On measuring 3D flow within inkjet droplet streams using a digital holographic microscope

With a view to measuring the internal and external flow fields in an inkjet droplet stream, this paper discusses the problem of imaging through droplet surfaces and more generally interfaces in two-phase flows. First the propagation of optical fields through interfaces between media of different refractive index is discussed with reference to scalar diffraction theory. Some approximations suitable for droplet imaging are then discussed and the use of a-priori information is then explained. The imaging technique is applied to holograms recorded using a digital holographic microscope and is illustrated in the synthesis of a 3D image that is reconstructed through a cylindrical telecommunication fibre. Subsequently we demonstrate for the first time, high resolution imaging throughout an inkjet droplet. The practical implementation and improvement of these imaging methods for the measurement of two-phase flows is then discussed.