Calibration of Mie scattering imaging for microbubble measurement in hydrodynamic test facilities

Calibration of the Mie Scattering Imaging (MSI) technique for microbubble size and concentration measurement in hydrodynamic test facilities is investigated. Monodisperse bubbles are generated by a microfluidic'T' junction, and individual bubbles simultaneously imaged with shadowgraphy and MSI. Nominal bubble diameters between 30 and 150μm were tested. The influence of fringe uniformity and intensity for each polarisation on measurement precision was investigated. Parallel polarisation was chosen over perpendicular for its more uniform spacing despite the lower intensity. The linear relation between fringe wavelength and bubble diameter was demonstrated at a measurement angle of 90°. The calibration was derived from constants for light scattering, and for the imaging optics. The wavelength of the scattered fringe pattern is predicted using the Lorentz-Mie theory. A practical method for the calibration of interference patterns is presented. Using this approach, the measured bubble diameters from the shadowgraphy and MSI compare to within 1μm. A method for determining the size-dependent measurement volume for axisymmetric and arbitrary beam profiles is also presented.