Experimental setup for simultaneous rainbow refractometry on multiple micrometric droplets

Aim of this thesis is the implementation of an experimental setup for the simultaneous analysis of the light scattered by two droplets illuminated by a laser beam. The analysis involved size and evaporation rate measurement and refractive index detection by means of interference pattern measurement and rainbow refractometry. Experimental investigation and data analysis were carried out at the Institute of Aerospace Thermodynamics of the University of Stuttgart (ITLR - Institut für Thermodynamik der Luft- und Raumfarth). The droplets, having sizes between 30 m and 80 m, were analyzed while falling freely through a glass-made probe chamber, illuminated from the bottom by the ray produced by a diode laser. The droplets, generated by two piezoelectric generators, were made of various n-alkanes. The scattered light was recorded by two CCD cameras, one for the interference fringes in the forward hemisphere, the other for the rainbow signal in the backward hemisphere. In order to get images of multiple droplets avoiding superimpositions, a close-focus arrangement was adopted for the optics. The signal was acquired and processed using specifically written software. Three kinds of experiments were conducted. Single droplets were analyzed to validate the experimental setup and the processing software. Doublets of droplets falling together were analyzed to perform the simultaneous observation of the signal scattered by multiple droplets, extending the common technique for size measurement of groups of droplets to the analysis of the rainbow signal. The collision of droplets was also studied, analyzing the light scattered during the coagulation and mixing of droplets. The results of the analysis of single droplets, obtained using the close-focus arrangement, were compared to the values obtained using the standard infinite-focus arrangement, and to the data found in literature. The matching of evaporation rate and refractive index proved the correct functioning of the experimental setup and the analysis code. The evaporation rate and refractive index were calculated correctly for several doublets of droplets of either same and different substances, proving the capacity to perform the simultaneous analysis on two droplets. The analysis of droplets during collision was also performed, showing the rainbow signal during the coagulation of droplets and measuring size and refractive index of the resulting mixed droplet.