Wave-induced droplet growth measurement by laser light M. Fransen, E. Sachteleben, and D. Smeulders 1 Introduction Wave experiments have been carried out in our pulse-expansion wave tube (PEWT) to study the fundamentals of water condensation. The results are of paramount im- portance for meteorological models and climate studies. The process of condensa- tion consists of two distinct phases: nucleation and droplet growth. The former is the formation of stable (critical) clusters by the vapor molecules in the gas-vapor mix- ture. Typically, only some tens of water molecules are needed to create such a stable cluster. When these stable nuclei are formed they gradually evolve into macroscopic droplets. The sizes of these water droplets can be detected by means of laser optics. In this paper we present an improved optical detection technique. 2 Wave Experiments The PEWT is specifically designed for studying nucleation and droplet growth [1]. The idea to use a shock tube for this originates from Peters [2]. The gas dynamic aspects of our shock tube are described in further detail elsewhere [1]. For clarity, we summarize the working principles here. A polyester diaphragm initially separates the high pressure section (HPS) with a length of 1.25 m from the low pressure section (LPS) with a length of 9.25 m. The HPS contains a mixture of compressed synthetic air and a well-defined amount of water vapor at 2 MPa. The measurement plane where the nucleation (pulse) is measured, is located 5 mm away from the HPS end wall, where thermal boundary layers do not play a role. The LPS is filled with nitrogen at roughly 1 MPa. It also contains a local widening section (W) that has a larger cross-sectional area than the rest of the tube [3]. The rupture of the diaphragm starts the wave experiment: it creates both an expansion fan (A) moving towards the Eindhoven University of Technology, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands 1