types of actuators. The most important diagnostic methods encompass high-speed photomicrography, laser interferometry and scanning laser Doppler velocimetry. High-speed thermography and flash microradiography have not yet been fully established for dynamic testing in the microscopic domain because of various technical limitations. The present state of the art of these methods is discussed and illustrated at various microactuator types. By the example of a microrelay it is shown that also material data can be derived from a motion analysis and that the relay operability can be improved by using methods of systems theory and control engineering.
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