A review of the methods of point measurement and spatial measurement on thin liquid film thickness

Liquid film phenomena exist widely in nature and in industry processes. In particular, fuel spray in combustion engines can always form the thin liquid film in sub-millimeter or micron scales. The high precision measurement of the liquid film thickness is vital for the design and improvement of combustion engines. In this paper, we briefly review the point measurement and spatial measurement methods that are commonly used in the thin liquid film thickness measurement. Point measurement methods mainly include electric methods and the total internal reflection (TIR) method for measuring the liquid film thickness at a single position, which are low cost and easily operational but have no spatial resolution. Spatial measurement methods mainly include electric methods, fluorescence intensity method and planar laser induced fluorescence (PLIF) method, which can simultaneously measure the liquid film thickness at multiple positions or in continuous regions and obtain the information about the film thickness distribution and film movement and development. Among spatial measurement methods, electric methods are easily operational and highly stable, but the electrodes can disturb the liquid film. On the other hand, the optical methods are always non-intrusive and suitable for the measurement of the liquid film in high speed motion.