This paper explores the use of a steady-state scanning laser Doppler vibrometer (LDV) system for the identification
of transition areas between solid, liquid, and gaseous substances in an enclosed container. This technique images
lateral surface velocity under the excitation of a single-frequency ultrasonic tone, produced by a piezoelectric actuator.
Differences in measured spatial wavenumber at discrete measurement points of a surface scan can be used to detect
the boundaries between solid, liquid and gaseous regions of material. We used the LDV system to compare the relative
distributions of solid wax, liquid wax, and air in a cylindrical container based on local changes in wavenumber.
Through the same methodology, we were able to distinguish the transition between solid and liquid epoxy in a
container. Finally, by repeatedly scanning the container during a phase-changing reaction within the container, we
established that the system can be used to monitor reactions as they progress.