Ultrasonic monitoring of interfacial motion of condensing and non-condensing liquid films

An ultrasonic technique was developed to monitor the dynamic behavior of condensing and non-condensing fluid films. Both stable (upwards-facing) and unstable (downwards-facing) liquid films were examined. The ultrasonic system utilized 5 MHz planar piston transducers operated in pulse-echo mode. The transducers were mounted underneath a copper block on which the films resided. The technique is capable of measuring film thicknesses ranging from approximately 50 μm to several centimeters. The thicknesses of the thicker films (>1000 μm) were determined by time-of-flight measurements. For the thin films, a Fourier Transform method was employed. A signal-processing algorithm automatically applied the appropriate method, based on the film thickness. The film configurations examined included both static and growing non-condensing films, a non-condensing film with surface waves, and stable and unstable condensing films. The ultrasound measurements of film thicknesses agreed closely with the known thicknesses of static and growing, planar films. The measurements of the wave speed and wavelength also agreed well with those observed by optical means. The measured thickness of unstable condensing films (downwards-facing cooled surface) exhibited the expected fluctuations owing to the formation, passage, and break-off of pendant fluid drops. A numerical model developed to compute the acoustics associated with a stagnant film agreed well with experimental results.