MEASUREMENT OF LIQUID FILM THICKNESS BY A FRINGE METHOD

摘要

A technique to measure the local thickness of a droplet or a liquid film by an interference fringe pattern that was formed by reflecting laser lights was developed and tested. Monochromatic epi-illumination through an objective lens of a microscope was provided by a 5mw-300mw laser and a filter to remove the noise caused by laser speckle. The incremental height difference of the liquid layer between neighboring maxima or minima of fringes was evaluated from the wavelength of the laser light and the refractive index of the liquid. Estimation error of a local inclination angle was discussed using ray tracing under parallel illumination approximation (Kuehner et al., 1996). Droplet profiles evaluated from the interferogram that were obtained by the present fringe method agreed well with those by Laser Focus Displacement Meter (LFD) (Fukamachi et al., 2003). Measurement was tried to make sure the usefulness of the present technique. It was made clear that a) contact angle of a liquid droplet could be obtained precisely and swiftly even in small size or small contact angle, and b) instantaneous three dimensional profile of a liquid film on a bubble moving in a micro-channel could be measured. The fringe method had sufficient potential to obtain more detailed information about three dimensional characteristics of liquid film in flow phenomena such as the generation, break down and growth of waves and the liquid film of a bubble at the beginning of movement.