Development of an optical imaging technique for particle number density

Abstract

Accurate measurements of particle number density along with particle diameters and velocities are strongly required both in academic and industrial fields. A new imaging technique, through the evaluation of the effective depth of field of a camera, is developed using standard solid particles with constant diameters. To measure the effective depth of field for a wide range of particle diameters, three optical setups, named microscale, mesoscale, and macroscale setups, are used for the diameters of 50 μm - 201 μm, 201 μm - 3.97 mm, and 3.97 mm - 15 mm, respectively. The measured effective depth of field is further applied to measure the size dependence of the number density of entrained bubbles by breaking waves in a wind-wave tank. The results show that the slopes of the number density of the entrained bubbles in the experiments corresponded to those measured by a phase Doppler particle analyzer under 500 μm, and was -5 over 500 μm in both fresh and salt waters. This emphasizes that the present imaging technique can measure the diameters and particle number density with high precision and is important for measurements of droplets, bubbles, and solid particles with a wide range of diameters.

Keywords

• image analysis
• particle
• bubble
• wind wave
• shadow sizing method