Secondary rainbow refractometry for droplet refractive index and size measurement

Abstract Accurate measurement of droplet size, temperature and concentration in sprays is of great importance but still challenging in applications. The secondary rainbow refractometry is investigated in this work to measure refractive index and size of transparent spherical droplets, with better sensitivity than the first rainbow. Inversion algorithm of the secondary rainbow is used and validated through processing the secondary rainbow signals simulated by the Lorenz-Mie theory. The secondary rainbow refractometry is applied to measure monodispersed spherical droplets of ethanol and N-heptane with different diameters in the isothermal conditions. Measured refractive indices of both liquids are 1.3621 and 1.3950 with diameter of about 145 µm and uncertainty of size measurement is down to 1 μ m with RMS error of 0.3 μ m . Compared with results from the microscopic imaging, the maximum of deviation as well as the relative error are within several microns and 4 % . This work confirms the feasibility and high accuracy of the secondary rainbow refractometry in spherical droplet size and refractive index measurement at room temperature of about 300 K .