High-spatio-temporal resolution molecular tagging velocimetry by photobleaching of rhodamine 6G

Molecular tagging velocimetry (MTV) is a non-intrusive optical diagnostic well suited to measure velocity gradients.  However, in liquids, the technique has suffered from limited temporal and spatial resolutions and the need for specialized lasers.  Here, rhodamine 6G dye is the molecular tracer in a two-laser write/read configuration.  A frequency-tripled, pulsed, Nd:YAG laser photobleaches the dye efficiently.  The read signal is a laser-induced fluorescence of the untagged dye by a green (frequency-doubled Nd:YLF) laser, leading to images of a dark pattern against a bright background.  The write process is prompt (order of nanoseconds) enabling very short read times; it is also permanent.  Additionally, very fine patterns (tens of micrometers) are generated by Talbot-effect structured illumination. With those improvements, MTV by photobleaching reaches unprecedented spatio-temporal resolutions and becomes a very flexible scheme to study a broad range of liquid flows. Here, it is demonstrated in a turbulent channel flow with spatial resolution of 25 micrometer and repetition rate of 10 kHz.