Using laser Doppler vibrometry to measure capillary surface waves on fluid-fluid interfaces

Capillary wave phenomena are challenging to study, especially for microfluidics where the wavelengths are short, the frequencies are high, and the frequency distribution is rarely confined to a narrow range, let alone a single frequency. Those that have been studying Faraday capillary waves generated by vertical oscillation have chosen to work at larger scales and at low frequencies as a solution to this problem, trading simplicity in measurement for issues with gravity, boundary conditions, and the fidelity of the subharmonic capillary wave motion. Laser Doppler vibrometry using a Mach–Zehnder interferometer is an attractive alternative: The interface’s motion can be characterized at frequencies up to 40 MHz and displacements of as little as a few tens of picometers.