A multi-frequency 3ω method for tracking moving phase boundaries

A technique has been developed to track a moving phase front using the electrothermal 3ω method and demonstrated by tracking the location of the phase boundary between air and dielectric oil. A fine wire 3ω sensor (diameter 10 µm, length 30 mm) is suspended in oil and excited at four frequencies simultaneously to gain more thermal information than a single-frequency approach. Measurements of the phase boundary location are compared to camera images to verify their accuracy. For slow front velocities which approximate quasistatic operation, the location of the oil-air front determined from the 3ω approach is found to be accurate to within an average error of under 18 µm (root mean square) for front distances between 12 and 360 µm. Frequency cross talk and other considerations unique to multifrequency measurements are also discussed.