Inversion of Critical Light Scattering.

This Ph.D. work deals with the scattering of light by a cloud of bubbles (i.e. particles with relative refractive index below unit) near the critical-scattering-angle. Under a single scattering assumption, a critical scattering pattern (CSP) can be accurately modeled with the Lorenz-Mie theory and a Fredholm integral of the first kind. A CSP is basically composed of several bows that are very similar to those observed in the forward diffraction zone and the rainbow region. By measuring the angular spreading, the visibility and the global position of these bows it is possible to deduce the bubbles mean size, poly-dispersion and mean refractive index (i.e. composition). We have developed several inverse methods and experiments to recover these properties, demonstrating the validity of what can be called the "Critical-Angle Refractometry and Sizing" (CARS) technique. Various effects like the laser Gaussian intensity profile or the laser beam spectrum width have also been studied. This new optical particle characterization technique is thought to be a useful tool to study real bubbly flows as well as for laboratory experiments requiring bubbles material recognition.